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A Practical Guide to
Noise and Vibration Control
for HVAC Systems

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digital form is not permitted without ASHRAE's prior written permission.

The first edition of this book was prepared under ASHRAE Research Project
RP-526, sponsored by TC 2.6, Sound and Vibration.

ABOUT THE AUTHOR
Mark E. Schaffer, PE, Member ASHRAE, is president of Schaffer Acoustics Inc.,
a southern California acoustical consulting and testing firm. He has been an acoustical consultant for 34 years and an HVAC subcontractor for 5 years. He is an active
member of ASHRAE Technical Committee 2.6, Sound and Vibration, and a copresenter of the ASHRAE Professional Development Seminar on HVAC noise and
vibration control. He has participated in the development of ASHRAE Handbook
chapters in that area.

Updates/errata for this publication will be posted on the
ASHRAE Web site at www.ashrae.org/publicationupdates.

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RP-526

A Practical Guide to
Noise and Vibration Control
for HVAC Systems
SECOND EDITION

Mark E. Schaffer

American Society of Heating, Refrigerating
and Air-Conditioning Engineers, Inc.

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ISBN 978-1-936504-02-2
© 2011 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.
1791 Tullie Circle, N.E., Atlanta, GA 30329
www.ashrae.org
All rights reserved.
First edition 1991; Second edition 2011
Printed in the United States of America
Cover design by Tracy Becker.
ASHRAE has compiled this publication with care, but ASHRAE has not investigated, and ASHRAE
expressly disclaims any duty to investigate, any product, service, process, procedure, design, or the like that
may be described herein. The appearance of any technical data or editorial material in this publication does
not constitute endorsement, warranty, or guaranty by ASHRAE of any product, service, process, procedure,
design, or the like. ASHRAE does not warrant that the information in the publication is free of errors, and
ASHRAE does not necessarily agree with any statement or opinion in this publication. The entire risk of the
use of any information in this publication is assumed by the user.
No part of this publication may be reproduced without permission in writing from ASHRAE, except by a
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any part of this publication be reproduced, stored in a retrieval system, or transmitted in any way or by any
means—electronic, photocopying, recording, or other—without permission in writing from ASHRAE.
Requests for permission should be submitted at www.ashrae.org/permissions.
Library of Congress Cataloging-in-Publication Data
Schaffer, Mark E., 1949A practical guide to noise and vibration control for HVAC systems / Mark E. Schaffer. -- 2nd ed.
p. cm.
Includes bibliographical references.
Summary: "A guide that presents rules for controlling the noise and vibration of HVAC Systems"-Provided by publisher.
ISBN 978-1-936504-02-2 (softcover)
1. Heating--Equipment and supplies--Noise. 2. Ventilation--Equipment and supplies--Noise. 3. Air
conditioning--Equipment and supplies--Noise. 4. Noise control. I. American Society of Heating, Refrigerating and Air-Conditioning Engineers. II. Title.
TH7015.S33 2011
697--dc22
2011002509

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Contents
Preface

Acknowledgments
Introduction

xvii

xix

Chapter 1—General Design Guidelines
Chapter 2—Airside Equipment

Chapter 3—Water-Side Equipment

Chapter 4—Packaged and Unitary Equipment
Chapter 5—Vibration Isolation
Chapter 6—Specifications

101

117

Chapter 7—Construction Phase Tasks

125

Chapter 8—Troubleshooting Noise and Vibration Complaints
Appendix A—Some Basics of HVAC Acoustics

167

Appendix B—Acoustical Rating Systems and Criteria
Appendix C—Measuring HVAC System Noise

191

Appendix D—Using Manufacturers’ Sound Data
Appendix E—Definitions and Abbreviations

177

197

201

Appendix F—Addresses of Agencies and Associations
Appendix G—Bibliography and Selected References

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207
209

143

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Illustrations
FIGURES
Introduction
A
B

Example of an air-handling unit room with several acoustical problems. xx
Example of an air-handling unit room with optimal acoustical features. xxii

Chapter 1
1-1
1-2
1-3
1-4
1-5
1-6
1-7
1-8
1-9
1-10
1-11
1-12
1-13
1-14

Guideline for duct chase, shaft, and enclosure sizing. 4
Acoustical comparison of various building core area layouts. 5
Guidelines for the preliminary selection of mechanical room walls. 6
Sample mechanical penthouse equipment layout. 7
Labyrinth air path used for sound attenuation at an equipment room ventilation
opening. 10
Upward noise control for mechanical rooms. 10
Downward noise control using an auxiliary ceiling. 11
Section views through two types of floating floor assemblies. 12
Sound transmission at perimeter mechanical rooms. 12
Electrical conduit routing into a mechanical room. 13
Pipe lagging for noise control. 14
Examples of rumbly and quieter parallel fan installations. 15
Guidelines for a basement built-up fan system. 16
Typical duct silencer arrangement at vane-axial fan. 17

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Illustrations

1-15
1-16
1-17
1-18
1-19
1-20
1-21
1-22

Reflected and refracted equipment sound at a building perimeter. 18
Structural support of rooftop equipment for vibration control. 19
Guidelines for mechanical room wall selection. 20
Duct and pipe penetrations through walls. 21
Plan view of return air shaft with supply duct takeoffs obstructing return
airflow. 22
Acoustical comparison of several duct chase, shaft, and enclosure
constructions. 23
Two typical duct laggings. 24
Noise control duct enclosure. 24

Chapter 2
2-1
2-2
2-3
2-4
2-5
2-6
2-7
2-8
2-9
2-10
2-11
2-12
2-13
2-14
2-15
2-16
2-17
2-18
2-19
2-20
2-21

Inlet and discharge octave band LW values for a 925 mm plenum fan. 27
Sound power level comparison for three types of centrifugal fans. 28
Guidelines for centrifugal fan installations. 29
Inline fan airflow patterns. 30
Cutaway view into a mixed flow fan. 31
Inline fan sound power level comparison. 31
Guidelines for ducted axial flow fan installations. 32
Guidelines for unducted axial flow fan installations. 33
Inlet side of a direct-drive plenum fan. 34
Discharge side of a belt-drive plenum fan. 34
Power roof ventilators (mushroom fans) mounted on intake duct silencers and
roof curbs. 35
Mushroom type exhaust fan on vibration-isolated roof curb. 36
Inlet octave band LW comparison for three propeller fans. 37
Propeller fan with a 12-socket aluminum hub and plastic blades. 37
Ultra-low-noise propeller fan with backswept airfoil blades. 38
Lined hood for propeller fan noise control. 38
Vibration isolation suspension for propeller fans. 39
Noisy and quiet installations of ceiling-mounted exhaust fans. 39
Minimum clearance at AHU and cabinet fan inlet. 41
Plenum AHU with supply ducts attached to the top of discharge plenum. 42
Cutaway sketch of a plenum fan air-handling unit. 42

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2-22
2-23
2-24
2-25
2-26
2-27
2-28
2-29
2-30
2-31
2-32
2-33
2-34
2-35
2-36
2-37
2-38
2-39
2-40
2-41
2-42
2-43
2-44
2-45
2-46
2-47
2-48
2-49
2-50
2-51
2-52

Guidelines for VAV unit installation. 43
Laboratory air valve and its “noise flow” directions. 45
Good inlet duct connection to a supply air ceiling diffuser. 47
Poor inlet duct connection to a supply air ceiling diffuser. 47
The effect of installing a damper behind a grille. 48
Attenuation for lined and unlined sheet metal ductwork. 49
Breakout transmission loss for three types of sheet metal ductwork. 50
Guidelines for minimizing regenerated noise in elbows. 52
Guidelines for minimizing regenerated noise in takeoffs. 52
Guidelines for minimizing regenerated noise in transitions and offsets. 53
Guidelines for minimizing regenerated noise in duct tees. 53
In-duct attenuation for various duct liner thicknesses. 54
The speaking tube (cross-talk) problem. 55
Attenuation of rectangular elbows with and without turning vanes (lined and
unlined). 57
Attenuation of rectangular and radius elbows (lined and unlined). 57
Flexible duct with spunbond nylon inner liner. 58
Cutaway view into a dissipative duct silencer. 59
Cutaway view of a duct silencer with film-lined baffles. 59
Cutaway view of a reactive (“packless,” “no-fill,” or “no-media”) duct
silencer. 61
Cutaway view of an elbow duct silencer. 61
In-duct attenuation of duct silencers and lined ductwork. 62
Comparative insertion loss of dissipative, film-lined, and reactive duct
silencers. 62
Guidelines for duct silencer placement near fans and duct fittings. 63
Duct silencer placement near a mechanical room wall. 64
General guidelines for sound-attenuating plenum design. 66
Acoustical louver cutaway. 66
Sound transmission loss of acoustical and weatherproof louvers. 67
Acoustical louver in a parking garage ventilation shaft. 67
Basis for fan selection in a VAV system. 68
Nested inlet vanes obstruct airflow. 70
Variable-frequency drive. 70

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A Practical Guide to Noise and Vibration Control for HVAC Systems

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Illustrations

Chapter 3
3-1
3-2
3-3
3-4
3-5
3-6
3-7
3-8
3-9
3-10
3-11
3-12
3-13
3-14
3-15
3-16
3-17
3-18

Water-cooled screw chiller with several noise and vibration control
treatments. 74
ARI-370 LW values for a 875 kW air-cooled chiller with and without factory
noise reduction options. 75
ATC-128 octave band LP values at 15 m from the air inlet side of three types
of 2800 kW cooling towers. 76
ATC-128 octave band LP values for cooling towers of the same fabrication
series but with fans of different diameters. 77
View of “standard” cooling tower induced-draft fan. 78
View of an induced-draft fan with wide-chord blades. This fan type can be as
much as 12 dBA quieter than a “standard” fan of the same diameter. 78
ATC-128 octave band LP values for cooling towers with “standard” and widechord fan blades. 79
Cooling tower basin with free-falling condenser water. 80
Honeycomb water basin silencers installed several centimeters above the
water surface. 80
Outdoor noise control barrier installation. 81
Close-up view of a sample of a sound-absorbing, outdoor noise barrier
panel. 81
Low-noise control sequence for a two-cell cooling tower. 82
Pump impeller sizing guideline for minimizing the strength of the blade
passage frequency tone. 83
Proper installation of an end-suction pump. 84
Proper installation of an inline pump. 85
Vibration isolation for piping riser. 87
Duct and pipe penetrations through walls. 87
Sealing pipe penetrations for sound isolation. 88

Chapter 4
4-1
4-2
4-3
4-4
4-5
4-6

Very noisy rooftop unit installation. 90
Moderately noisy rooftop unit installation. 90
Moderately quiet rooftop unit installation. 91
Quietest rooftop unit installation. 91
Guidelines for suspended heat pump units. 94
Guidelines for floor-mounted heat pumps. 94

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A Practical Guide to Noise and Vibration Control for HVAC Systems

4-7
4-8
4-9
4-10
4-11
4-12

Small condensing unit noise control. 96
Guidelines for fan coil unit installations. 97
Guidelines for vibration isolation of split systems. 98
Indoor fan coil section of a ductless split system. 98
Outdoor condensing unit typically used with ductless split systems; it can also
be used with ducted fan coil units. 99
Remote radiator for engine-generator sets can be quiet with an oversized,
variable-speed cooling fan. 100

5-1
5-2
5-3
5-4
5-5
5-6
5-7
5-8
5-9
5-10
5-11
5-12
5-13

Elastomeric pads. 106
Elastomeric or compressed fiberglass isolation mounts. 107
Seismically rated elastomeric mounts. 108
Two types of spring floor mounts. 108
Spring hanger installation. 109
Two types of spring floor mounts with seismic/wind-loading standby
restraints. 110
Pneumatic isolators (“air bags”) supporting a rooftop air-cooled chiller. 111
Cast-metal floor mount. 111
Thrust restraint at mid-height of fan inlet panel. 112
Flanged and threaded flexible pipe (pump) connectors. 113
Braided metal pump connector is not an effective vibration isolator. 113
Floor mount spring isolator under a height-saving bracket with a separate
seismic restraint. 114
Pump mounted on combination isolator/restraint. 115

Chapter 7
7-1
7-2
7-3
7-4
7-5
7-6
7-7

Overhead plan views of AHU rooms showing the effects of a duct
offset. 132
Properly installed dual-duct variable air volume unit. 135
Check of large duct elbow verifying screw attachment of turning vanes. 136
Conduit debris short-circuiting isolator effectiveness. 137
Overloaded spring hanger. 138
Overloaded free-standing floor mount. 138
Short-circuited floor mount isolator whose shipping shims have not been
removed. 139

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Chapter 5

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Illustrations

7-8
7-9
7-10

Faulty spring hanger installation with hanger rod touching the hanger
box. 139
Taut outdoor “flexible” conduit forms a vibration “short-circuit” at cooling
tower. 141
Pipe risers without vibration isolation. 141

Chapter 8
8-1
8-2
8-3
8-4
8-5
8-6
8-7
8-8
8-9
8-10
8-11
8-12
8-13
8-14
8-15
8-16
8-17
8-18
8-19
8-20
8-21
8-22
8-23

Frequency ranges of the most likely sources of common acoustical
complaints. 144
Example of poor fan discharge duct design. 150
Fan installation with poor discharge duct system aerodynamics. 151
Restrained spring isolator with a “short circuit” between its baseplate and
equipment mounting plate. 155
View into the fan section of a rooftop unit. 156
Excellent rooftop package unit installation. 156
Improperly installed fan-powered variable-air-volume unit. 157
Proper installation of an indoor self-contained packaged HVAC unit. 157
Closely spaced circular duct fittings produce turbulence and noise. 158
Closely spaced rectangular duct fittings produce turbulence and noise. 158
Improper duct transition at fan inlet. 159
Vane-axial fan intakes too close to wall. 159
Duct split using radius elbows. 160
Faulty installation of a large equipment isolator with stanchion
restraints. 160
Proper installation of a large equipment isolator with stanchion
restraints. 161
Braided metal pump connectors do not provide significant vibration
isolation. 161
Neoprene pump connectors provide better isolation of pump vibration from
attached piping. 162
Incomplete vibration isolation at cooling tower. 162
Taut “flexible” conduit forms a vibration short-circuit at vane-axial fan. 163
Correctly installed flexible conduit between electrical disconnect and
motor. 163
View of pipe penetration from below roof. 164
Non-isolated pipe penetration. 164
Improperly placed neoprene hanger. 165

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A Practical Guide to Noise and Vibration Control for HVAC Systems

Appendix A
A-1
A-2
A-3
A-4
A-5
A-6

Airborne and structure-borne sound transmission. 168
Airborne and structure-borne sound transmission from equipment. 169
Chart for adding decibel values. 170
Everyday sound sources—their frequencies and wavelengths. 171
Frequencies at which various types of HVAC equipment generally control
their sound spectra. 172
Sound pressure levels of some everyday activities. 173

Appendix B
B-1

Frequency weighting curves.

178

B-2

Blank RC chart.

B-3

Blank RC mark II chart.

B-4

Blank NC chart.

B-5

Octave band spectrum rated at NC-45.

B-6

Blank NCB chart.

B-7

Quality of speech communication in background noise.

180
181

183
184

185
189

Appendix C
C-1

Sound level meter.

192

C-2

Sound measurement plan.

C-3

Blank sound measurement data sheet.

C-4

Completed sound measurement data sheet.

193
194
195

TABLES
Chapter 1
1-1

Maximum Mid-Span Deflections for Above-Grade Structures that Support
Vibration-Isolated HVAC Equipment 7

1-2

Selection Guidelines for Slabs Separating Mechanical Equipment Rooms
from Noise-Sensitive Occupied Spaces 9

Chapter 2
2-1

Suggested Maximum Airflow Velocities for Various Ductwork
Installations 51

2-2

Suggested Maximum Airflow Velocities in Elbows for Rectangular

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Illustrations

Ductwork
2-3

Acoustical Characteristics of the Various Types of Ductwork

Chapter 3
3-1

Maximum Recommended Waterflow Rates

Chapter 5
5-1

Vibration Isolation Selection Guide

102

Chapter 7
7-1
Common Value Engineering Proposals and Their Potential Acoustical
Impacts 126
7-2
Procedure for Converting from A-Weighted LW Values to Unweighted LW
Values 127
Appendix A
A-1
A-2

Octave Band Center Frequencies and Their Frequency Ranges
Subjective Impressions of Sound Level Differences 174

172

Appendix B
B-1
B-2
B-3

Recommended Indoor Sound Criteria 187
Industrial Noise Levels Requiring Employer Action
Sample Municipal Code Limits 189

188

Appendix C
C-1

Adjustment Values for Determining Equipment Sound Levels in the Presence
of Constant Background Noise 196

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Preface
Since the publication of the 1991 edition of this guide, building owners and
managers have been paying more attention to occupant comfort, and municipal
codes have become more attentive to property-to-property noise emissions. As a
result, the HVAC community has become much more sensitive to the acoustical
design of its products and systems. Many equipment manufacturers are producing
quieter products and sometimes use the acoustical benefit as the primary marketing
feature. Increased attention to product acoustical performance is evidenced by the
fact that the number of HVAC acoustical testing laboratories has almost doubled
since 1991. Also, increased internationalization has permitted access to low-noise
HVAC products from Europe and Asia.

Unfortunately, not all industry changes have been for the quieter. Energy-efficient
screw (rotary) compressors have been the source of many noise and vibration
complaints, and the momentum to remove internal acoustical liner from ductwork, or
to cover it with solid sheet metal in air-handling units and terminal boxes, has reduced
the palette of noise reduction strategies that can be used in system designs. These and
other factors increase the importance of system design in the form of more careful
attention to vibration isolation and the airflow aerodynamics in air distribution
systems. Therefore, the main theme of this guide has not changed over the past 20
years—that is, most HVAC system noise and vibration problems are system problems
that are due to the improper selection, design, or installation of the components into
a complete system. More careful attention to these factors will greatly reduce the
number and severity of noise and vibration complaints.

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System designers are paying closer attention to the acoustical performance of
the products and systems that they specify, and many building owners now require
acoustical consultants on project design teams.

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Acknowledgments
The information in this guide was taken from hundreds of publications and was
produced with the help of dozens of people. The author wishes to particularly thank
the following people:
The authors of the many articles, manuals, bulletins, and books from which many
of the guidelines and figures were taken;
The members of ASHRAE Technical Committee 2.6, Sound and Vibration, for
their direct contributions, review comments, encouragement, and patience;

The staffs of the acoustical consulting firms of McKay Conant Brook, Westlake
Village, CA, and Paul S. Veneklasen & Associates, Santa Monica, CA, for their
assistance;
Ms. Barbara A.J. Schaffer for her unending patience and support;
Mr. Eli Schaffer of Schaffer Mechanical, Inc., for his help in understanding HVAC
systems;
Mr. David Peters of Southland Industries, Long Beach, CA, for his review
comments of the original publication;
Messrs. Jerry Lilly, Karl Peterman, Rob Lilkendey, John Paulauskis and Mark Fly
for their very helpful and thorough reviews of the second edition;

xvii

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The staff of Gary Davis & Associates, Santa Monica, CA, for much of the artwork;

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Dr. David T. Blackstock of the Applied Research Laboratories, Austin, TX, for his
insistence on the correct usage of the English language; and
The entire membership of ASHRAE for its support of technical research in all areas
of HVAC system design.

xviii

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Introduction
Over the past few decades, building design teams have become more aware of
the potential noise and vibration problems from HVAC systems. Mechanical engineers are specifying sound traps (duct silencers), acoustical louvers, sound-absorbing duct liners, and vibration isolators, while architects are designing mechanical
room walls and slabs with high sound transmission class (STC) ratings. Despite the
addition of these noise and vibration control features in more and more building
designs, complaints of excessive HVAC system noise and vibration are still
common. Investigations into these complaints by acoustical professionals have
found that, in many cases, the correct equipment and materials were used, but they
were not properly integrated into a quiet system, or some seemingly insignificant
detail was omitted that negated the expected acoustical benefit.
A TYPICAL APPROACH
Figure A shows a typical air-handling unit (AHU) mechanical room in which
some of the most common system-related acoustical faults are noted. Of the 12
faults noted, very few have to do with the specific equipment or materials used.
Most of the faults are system related; that is, the individual components are not
designed and installed as if they were part of a whole system. Airflow through the
AHU/ductwork layout shown in the figure would generate excessive turbulence,
noise, and rumble. The small mechanical room would have restricted airflow in
places, resulting in locally turbulent airflow and unexpected noise and rumble in the
tenant space. The vibration isolators would not perform as expected because they
are resting on a flexible floor structure. The designers of this hypothetical system
did not consider all of the system (interactive) effects of the various components
and how they would control the amount of noise and vibration that enters the building’s occupied spaces.

xix

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Introduction

1. AHU panel vibration “couples” to the lightweight, flexible gypsum wall just a few centimeters
away. This coupling lets low frequency noise pass easily through the wall.
2. The counterclockwise rotation of the fan’s discharge airstream is forced to change its spin
direction at the downstream elbow. The turbulence generatred at the change can produce
unstable flow with a very high, fluctuating pressure drop, thereby resulting in fan instability
that is heard as rumble.
3. Problem 2 is aggravated if the elbow’s turning vances do not have long trailing edges to
straighten the airflow and control the turbulence.
4. The duct silencer is too close to the elbow. This compounds the turbulence problem.
5. Rectangular ductwork and duct silencers do not control the rumble produced by turbulent
airflow.
6. The AHU’s air inlet is too close to the wall. This causes two acoustical problems: unstable
fan operation leading to surge and rumble, and direct exposure of the inlet noise to the
mechanical room wall.
7. The lack of a duct silencer in a mechanical room return air opening allows fan noise to travel
into the ceiling cavity, then through the lightweight acoustical ceiling into the occupied space.
8. The unit is resting on thin cork/neoprene isolation pads that are too stiff to adequately isolate
the fan vibration.
9. The poorly isolated unit is resting on a relatively flexible floor slab without sufficient structural support. This arrangement allows unit vibration to enter the slab.
10. The chilled water piping is rigidly attached to the slab above, thereby letting unit vibration
enter the slab.
11. Ductwall vibration in the duct silencer (or any other part of the trunk duct system) touching
the drywall partition can cause the partition to act as a sounding board and radiate low
frequency noise into the occupied space.
12. Suspending ceiling from supply duct causes ceiling to be a sound radiator.

Figure A

Example of an air-handling unit room with several acoustical
problems.

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A Practical Guide to Noise and Vibration Control for HVAC Systems

A BETTER APPROACH
Correcting a noise or vibration problem usually costs much more than preventing one. The real costs include not only the direct payments to the retrofitting
contractor but also the time required to coordinate the investigation and retrofit and
the loss of goodwill from the complaining tenants. Therefore, in most cases the
slight extra cost for prevention (usually about 1% to 2% of the total HVAC system
cost) is money well spent.
Specifying quiet equipment and adding noise control materials to an HVAC
system design are necessary initial steps of the design process. Calculations during
the initial steps can be used to estimate the sound levels in a room or to select noise
control materials to achieve a design goal. Comparing equipment sound data for
competing products can help in the selection of quiet equipment. However, design
decisions based on such work lose their value if the equipment and materials are
not integrated into a properly designed system or if certain system detailing is
ignored.
Figure B shows one version of a typical AHU room in which basic construction
materials are integrated into a quiet system. The AHU and ductwork are selected
and laid out for the best aerodynamics possible (this also reduces pressure drop and,
therefore, fan horsepower). The AHU room is large enough for good aerodynamics
in the room and for adequate clearance between the unit and the walls. The supporting structure has been stiffened with a housekeeping pad and beam to permit better
vibration isolation.
SUMMARY OF THIS GUIDE
This guide presents information that will help engineers, architects, contractors, and others with little acoustical experience to develop project designs that
avoid the kinds of problems shown in Figure A. Chapter 1 includes general noise
control design guidelines to be addressed during the various design phases and
includes some information on the architectural and structural aspects of HVAC
system design. Chapter 2 addresses the selection and use of airside system components. Chapter 3 looks at the selection and use of waterside and central plant equipment.
Packaged and unitary equipment are addressed in Chapter 4. Guidelines for
selecting and using vibration isolation systems are included in Chapter 5. Sample
specifications for acoustical materials and the acoustical performance of HVAC
equipment are given in Chapter 6, along with some examples of poorly written
specifications. Reviewing submittals and making site inspections are addressed in
Chapter 7. Chapter 8 gives suggestions for troubleshooting HVAC noise and vibration complaints.

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Introduction

The appendices present background information, including some basics of
HVAC system acoustics, the use of criteria and manufacturer’s published sound
data, and how to make sound level measurements. Appendices with definitions of
acoustical terminology, abbreviations, and addresses of relevant agencies and associations are also included.

1. Keeping a minimum 600 mm clearance reduces coupling between AHU and wall. Masonry
wall provides excellent low frequency sound isolation.
2. Use of a horizontal discharge AHU eliminates the need for a turbulence-producing airflow.
3. Gradual transition at AHU outlet minimizes turbulence.
4. Duct silencer is far enough away from AHU outlet to avoid excessive regenerated noise and
turbulence.
5. Circular ductwork controls the transmission of low frequency noise and rumble into the
occupied space.
6. The large clearance at the AHU inlet keeps the unit away from the wall and avoids excessive
inlet turbulence.
7. The return air duct silencer controls AHU noise via the return air path.
8. The unit is resting on high-deflection, steel spring vibration isolators.
9. The floor assembly supporting the unit has a housekeeping pad and at least one major beam
under the unit. Additional stiffness and mass help to control the transmission of unit vibration
into the slab.
10. The chilled water pipes are suspended by vibration isolation hangers.
11. The supply trunk duct does not touch the wall. A 13 mm gap surrounding the duct is filled
with a non-hardening sealant.
12. Ceiling not suspended from supply duct.

Figure B

Example of an air-handling unit room with optimal acoustical
features.

xxii

General Design Guidelines
Virtually every survey on office comfort finds that, after temperature problems, excessive HVAC system noise levels are responsible for more complaints
than any other aspect of the office environment. Some of the many causes of the
excessive noise can be traced to one of more of the following:
•
•
•
•
•

Incorrect design of the HVAC systems and their equipment area walls and
slabs
Cost-cutting without regard for the noise and vibration implications
Improper installation
Improper start-up or commissioning
Ignoring the selection and/or installation guidelines published by HVAC
equipment and noise/vibration control product manufacturers

To minimize the possibility that design decisions could cause noise or vibration problems, a project’s design team must consider the acoustical impacts of all
design decisions, whether they are part of the schematic, design development,
construction documents, value engineering, or construction administration phases
of a project. The team members should also follow the selection and installation
guidelines that are published by most equipment and noise/vibration control product manufacturers.
All too often the area of HVAC acoustical design is limited to the addition of
duct silencers, duct lining, plenum lining, and vibration isolators at the end of the
Construction Documents (Working Drawings) phase, after the mechanical engineer has virtually completed the HVAC system design and long after the structural
system has been designed. These “add-on” acoustical treatments may control the

General Design Guidelines

noise and vibration, but if they are not properly integrated into the system, they can
reduce the system’s performance and energy efficiency. If selected or installed
improperly, they can even cause noise or vibration problems. Therefore, noise and
vibration control design should begin during the project’s Schematic Design phase
and continue throughout the entire design process. This is even more critical if the
use of fibrous materials, such as fiberglass and mineral wool, is not permitted.
Some “add-on” acoustical treatments may be required no matter how well a
system is designed, but the use of some, such as duct liner and duct silencers, can
sometimes be minimized if the HVAC system is designed with noise and vibration
control in mind from the start.
The more noise and vibration control you DESIGN IN, the less you have to
ADD ON.
The design phase of a new facility occurs in sub-phases with titles such as Preschematic Design, Schematic Design (SD), Design Development (DD), and
Construction Documents (CD). The placement and sizing of mechanical equipment rooms and duct shafts are typically set during the Schematic Design phase.
Therefore, many critical decisions for HVAC system noise and vibration control are
needed in this early design phase. More detailed noise and vibration control decisions can wait until the DD and CD phases.
One crucial SD phase milestone is the completion of the structural system
design. Delaying the acoustical design until after the structural system design is
essentially complete sometimes leaves the design team with little flexibility in
selecting and locating cost-effective noise and vibration control treatments. Troubleshooting investigations into HVAC system acoustical problems often find that
the locations of shear walls, beams, columns, and cross-bracing make effective
retrofit solutions either very expensive or, in some cases, impossible. Good acoustical design decisions made in cooperation with the structural engineer can avoid
this kind of problem and minimize the cost of noise and vibration control treatments.
Proper acoustical design requires broad cooperation in the areas of architecture, structural engineering, mechanical engineering, electrical engineering, and
acoustics. To that end, the design team should begin by working together in the
areas of (1) system type determination, (2) preliminary equipment selection, (3)
mechanical room and duct shaft sizing, and (4) space planning.

A Practical Guide to Noise and Vibration Control for HVAC Systems

SYSTEM TYPE DETERMINATION
Each type of HVAC system has its own set of layout and operating features that
determine which noise and vibration control measures are most cost-effective.
Therefore, the choice of one system type over another should not be made without
considering the cost of controlling noise and vibration. For example, a built-up
rooftop penthouse HVAC system would generate almost all of its noise and vibration in the penthouse and would, therefore, require most attention there to prevent
complaints from the occupants below the equipment rooms and near the duct
shafts. On the other hand, a system using water-source heat pump units distributed
throughout the building’s ceiling plenums would require much less concern for
noise and vibration control at the central plant but much more care in the selection,
placement, and installation of the dozens of noise and vibration sources (the heat
pump units) that are scattered throughout the building. It is, therefore, essential that
the acoustical properties of all potential system types be considered before finalizing the selection.
PRELIMINARY EQUIPMENT SELECTION
After determining the type of system, the mechanical engineer should make
preliminary equipment selections as soon as possible to allow for a preliminary
noise analysis and to determine the probable sizes of the mechanical equipment
rooms and duct shafts. The primary acoustical guideline is to select the quietest
equipment possible, which usually means selecting the most efficient equipment
(i.e., the lowest kilowatt rating at the maximum operating duty point). Since the
original publication of this guide, virtually all of the major equipment manufacturers have updated their equipment selection procedures to include acoustical data
that have been determined in accordance with internationally recognized testing
procedures. Therefore, obtaining the acoustical information necessary for comparing equipment alternatives is much easier than it was in the past.
MECHANICAL ROOM SIZING
Economic pressure to maximize a building’s usable/rentable space has
resulted in less space being available for the HVAC systems and other building
services. This reduction in equipment room size often forces the mechanical engineer to either select small, inefficient equipment or to shoehorn properly sized
equipment into a restricted space. A similar problem occurs at duct shafts, which
often contain either undersized ducts with excessive air velocities or properly sized
ducts that are too close to the shaft walls. All such cases can lead to excessive noise.
To minimize the possibility of this problem, mechanical rooms and duct shafts
should be sized as follows:

General Design Guidelines

All HVAC equipment rooms should have a floor area large enough to allow a
clearance of at least 610 mm around all equipment. Building codes and equipment maintenance requirements may necessitate larger clearances in some cases.

Ductwork should be sized for the proper air velocity (see the section titled “Duct
System Components” in Chapter 2) and shafts should be sized to provide sufficient clearances around the ducts (see Figure 1-1 for guidelines).

SPACE PLANNING

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The primary acoustical goal of space planning is to locate the noisy equipment
as far as possible from the building’s noise and vibration-sensitive areas. For HVAC
system design, this is most important in the location of chiller rooms, fan/AHU
rooms, cooling towers, and rooftop package units.
A common space-planning problem involves the placement of an AHU room
in the core area on a typical floor of a multistory office building. The best core area
plans surround the AHU room with buffer zones such as toilet and storage rooms,
as well as elevator, stair, and duct shafts as shown in Figure 1-2.
Effective space planning can also help minimize the construction costs of
mechanical room walls and slabs. For instance, if a mechanical equipment room is
adjacent to a non-sensitive room, then the construction of the common wall is not

Figure 1-1

Guideline for duct chase, shaft, and enclosure sizing.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 1-2

Acoustical comparison of various building core area layouts.

General Design Guidelines

Figure 1-3

Guidelines for the preliminary selection of mechanical room walls.

critical, so lightweight gypsum board walls are typically acceptable. However, if the
adjacent room is a conference room or private office, then a double-studded partition, or perhaps even a fully grouted masonry wall, may be needed. Figure 1-3 shows
the concept.
After the preliminary space plans for the HVAC equipment areas has been determined, the mechanical engineer should distribute equipment layouts to the rest of the
design team showing approximate weights and sizes of all equipment. A sample
layout is shown in Figure 1-4. The structural engineer should use this information
to select beams with maximum loaded deflections as listed in Table 1-1. The recommended deflection limits may be somewhat smaller than structural engineers would

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 1-4

Table 1-1.

Sample mechanical penthouse equipment layout.

Maximum Mid-Span Deflections for Above-Grade Structures
that Support Vibration-Isolated HVAC Equipment

Equipment to be Mounted
on Vibration Isolators

Maximum Structural Deflection
due to Equipment Load
(Composite Action for Concrete/
Metal Deck Floor Systems)

Cooling towers

8 mm

All other HVAC equipment

6 mm

General Design Guidelines

typically select for floor and roof beams. The equipment sizes and manufacturerrecommended maintenance clearances should be given to the architect, who can
allocate equipment rooms of the proper size.
Slab Selection
The schematic design phase is also the proper time to determine the thicknesses
(and, therefore, the weights) of the slabs that provide sound isolation between equipment rooms and any occupied spaces that are either above or below the equipment
room. Table 1-2 gives preliminary slab thickness suggestions for various types of
equipment.
MECHANICAL EQUIPMENT ROOMS
AND OUTDOOR EQUIPMENT AREAS
Central Plants
Some of the noisiest equipment in a commercial HVAC system is located in the
central plant. The most frequently used chillers—centrifugal, rotary, and screw—
produce enough noise in some rooms to require hearing protection per OSHA
requirements. Air compressors and vacuum pumps can also generate high noise
levels. The ventilation openings that serve these rooms are acoustical “holes” that
let equipment noise transfer to nearby areas that may be noise-sensitive.
Noise and Vibration Control Guidelines for Central Plants
1.

For a large enough building site, locate the central plant away from any building
with perimeter noise-sensitive areas.

Treat ventilation passages with acoustical louvers, duct silencers, or acoustically lined plenums to control the noise that escapes to the community (see
Figure 1-5 and the section titled “Duct Silencers, Plenums, and Acoustical
Louvers” in Chapter 2 for more information).

If the central plant is in an occupied building, install all equipment and piping
with vibration isolators in accordance with the “Vibration Isolation Selection
Guide” table in Chapter 5.

Make electrical connections to all central plant equipment with slack, flexible
conduits.

For noise control to noise-sensitive areas, either above or below, consider the
following options:
a.

For upward noise control, use a building standard slab and a noise control enclosure around the chillers and air compressors (see Figure 1-6).

A Practical Guide to Noise and Vibration Control for HVAC Systems

Slab Thickness in Millimeters (2300 kg/m3 concrete)*

Equipment Type

Total Size or Total Motor
Kilowatt (kW) Rating in
Equipment Room

Over a Noise-Sensitive
Occupancy
(e.g., NC-35)

Over Somewhat Sensitive
Occupancy
(e.g., NC-40)

Reciprocating chiller

All sizes

200**

200

Up to 1000 kW

200

150

1000 kW and larger

200**

200

All sizes

200**

200

Up to 7.5 kW

100

7.5 to 22.5 kW

160

100

Over 22.5 kW

200**

150

Up to 22.5 kW

100

22.5 kW and larger

150**

100

Up to 18.5 kW

100

18.5 kW to 75 kW

150

100

Over 75 kW

200

160

Up to 22.5 kW

150

100

22.5 kW to 75 kW

200**

150

Over 75 kW

200**

200

Centrifugal chiller

Screw chiller

Unducted fan or AHU
(either side unducted)

Ducted fan or AHU
(both sides ducted)

Pump

Cooling Tower,
Evaporative Cooler,
Air-cooled Condenser,
Air-cooled Chiller
*

For mechanical equipment rooms housing multiple pieces of equipment and different types of equipment, determine the slab thickness for each equipment type and use the largest value for the entire equipment room. Increases
slab thickness by 33% if 110 pcf concrete is used.
** Some installations of this type, and equipment rooms with several types of equipment, each requiring a 200 mm
thick slab, will require a “floating floor” assembly per Figure 1-10.
Note: Do not locate mechanical equipment adjacent to room with a criteria rating of NC-30 or less.

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Table 1-2. Selection Guidelines for Slabs Separating
Mechanical Equipment Rooms from Noise-Sensitive Occupied Spaces

General Design Guidelines

Labyrinth air path used for sound attenuation at an equipment
room ventilation opening.

Figure 1-6

Upward noise control for mechanical rooms.

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Figure 1-5

A Practical Guide to Noise and Vibration Control for HVAC Systems

c.
d.

For downward noise control, use a building standard slab with a gypsum board ceiling suspended below using spring hangers (see
Figure 1-7). The suspended ceiling is also an alternative to an equipment enclosure for upward noise control, but it can be difficult to
implement where the room has a lot of suspended piping and conduits.
For both upward and downward noise control, consider a monolithic
slab weighing at least 500 kg/m2.
In the most critical cases, including screw chiller and vane axial
installations, use a 500 kg/m2 floating floor assembly. Figure 1-8
shows section views of two types of floating floors.

For rooftop or mid-level central plants, consider the following additional guidelines:
a.

Figure 1-7

Do not extend the central plant to the edge of the slab. This lets
equipment noise leak through the small gap between the slab edge
and the building skin. See Figure 1-9 for a suggested noise control
method.

Downward noise control using an auxiliary ceiling.

General Design Guidelines

Section views through two types of floating floor assemblies.

Figure 1-9

Sound transmission at perimeter mechanical rooms.

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Figure 1-8

Electrical conduits serving central plant equipment should not penetrate a floor or ceiling slab that is common to a noise-sensitive area.
These conduits should penetrate the slab in non-sensitive areas and
enter the plant through wall penetrations (see Figure 1-10).

If the central plant is located next to a stair, duct, or elevator shaft, the
common wall should be either masonry or a multi-layer gypsum
board construction, depending on the noise levels of the central plant
equipment.

In critical installations, the extensions of floor drains beneath the
central plant floor (above an occupied space ceiling) may require
either a gypsum board enclosure or pipe lagging. Figure 1-11 shows
a photo of a pipe lagging product. The effectiveness of pipe lagging
depends on the thickness of the compressible spacer layer, available
from 13 to 50 mm thick (thicker is better), and the surface density
of the outer barrier material, usually barium-loaded vinyl that is
available in surface-densities that range from 2.5 to 10 kg/m2
(denser is better).

Figure 1-10 Electrical conduit routing into a mechanical room.

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A Practical Guide to Noise and Vibration Control for HVAC Systems

General Design Guidelines

Air-Handling Unit Rooms
The system aspect of HVAC noise and vibration control is most important in
the design of air-handling unit (AHU) rooms. Figures A and B in this guide’s introduction compare noisy and quieter AHU room installations. Consider the following
guidelines for AHU equipment rooms.
Noise and Vibration Control Guidelines for AHU Rooms
1.

The room’s floor area should be at least 1 m2 per 340 L/s of total maximum fan
airflow. For example, an AHU with a pair of 9400 L/s fans would need an equipment room floor area of at least 55 m2. A larger floor area would be needed for
a unit with an oversized cabinet that houses special filters or a humidifier.

The placement of the unit and ductwork in the room should allow smooth airflow
from the return air openings to the AHU inlet.

The type of unit and the location of its discharge opening(s) should allow the use
of a supply duct system that meets the requirements of SMACNA for smooth
airflow from the AHU discharge and through the supply ducts.

In VAV systems, fan capacity should be controlled with a variable frequency
drive (VFD) motor speed controller.

Where possible, allow for a vestibule between the AHU room and the occupied
space. The doors serving the vestibule should each weigh at least 34 kg/m2 and

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Figure 1-11 Pipe lagging for noise control includes a compressible spacer layer
and a heavy barrier layer.

A Practical Guide to Noise and Vibration Control for HVAC Systems

have airtight, full-perimeter seals and automatic door bottoms. The doors should
hinge outward so that the negative room pressure will help pull the door tight to
the jamb seals.
6.

If a vestibule is not possible, then specify a sound-rated door assembly (including
door, frame, and seals) whose Sound Transmission Class (STC) rating is equal to
that of the wall, typically STC-50 or higher.

Specify a duct silencer at all AHU room wall openings above the ceiling used to
collect common plenum return air.

Built-Up Air-Handling Systems
Because built-up air-handling systems typically use large, noisy equipment
and handle very large air quantities, attention to noise and vibration control is
required in all of the design phases, especially the early phases.
General Guidelines for Built-Up Basement and Rooftop AHU Installations
1.

Arrange the equipment and duct shaft locations for smooth airflow at the recommended velocity and pressure drop.

When two supply fans (centrifugal or vane-axial) are to operate in parallel, arrange
them in a “Y” discharge arrangement as shown in Figure 1-12.

Figure 1-12 Examples of rumbly and quieter parallel fan installations.

General Design Guidelines

Equipment and piping should be suspended or supported with vibration isolators
according to the “Vibration Isolation Selection Guide” table in Chapter 5.

Electrical connections to equipment should be made using slack, flexible
conduits.

Acoustical louvers, or duct silencers behind standard louvers, may be required at
fresh air intakes and exhaust air outlets to control noise transmission to the
outside.

If the equipment room is directly above a noise-sensitive space, consider using
either an auxiliary equipment enclosure or a floating floor (refer to Figures 1-8
and 1-9).

If the equipment room is directly below a noise-sensitive space, consider using an
upgraded cabinet construction or an auxiliary enclosure around the entire builtup assembly, as shown in Figure 1-13.

In general, when vane-axial fans are used, duct silencers are required on both the
inlet and discharge sides of the fans (see Figure 1-14). Extreme cases will require
an auxiliary sheet metal housing around the fan to control its case-radiated noise.

Figure 1-13 Guidelines for a basement built-up fan system.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 1-14 Typical duct silencer arrangement at vane-axial fan.

Additional Guidelines for All Above-Grade Equipment Rooms:
1.

Do not locate the equipment adjacent to a noise-sensitive area. For example,
don’t place the equipment at the roof perimeter where sound can refract around
the edge of the building and enter through the perimeter glass (see Figure 1-16).

If the equipment must be over a noise-sensitive area, then the equipment room
floor construction and the interior ceiling assembly, if any, must provide adequate
sound isolation. See Table 1-2 for general slab selection guidelines.

The floor structure supporting the equipment must be very stiff. Figure 1-16
shows various support methods. For all methods, the supporting structure should
deflect less than 6 mm due to the combination of the dead and operating equipment loads.

Select vibration isolators for the equipment and its attached piping in accordance
with the “Vibration Isolation Selection Guide” table in Chapter 5.

EQUIPMENT ROOM WALLS AND SLABS
Wall Selection
Mechanical room walls should not be selected solely on the basis of their STC
ratings. The STC rating system was developed to rate the ability of walls, doors, and

Figure 1-15 Reflected and refracted equipment sound at a building perimeter.
Do not locate noisy equipment near a roof perimeter where this
can happen.
windows to control the transmission of speech and other noises that have very little
low-frequency sound energy. It does not rate the acoustical performance in the lowfrequency range that is prevalent in HVAC system noise spectra. Low-frequency
performance is determined by a wall’s mass and stiffness, and masonry walls are
much more massive and stiff than gypsum board walls. Therefore, where lowfrequency noise is a potential problem, use masonry construction. Figure 1-17
includes guidelines for mechanical room wall selections.
Wall Penetrations
Where ductwork or piping penetrates a wall, the penetration should be made
with the duct or pipe floating inside a sleeve, and with a resilient sealant filling the
gap (see Figure 1-18).
DUCT CHASES · SHAFTS · ENCLOSURES · LAGGINGS
Duct chases and shafts are required for a variety of architectural purposes.
Enclosures and laggings are add-on elements that are used strictly for noise control.
Keep these distinctions in mind when reviewing the guidelines given below.

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General Design Guidelines

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A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 1-16 Structural support of rooftop equipment for vibration control.

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General Design Guidelines

Figure 1-17 Guidelines for mechanical room wall selection.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 1-18 Duct and pipe penetrations through walls.

Location
Locate shafts and chases, especially those whose ducts are large and carry
high-velocity airflow, as far as possible from noise-sensitive areas.
Sizing
To be effective at the low frequencies where noise control is usually needed,
chases, shafts, and enclosures should be sized so that there is a minimum clearance
around each enclosed duct of at least 10% of its larger dimension or 150 mm,
whichever is larger. Figure 1-1 shows an example.
Return air shafts that also contain supply air ducts and their branch takeoffs
should be sized for no more than 2.5 to 5 m/s return air velocity. The 5 m/s upper limit
applies where the supply duct riser and its takeoffs are localized restrictions to the
return airflow, as shown in Figure 1-19.

General Design Guidelines

Figure 1-19 Plan view of return air shaft with supply duct takeoffs obstructing
return airflow.

Construction
Figure 1-20 generically compares the sound isolation performance of several
shaft, chase, and enclosure constructions. In general, higher wall mass and larger
clearances around ducts provide better sound isolation. The higher stiffness of a
masonry shaft provides excellent low-frequency sound isolation.
In installations where shaft walls are used for architectural masking, but not for
fire protection, the shaft construction sometimes extends only a few centimeters
above the ceiling, leaving the ductwork exposed to the ceiling plenum. Verify that
in such a situation, the shaft wall is not needed for noise control purposes. If it is,
make sure that the shaft wall construction extends from slab to slab.
Figure 1-21 shows typical duct lagging assemblies. Laggings provide good
mid and high frequency noise reduction but provide only small amounts of lowfrequency noise reduction. Instead of lagging, use an airtight gypsum board enclosure where significant low-frequency noise reduction is needed.
Figure 1-22 shows a section through a gypsum board duct enclosure. Note that
the figure caption shows that the enclosure framing does not touch the duct. This
is very important because contact between a duct and any part of its enclosure can
allow duct wall vibration to be amplified by the enclosure, which then uses the
“sounding board effect” to convert the vibration into noise that is then radiated into
the adjacent area.
An enclosure for low-frequency noise control may not be necessary if circular,
instead of rectangular, ductwork is used.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 1-20 Acoustical comparison of several duct chase, shaft, and enclosure
constructions.

General Design Guidelines

Figure 1-21 Two typical duct laggings.

Figure 1-22 Noise control duct enclosure.

Airside Equipment
FANS
Because most HVAC noise complaints are associated with fans, selecting and
integrating them properly with the air distribution system will preclude a large
percentage of possible noise problems. A fan can be used as the central element in
a ventilation system or as a component in a piece of packaged equipment, such as
an air-handling unit (AHU), cooling tower, air-cooled chiller, generator radiator,
etc. This section of the guide deals specifically with the selection of fans in all of
their likely applications. The acoustical performance of AHUs is addressed later in
this chapter.
The first step in selecting a fan for a noise-sensitive application is to select the
quietest equipment available for the most likely operating condition. In a constant
volume system, this is simply the set of design operating conditions, e.g., airflow
and system static pressure. In variable air volume (VAV) systems, the selection
should be made for the most likely range of part-load operating conditions (typically in the range from 60% to 85% of full capacity).
All of the major fan manufacturers offer fan selection computer programs that
include octave band sound power level (LW) values based on laboratory measurements conducted in accordance with AMCA Standard 300, Reverberant Room
Method for Sound Testing of Fans. The LW values are typically given for the octave
bands with center frequencies from 63 to 8000 hertz, inclusive. (Refer to Appendix
D for more information on the use of manufacturers’ published and submitted
acoustical data.) There is no longer a need to use the LW estimating procedures that
were included in early editions of the ASHRAE Handbook. Those early estimating
procedures have been proven to be inaccurate, with errors as large as 30 dB in some

cases. Only LW values measured in accordance with AMCA 300 should be used in
calculations and specifications.
In general, fan noise at a particular operating condition correlates loosely with
energy usage at that condition, so when comparing fans of the same type, the selection that operates at the lowest motor kW rating will usually be the one that
produces the least noise.
All fans produce both broadband noise as well as tonal noise at the fan’s fundamental blade passage frequency (BPF) and its first few integer multiples, which are
referred to as “overtones” or “harmonics.” The fundamental BPF is easily calculated from the following equation:
BPF = RPM ⋅ N ⁄ 60

where RPM is the fan rotation rate and N = the number of fan blades.
For instance, a 1000 rpm fan with nine blades would have a fundamental BPF of
150 hertz, which is in the 125 hertz octave band. This fan would also produce BPF overtones at 300 hertz, 450 hertz, 600 hertz, etc., but the overtones’ LW values are generally
lower than those of the fundamental BPF. In general, the strength of a fan’s BPF tone
is proportional to its total pressure performance, so a fan that operates at a high pressure
will usually have a louder BPF tone than if it were operating at a lower pressure.
Most fan manufacturers measure only inlet LW values, but a few also measure the
discharge LW values for some of their product lines. The difference between the inlet
and discharge LW values can be as large as 10 dB in some octave bands, depending on
the fan’s operating conditions; discharge values are typically higher, as shown in Figure
2-1, which compares the inlet and discharge LW values for a 925 mm diameter plenum
fan operating at 11,750 L/s at 1250 Pa total pressure. Each fan type and operating condition will have its own unique pair of inlet and discharge acoustical spectra. Therefore,
always request both the inlet and discharge LW values for a fan and assume that, unless
otherwise noted, any submitted values are valid for only the inlet side of the fan.
Centrifugal Fans
Centrifugal fans discharge their air at an angle of 90 degrees relative to its inlet
flow direction. These fans are either single or dual inlet, with several possible blade
shapes, each with its own set of acoustical characteristics. Of the three blade shapes
used in fans in most commercial systems, the forward-curved (FC) blades are the
most cost-effective in systems with low airflow and low static pressure, whereas
fans with backward-inclined (BI) and airfoil (AF) blades are more efficient and
quieter where higher airflow and static pressure performance are needed. Industrial
fans and material transfer fans are very noisy because their blade shapes are
designed for very high pressure performance that is not needed in commercial and
residential systems.

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Airside Equipment

Figure 2-1

Inlet and discharge octave band LW values for a 925 mm plenum fan.

Figure 2-2 compares the inlet LW values of three commercial 925 mm diameter
fans operating at 9400 L/s and 750 Pa total pressure of similar motor kW rating, each
of which has a different blade shape. The 125 hertz octave band “spikes” in the BI and
AF spectra are due mainly to the strengths of their BPF tones. The FC fan’s LW spectrum does not show a strong BPF tone, but its higher 63 hertz band LW value can be
a problem because of the expense required to control noise in that frequency band.
Centrifugal fans of all types will produce unexpectedly high noise levels if duct
fittings or other airflow obstructions are too close to the fan inlet or discharge. The
amount of excessive noise cannot be quantified; ongoing ASHRAE-sponsored
research is attempting to quantify the effect for all types of fans. Figure 2-3 shows
the current “best practice” guidelines for inlet clearance and discharge ductwork
configurations for centrifugal fans.
Inline Fans
Inline fans do not change the direction of airflow and are typically used in
ducted applications and include the vane-axial, tube-axial, propeller, and inline
centrifugal types, as well as others. Each type of inline fan is best for a specific
range of operating conditions. They all produce significant BPF tones, with vaneaxial and tube axial fans producing the strongest, while the tones from inline
centrifugal fans are not as prominent.

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A Practical Guide to Noise and Vibration Control for HVAC Systems

Airside Equipment

Mixed Flow Fans
Mixed flow fans are inline fans, but the airflow direction through them is
midway between axial and centrifugal flow. Figure 2-4 compares the airflow paths
through typical inline fans and a mixed flow fan. Figure 2-5 shows a cutaway
rendering of a mixed flow fan, whose main acoustical benefit is the relatively low
strength of its BPF tones. Figure 2-6 compares the inlet LW spectra of vane-axial,
inline centrifugal, and mixed flow fans of equal physical size and motor kW operating at 23,500 L/s and 1000 Pa total pressure.
Figures 2-7 and 2-8 show recommendations for ducted and unducted inline
fans of all types, keeping in mind that these fans are very sensitive to distorted inlet
airflows.
Plenum Fans
Plenum fans are single-inlet centrifugal fans whose impeller designs have been
optimized for maximum efficiency in a discharge plenum. Their discharge air
leaves the impeller in a 360 degree radial pattern. They are typically used in custom

Figure 2-2

Sound power level comparison for three types of centrifugal fans.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 2-3

Guidelines for centrifugal fan installations.

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Airside Equipment

Sketches courtesty of the Greenheck Fan Corporation.

Figure 2-4

Inline fan airflow patterns.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Rendering courtesy of the Greenheck Fan Corporation.

Figure 2-5

Cutaway view into a mixed flow fan.

Figure 2-6

Inline fan sound power level comparison.

Airside Equipment

Figure 2-7

Guidelines for ducted axial flow fan installations.

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A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 2-8

Guidelines for unducted axial flow fan installations.

Airside Equipment

and semi-custom (modular) air-handling units (see Chapter 4 titled “Packaged and
Unitary Equipment”). Figures 2-9 and 2-10 show inlet and discharge views, respectively, of direct-drive and belt-drive plenum fans. These fans are typically available
with either 9, 10, or 12 blades.
Power Roof Ventilators
Power roof ventilators (mushroom fans) are usually used for exhaust applications but are sometimes used for supply or makeup air. Because they use centrifugal
impellers, they can tolerate moderate amounts of static pressure, so duct silencers
can be used with these fans where no other noise control mitigation is feasible and
care is taken to have two to three equivalent diameters of straight duct between the
fan and the silencer. Figure 2-11 shows a fan installation with a duct silencer
inserted between the fans and their curbs. Large mushroom fans installed near
noise-sensitive or vibration-sensitive areas may be mounted on roof curbs with
internal isolation. A sketch of such a curb is shown in Figure 2-12.
Panel Fans
Panel fans, which use propeller fan wheels, are typically selected for unducted,
low static pressure applications. There are several propeller blade shapes (e.g.,
stamped steel, cast aluminum, backswept airfoil, etc.), so be sure to consider them
all when selecting a propeller fan for a noise-sensitive application.

Photos courtesy of Energy Labs Inc.

Figure 2-9

Figure 2-10 Discharge side of a
belt-drive plenum fan.

Inlet side of a directdrive plenum fan.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 2-11 Power roof ventilators (mushroom fans) mounted on intake duct
silencers and roof curbs.

Since “add-on” noise control products, such as duct silencers or even ductwork, are often not feasible because of the static pressure losses that they create,
selection of the quietest propeller fan available is often needed for a noise-sensitive
application. Figure 2-13 compares the inlet LW values of three 1200 mm propeller
fans operating at 9400 L/s and 125 Pa static pressure. The noisiest of these fans uses
cast aluminum blades, while the quietest uses backswept, airfoil-shaped, fiberglass
blades that are either form-cast or handmade. Figures 2-14 and 2-15 show two types
of propeller fans that can be optimized for high performance and low noise in a
panel fan arrangement.
If the quietest panel fan selection still creates excessive outdoor noise levels,
for instance, at a nearby residential property line, a sound-attenuating hood similar to that shown in Figure 2-16 can be used to provide attenuation in the range
of 3–5 dBA.
Few panel fans are fabricated with sufficient vibration isolation for noisesensitive applications. For those cases, the panel fan may be suspended from four
isolation hangers in arrangement similar to that shown in Figure 2-17.

Airside Equipment

Figure 2-12 Mushroom type exhaust fan on vibration-isolated roof curb.

Ceiling Exhaust and Cabinet Fans
Ceiling exhaust and cabinet fans used to be standard in conference rooms to
remove cigarette and cigar smoke; however, with many fewer buildings permitting
smoking, the dominant use for these fans is now in residential bathrooms, kitchens,
and utility rooms. These fans are typically rated in terms of sones, where 1 sone is
roughly equivalent to a sound level in the 35 to 40 dBA range. Doubling the number
of sones is roughly equivalent to adding 10 dBA to the sound level, so 2 sones is
typically in the 45 to 50 dBA range, 4 sones is in the 55 to 60 dBA range, etc. Most
of these fans have ratings of several sones, but some manufacturers have recently
developed low-capacity fans that are as quiet as 1 sone. For rooms with cabinet fans,
Figure 2-18 shows noisy and quieter installations of this fan type.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 2-13 Inlet octave band LW comparison for three propeller fans.

Photo courtesy of Multi-Wing America.

Figure 2-14 Propeller fan with a 12-socket aluminum hub and plastic blades.
This fan can be selected for use with 2, 3, 4, 6, 8, 9, or 12 blades
of any custom length or blade twist to optimize airflow and
acoustical performance.

Airside Equipment

Figure 2-15 Ultra-low-noise propeller fan with backswept airfoil blades.
Versions of this fan are available with 2, 3, or 4 blades of adjustable
blade twist and diameters up to 12.3 m for very large capacities.

Figure 2-16 Lined hood for propeller fan noise control.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 2-17 Vibration isolation suspension for propeller fans.

Figure 2-18 Noisy and quiet installations of ceiling-mounted exhaust fans.

Airside Equipment

General Guidelines for Controlling Fan Noise and Vibration

Select the most efficient type and size of fan for the application. This will also, in
most cases, be the least noisy. Low discharge velocity is also preferred. For fans
in VAV applications, see the fan selection guidelines given in the section titled
“Special Variable-Air-Volume (VAV) System Concerns” later in this chapter.
Select the fan to operate on the right side of the fan curve, safely away from the
stall region and near the peak of the kilowatt curve.

Allow a clearance of at least 1 fan wheel diameter at all unducted fan inlets and
1.5 wheel diameters at all unducted fan outlets.

Use vibration isolation mounts or hangers, with auxiliary bases, if required, for
all fans over 0.75 kW located near noise-sensitive areas.
Attach ductwork to fans with canvas or elastomeric flexible connectors.
Inlet and discharge duct transitions should be gradual to minimize pressure drop
and maximize static regain. The total included angle within the transition should
be no more than 15 degrees (1:4 slope) for discharge transitions and 45 degrees
(1:1 slope) for inlet transitions.
In ducted discharge installations, the nearest downstream damper, duct silencer,
elbow, offset, transition, or takeoff should be at least three equivalent duct diameters from the fan outlet. The equivalent duct diameter, De, for a rectangular duct
is found from the equation

5.
6.

De =

4-----------------⋅ A ⋅ Bπ

where A and B are the rectangular duct’s cross-section dimensions.
8. All duct system fittings, especially near a fan inlet or discharge, should be
designed for the lowest practical pressure drop.
9. Because noise travels upstream and downstream from a fan, duct silencers and/or
acoustical duct liner are sometimes required in both the inlet and discharge air
paths.
10. Refer to the section titled “Duct System Components” later in this chapter for
additional guidelines.
AIR-HANDLING UNITS (AHU)
AND FAN-COIL UNITS (FCU)
Air-Handling Units (Draw-Through and Blow-Through)
All of the general guidelines and special guidelines for centrifugal fans apply
to air-handling units with one variation: the distance between the unit intake and the
nearest wall should be at least the height of the unit (see Figure 2-19). Also note that
factory acoustical data for AHUs should be determined in accordance with the most

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A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 2-19 Minimum clearance at AHU and cabinet fan inlet.
recent revision of AHRI 260—not an obsolete revision and not per AMCA 300,
which is for fans only.
Air-Handling Units Using Plenum Fans
Systems with this type of AHU can sometimes be designed with short supply
air path duct silencers, or sometimes without any silencers. Originally, plenum fans
were simply single-inlet, single-width centrifugal fans without a fan scroll. Their
designs are now optimized for use in open plenums, such that these fans are now
quieter and more efficient than before. The quietest variation of a plenum fan AHU
uses a fan that is selected for the lowest possible Lw values, and has 100 mm thick
sound-absorbing insulation behind perforated cabinet liner in all cabinet sections.
This unit’s discharge plenum can be outfitted with several discharge duct openings
to help minimize the number of duct fittings in and near the mechanical room.
Figure 2-20 shows an external view of a plenum AHU with two top outlets with
radius elbows that direct the supply air in opposite directions. This arrangement
precludes the need for a T-split discharge fitting. Figure 2-21 shows a cutaway
sketch of a blow-through plenum AHU.
Fan-coil Units
Motor hum from single-phase motors is a common noise problem in fan-coil units
because this kind of motor is prone to high internal vibration, and because this type of
motor is typically attached directly to the unit housing, which acts a sounding board
for the motor’s vibration. Therefore, a fan-coil unit in a noise-sensitive application
should be specified with a three-phase motor and a fan-motor sub-assembly that is

Airside Equipment

Figure 2-20 Plenum AHU with supply ducts attached to the top of discharge
plenum.

Figure 2-21 Cutaway sketch of a plenum fan air-handling unit.

A Practical Guide to Noise and Vibration Control for HVAC Systems

mounted on spring isolators within the unit housing. An electronically commutated
motor (ECM) will provide additional noise and vibration reduction because these
motors produce less vibration than other low-kW motors.

Experience has shown that the problems with this type of equipment typically
occurs because of excessive inlet static pressure, excessive inlet air velocity, incorrect installation, or a misunderstanding of the acoustical performance data given in
manufacturers’ catalogs. Past ASHRAE research has shown that the acoustical testing and rating procedures used for this equipment has resulted in published Noise
Criteria (NC) rating estimates that are almost impossible to achieve in a real-world
installation. Therefore, the specifying engineer should understand how to use the
cataloged ratings for a specific application. Refer to Appendix D, “Using Manufacturer’s Sound Data.” The guidelines shown in Figure 2-22 and in the checklist
below will help minimize the chances of excessive terminal unit noise. The guidelines apply to single-duct, dual-duct, and induction units, as well as parallel and
series fan-powered units.

Figure 2-22 Guidelines for VAV unit installation.

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TERMINAL UNITS
(CAV, VAV, AND FAN-POWERED VAV BOXES)

Airside Equipment

Establish the Room Criteria (RC), Noise Criteria (NC), or Balanced Noise
Criteria (NCB) rating for each room with the help of the table in Table B-1 (see
Appendix B).

Use the manufacturers’ published octave band discharge and radiated LW data for
a static pressure difference of 250 Pa to select the quietest unit that will meet the
design objectives; this will typically occur for a maximum inlet velocity of about
9 m/s for single-duct and dual-duct boxes and 6 m/s for fan-powered boxes.

For a terminal unit in a return air ceiling plenum with distributed return air grilles
in a mineral tile or fiberglass ceiling:
a.

b.
c.

Verify that the unit’s 125 hertz radiated octave band LW value is no
more than 32 points above the NC, RC, or NCB rating that was
selected for the room below the terminal unit. For instance, the 125
hertz radiated octave band LW value for a terminal unit above a room
with an NC/RC/NCB rating of 40 should not exceed 72 dB.
Locate the unit as high in the plenum space as possible and at least
1.5 m from any return air grille.
If a unit must be located less than 1.5 m from a return air grille, it
may be necessary to install a fiberglass-lined sheet metal elbow on
top of the grille with its discharge opening aimed away from the terminal unit.

If the terminal unit is installed above a room with a gypsum board ceiling having
no openings, its 125 hertz radiated octave band LW value can be as much as 40
points higher than the room’s NC/RC/NCB rating. For instance, a terminal unit
above a room with a solid gypsum board ceiling may produce a 125 hertz radiated
sound power level of 80 dB if it is installed above a room with an NC-40 design
criteria rating.

Do not locate a terminal unit over a space with an NC, RC, or NCB rating less
than 35 (e.g., private offices, conference rooms, classrooms, etc.).

Do not locate a fan-powered terminal unit over a space with an NC, RC, or NCB
rating less than 40. Instead, locate it over a nearby nonsensitive space, such as a
corridor, toilet, or storage room.

The high- or medium-pressure ductwork entering the terminal unit should be
straight, with no fittings or dampers, for at least three equivalent duct diameters
upstream of the unit.

Connect high- or medium-pressure ductwork to the unit’s inlet with a short, flexible, canvas duct connector, not a length of flexible ductwork.

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Terminal Unit Selection and Application Checklist

A Practical Guide to Noise and Vibration Control for HVAC Systems

The low-pressure discharge duct should be fiberglass-lined sheet metal, not
unlined sheet metal or fiberglass ductboard.

10. Duct taps into the unit’s discharge plenum should be at least 1200 mm downstream from the unit outlet.
Terminal units that are exposed in occupied areas without ceilings are common
sources of noise complaints. Ceilings are almost always needed to control terminal
unit radiated noise.
LABORATORY AIR VALVES
Cylindrical venturi air valves with plunger valve assemblies are often used in
laboratory makeup and exhaust air duct systems. A photo of such an air valve is
shown in Figure 2-23. These valves have no particular acoustical advantage over
traditional terminal units, and their selection and installation guidelines are similar
to those for terminal units with the additional warning that any acoustical analysis
should consider the valve’s discharge, radiated, and exhaust LW performance
ratings. Discharge noise travels in the direction of airflow, while exhaust noise travels in the direction opposite to the airflow.

Photo courtesty of Phoenix Controls Corporation.

Figure 2-23 Laboratory air valve and its “noise flow” directions.

Airside Equipment

GRILLES, REGISTERS, AND DIFFUSERS (AIR DEVICES)
Because air devices are directly exposed to the occupied space, noise reduction
treatments cannot be added between them and the space occupants. Therefore, they
must be selected with the appropriate acoustical ratings.
1.

Establish the NC, RC, or NCB ratings for all noise-sensitive areas in the building
with the help of Table B-1 (see Appendix B).

Select each supply and return air device to have a manufacturer’s NC rating at
least five points below the room’s established NC, RC, or NCB rating. Lower NC
ratings will be needed if the room is served by several devices.

The ductwork serving a supply air device should be straight for at least three
equivalent duct diameters upstream of the device’s duct collar. Wavy ductwork
will cause uneven airflow into the device, creating extra pressure drop and noise.
See Figures 2-24 and 2-25 for a comparison of good and bad airflow entry conditions.

The ductwork serving a supply air diffuser or return air grille should be at least
the same size as the device’s inlet collar. Using a short collar transition to adapt
the device’s collar to either a larger or smaller duct can result in unexpectedly high
noise due to turbulence at the transition adapter. If a transition is needed to match
a duct to its air device, the transition should conform to the guidelines given in the
section below titled “Duct System Components.”

Do not attach a balancing damper to a device’s duct collar. It should be at least
three equivalent duct diameters away from the device. Figure 2-26 shows the
effect of adding a balancing damper close to a supply outlet’s duct collar.

Pay close attention to the catalog data when selecting a linear diffuser. If the
diffusers will be installed with close-fitting inlet plenums, verify that the catalog
data used to make the selection are based on the same inlet plenum configuration.
DUCT SYSTEM COMPONENTS
While duct systems must be designed to deliver air within a given space and
at a reasonable cost, they must not be allowed to generate or transmit excessive
noise in the process. The excessive turbulence that often results from an inefficient
duct layout causes excess pressure drop and can lead to roar and rumble that can
be difficult and expensive to control. Also, the use of improper duct materials for
a given application can allow excessive levels of other system noises (usually from
fans or variable-volume terminal units) to be transmitted into occupied spaces.

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Air Device Selection and Application Checklist

Figure 2-24 Good inlet duct connection to a supply air ceiling diffuser. With a
straight flex duct path, the diffuser will produce the manufacturer’s
cataloged noise levels.

Figure 2-25 Poor inlet duct connection to a supply air ceiling diffuser. The
winding flex duct path causes excessive air turbulence and noise at
the diffuser’s inlet collar. This diffuser could produce noise levels as
much as 15 dB higher than the manufactuerer’s catalog data.
The main acoustical characteristics of duct system components are in-duct
attenuation, breakout attenuation, and self-noise generation. All of these characteristics should be considered when selecting, sizing, and routing ductwork.
In-Duct Attenuation
In-duct attenuation refers to the reduction of sound as it travels in the airstream
inside of a duct. Unlined metal ducts provide limited amounts of low-frequency in-duct

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A Practical Guide to Noise and Vibration Control for HVAC Systems

Airside Equipment

Figure 2-26 The effect of installing a damper behind a grille.

attenuation and almost no in-duct attenuation at high frequencies. Adding an internal
fibrous or foam liner that conforms with all of the NFPA Standard 90A limits for smoke
and combustibility will have little effect on low-frequency attenuation but will greatly
improve the attenuation at mid and high frequencies. Figure 2-27 shows a sample
comparison of the in-duct attenuation in lined and unlined sheet metal ductwork. Fiberglass ductboard and flexible ducts exhibit very high in-duct attenuation performance at
all frequencies because of their breakout transmission loss characteristics. See the
section on breakout transmission loss below.
Some publications, including early versions of the ASHRAE Handbook, stated
that external thermal insulation increases the low-frequency in-duct attenuation of
metal ductwork. That statement was never corroborated, and recent testing has
shown that external wrapping has no significant effect on the in-duct attenuation of
metal ductwork.
Breakout Transmission Loss (BTL)
Breakout transmission loss (BTL) refers to the reduction of noise that is transmitted through a duct wall. The BTL at all frequencies increases with ductwall mass,

Figure 2-27 Attenuation for lined and unlined sheet metal ductwork.

i.e., lower gauge number means that less noise will break out of the ductwork.
However, such improvements quickly reach a point of diminishing returns. The best
way to improve low-frequency breakout performance is to increase the duct’s stiffness.
Spiral-wound circular ductwork is very stiff, so it is the preferred duct type where lowfrequency breakout noise control is needed. Longitudinal-seam circular ductwork is
slightly less stiff, so it provides somewhat less breakout transmission loss. Flat-oval
ducts are significantly less stiff than either spiral-wound or long-seam circular and,
therefore, provide much less low-frequency breakout noise control. The relative flexibility of rectangular duct walls makes them even more prone to poor duct breakout
noise control. Figure 2-28 shows a sample comparison of breakout transmission loss
through rectangular, spiral-wound circular, and flat-oval ductwork. Higher values of
BTL will result in lower sound levels outside of the ductwork.
Self-Noise
Self-noise (also called self-generated or regenerated noise) refers to the turbulenceinduced noise that is produced in duct fittings and other duct system components that

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A Practical Guide to Noise and Vibration Control for HVAC Systems

Airside Equipment

Figure 2-28 Breakout transmission loss for three types of sheet metal
ductwork.

obstruct or divert airflow, e.g., elbows, takeoffs, transitions, dampers, duct silencers, etc.
The amount of noise generated at each duct system component depends on the duct
type, localized airflow velocity, and type of fitting. Self-noise is not usually a problem
where the trunk duct velocity is below 7.5 m/s and if diffuser branch ducts are sized to
match the diffuser’s duct collar. Following the guidelines given in the SMACNA
“HVAC Duct Construction Standards—Metal and Flexible” (1995) and designing the
duct system for the lowest possible airflow velocities and pressure drop will avoid most
self-generated noise and rumble problems. Tables 2-1 and 2-2 give recommended
maximum airflow velocities for various types of ductwork. Figures 2-29 through 2-32
compare the scale of self-generated noise produced by various types of elbows, takeoffs, transitions, tees, and offsets.
Duct and Plenum Linings
Duct and plenum linings are semi-rigid fibrous or foam boards that are
attached to the inner walls of ducts and plenums to attenuate sound. They also
provide thermal insulation. The typical 25 mm thick liner provides good highfrequency sound attenuation but has almost no effect on low-frequency sound.
Better low-frequency attenuation occurs with an increase in the lining thickness to
50 or 100 mm because the lining occupies a larger percentage of the cross section

A Practical Guide to Noise and Vibration Control for HVAC Systems

Table 2-1. Suggested Maximum Airflow Velocities
for Various Ductwork Installations

Duct
Location

RC or NC Rating
in Adjacent
Occupancy

Max. Airflow Velocity
in m/s
Rectangular

Circular

In shaft or above
solid drywall
ceiling

45
35
25 or less

17.5
12.5
7.5

25
22.5
12.5

Above suspended
acoustical ceiling

45
35
25 or less

12.5
8.5
5

22.5
17.5
10

Table 2-2. Suggested Maximum Airflow Velocities
in Elbows for Rectangular Ductwork

Airside Equipment

Figure 2-29 Guidelines for minimizing regenerated noise in elbows.

Figure 2-30 Guidelines for minimizing regenerated noise in takeoffs.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 2-31 Guidelines for minimizing regernated noise in transitions and
offsets.

Figure 2-32 Guidelines for minimizing regenerated noise in duct tees.

of the duct’s sheet metal face area. Figure 2-33 compares the sound attenuation in
a 610 by 610 mm duct for various lining thicknesses.
Duct lining is very effective in controlling the “cross-talk” or “speaking tube”
effect between rooms that share a common duct. For this use, a lined elbow can have
the same acoustical effect as 600 to 2400 mm of straight, lined ductwork or up to
30 m of straight, unlined duct. Figure 2-34 shows how the “speaking tube” problem
can occur and how to mitigate it.
Fiberglass Ductboard
Fiberglass ductboard is rigid fiberglass board with a thin external aluminum
vapor barrier. It is typically used in residential construction because it is relatively
inexpensive and is easy to modify in the field with a knife. Due to its very low
density it can permit breakout noise transmission problems, a feature that also
results in high in-duct attenuation.

Figure 2-33 In-duct attenuation for various duct liner thicknesses.

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A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 2-34 The speaking tube (cross-talk) problem.

Airside Equipment

Elbows and Takeoffs
Elbows and takeoffs provide in-duct attenuation, especially at mid and high
frequencies, due to reflection from the duct walls and other mechanisms. Figure 2-35
compares the in-duct attenuation of square elbows with and without internal duct liner.
Radius elbows provide less attenuation because they guide the air (and the noise) more
smoothly through the turn. Figure 2-36 compares the attenuation of square and radius
elbows.
Flexible Ducts
Flexible ducts are typically used for duct connections to ceiling diffusers. They
provide significant amounts of in-duct attenuation, mainly because their lightweight construction provides substantial breakout (i.e., they are very poor for
controlling breakout sound transmission). Flexible ducts with a spunbond nylon
inner liner provide higher in-duct attenuation than flex ducts with polyethylene
liner. Figure 2-37 shows a flex duct with spunbond nylon liner.
Table 2-3 can help you select the best type of ductwork as a function of its
required acoustical properties. For example, if low-frequency breakout noise
control is important, the table would suggest using either lined or unlined circular
ductwork; however, note that the lined version would be needed if mid-frequency
in-duct attenuation is also important.
DUCT SILENCERS, PLENUMS,
AND ACOUSTICAL LOUVERS
Duct Silencers
(Also Called Sound Traps, Duct Attenuators, Mufflers)
Duct silencers are prefabricated sections of ductwork with internal baffles
made of light gauge, perforated sheet metal. Their acoustical performance depends
mainly on their baffle design. Each type of duct silencer has a unique material filling its baffles, as follows:
•

•

Dissipative: The baffles are filled with a sound-absorptive medium, such as
fiberglass, mineral wool, or treated cotton. Specially designed open-cell
foams that conform with all of the NFPA 90A requirements have a very large
cost premium but can be used in duct silencers where fibrous materials are not
permitted. Figure 2-38 shows a view into a dissipative silencer before its final
fabrication step.
Film-Lined: Similar to dissipative, except that the acoustic fill in the baffles
is encapsulated in thin film bags to control moisture absorption and prevent
erosion of the fibers into the airstream (see Figure 2-39). The film lining

A Practical Guide to Noise and Vibration Control for HVAC Systems

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Figure 2-35 Attenuation of rectangular elbows with and without turning vanes
(lined and unlined).

Figure 2-36 Attenuation of rectangular and radius elbows (lined and unlined).

Airside Equipment

Figure 2-37 Flexible duct with spunbond nylon inner liner.

Table 2-3.

Acoustical Characteristics of the Various Types of Ductwork
In-Duct Attenuation

Ductwork
Type

Breakout Attenuation

Mid and
Mid and
Low
High
Low
High
Frequency Frequencies Frequency Frequencies

Rectangular, unlined

Fair

Poor

Fair

Excellent

Rectangular, lined

Fair

Excellent

Fair

Excellent

Circular, unlined

Poor

Excellent

Good

Circular, lined

Poor

Excellent

Good

Fiberglass ductboard

Excellent

Poor

Fair

Flexible acoustical duct

Excellent

Poor

Fair

Unlined elbow, all duct types

Poor

Fair

—

Lined elbow, all duct types

Poor

Excellent

—

A Practical Guide to Noise and Vibration Control for HVAC Systems

Photo courtesy of Dynasonics, a division of PCI Industries Inc.

Figure 2-38 Cutaway view into a dissipative duct silencer.

Photo courtesy of Vibro-Acoustics, a division of BVA Systems Ltd.

Figure 2-39 Cutaway view of a duct silencer with film-lined baffles.

Airside Equipment

•

reduces the silencer’s acoustical performance at mid and high frequencies
and may also affect low-frequency performance, depending on the type of
film and how it is installed in the baffles. Care must be taken when specifying the film material, as many films do not conform with the flame spread
and smoke rating limits given in NFPA Standard 90A.
Reactive: Also called “no fill,” “packless,” or “no media;” the baffles are
empty but are formed from sheet metal with special perforation patterns that
provide moderate amounts of insertion loss over a narrow frequency range
using the Helmholtz resonator effect (see Figure 2-40).

Duct silencers can be built in any shape that can be fabricated using sheet
metal. Figure 2-41 shows an “elbow silencer,” a combination duct silencer/duct
elbow. Double-elbow, T-shaped, and transition silencers are also available.
Because its internal baffles obstruct airflow, a duct silencer also generates
airflow turbulence that can lead to significant self-noise and static pressure loss if
it is not incorporated properly into the air distribution system. Silencers are available in many different configurations whose acoustical and aerodynamic performance depend on the size, shape, and length of the baffles. Long, wide baffles
provide more attenuation than short, narrow ones. High sound insertion loss performance usually requires either a silencer that is very long or a shorter one that has
a high static pressure drop and self-noise. Since silencers should generally be
selected for a static pressure loss of no more than about 70 Pa, proper silencer selection is an iterative process in which silencers of various lengths and baffle widths
are compared to find the one(s) that will provide the required sound insertion loss
at an acceptable static pressure loss. Typically, higher velocity systems will require
longer silencers with narrower baffles to provide acceptable performance. Ultimately, silencer selection is an optimization process that considers (1) the insertion
loss required to meet the specified acoustical criteria, (2) the space available, and
(3) the maximum permissible pressure drop (including aerodynamic system
effects).
Figure 2-42 compares the insertion loss performance of two types of dissipative duct silencers with that for 3 m of lined ductwork. The duct silencers’ insertion
losses shown as the upper and lower curves in the figure bracket the wide range of
insertion loss values that are typically available. Note the correlation of insertion
loss and pressure drop (PD). Figure 2-43 compares the insertion loss performance
of 1.5 m long duct silencers of the dissipative, film-lined, and packless types.
To perform at or near the ratings given in manufacturers’ catalogs, duct silencers should be installed in accordance with the guidelines given in Figures 2-44 and
2-45. Closer proximity to a fan, AHU, or duct fitting can cause excess turbulence
that can lead to higher self-noise and higher pressure drop and could force the fan
to operate in rotating stall, thereby creating a low-frequency rumble that the

A Practical Guide to Noise and Vibration Control for HVAC Systems

Photo courtesy of Vibro-Acoustics, a division of BVA Systems Ltd.

Figure 2-40 Cutaway view of a reactive (“packless,” “no-fill,” or “no-media”)
duct silencer.

Photo courtesy of Vibro-Acoustics, a division of BVA Systems Ltd.

Figure 2-41 Cutaway view of an elbow duct silencer.

Figure 2-42 In-duct attenuation of duct silencers and lined ductwork.

Figure 2-43 Comparative insertion loss of dissipative, film-lined, and reactive
duct silencers.

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Airside Equipment

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 2-44 Guidelines for duct silencer placement near fans and duct fittings.

Airside Equipment

Figure 2-45 Duct silencer placement near a mechanical room wall.

A Practical Guide to Noise and Vibration Control for HVAC Systems

silencer will not attenuate. The author was recently called to a resolve a complaint
of excessive rumble in a college library where a silencer with a catalogued PD of
70 Pa generated an actual PD of more than 250 Pa because it was “shoehorned”
between an AHU outlet and a duct elbow. The silencer’s extra PD threw the AHU’s
fan into rotating stall, thereby causing rumble complaints in the library. A complete
reconfiguration of the duct system was needed to eliminate the rotating stall condition and resolve the complaint.

If space is available, a plenum with an interior sound-absorbing lining (50- to
400-mm-thick) can provide significant fan noise reduction at all frequencies.
Recently completed ASHRAE-sponsored research shows that the plenum attenuation equation in the 2003 ASHRAE Handbook—Applications is conservative at
low frequencies. Later editions of the Handbook will include a more accurate
method for estimating the insertion loss as a function of plenum dimensions and
interior wall surface finish. Figure 2-46 gives some general guidelines for maximizing plenum insertion loss.
Acoustical Louvers
Acoustical louvers are used primarily in equipment rooms and air shafts where
noise control is needed at a ventilation opening. These louvers use fiberglass-filled
baffles and are available in thicknesses from 100 to 600 mm. The upper surface of
each baffle is solid, while the lower surface is perforated. Figure 2-47 shows a
cutaway view of an acoustical louver with airfoil-shaped baffles. Parallelogramshaped baffles with flat upper and lower surfaces and herringbone “sightproof”
blades are also available. Figure 2-48 compares the octave band sound transmission
loss performance of a typical 300 mm thick acoustical louver with that of a standard
weatherproof louver with a 50% net open area. Figure 2-49 shows a photo of an
acoustical louver in a garage ventilation system.
Because the free area of an acoustical louver can be as small as 15% of its face area,
care must be taken when sizing these louvers since a face area velocity of 2.5 m/s could
create a free area velocity of as much as 15 m/s, which would generate a high static pressure loss and high levels of self-generated noise and would entrain rain at an intake
louver. Controlling water entrainment and excessive air pressure drop often requires
selecting acoustical louvers for face velocities in the 1 to 1.5 m/s range.
Don’t confuse a louver’s Transmission Loss (TL) ratings with its Noise Reduction (NR) ratings, which are always 6 dB higher than the TL values. The TL values
indicate the attenuation of the louver if it is installed in an opening between two
adjacent rooms that are both very reverberant. The NR values are calculated from
the TL values and assume that the “quiet” side of the louver opens to the outdoors.
Conservative acoustical calculations typically use TL values since the exterior

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Acoustical Plenums

Airside Equipment

Figure 2-46 General guidelines for sound-attenuating plenum design.

Figure 2-47 Acoustical louver cutaway.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 2-48 Sound transmission loss of acoustical and weatherproof louvers.

Figure 2-49 Acoustical louver in a parking garage ventilation shaft.

Airside Equipment

landscape often has nearby reflective surfaces of some kind (e.g., the ground, other
buildings).
SPECIAL VARIABLE-AIR-VOLUME (VAV)
SYSTEM CONCERNS
Fans and AHUs
In an earlier section, this guide recommended that fans should be selected to
operate at maximum efficiency at the design airflow rate. That guideline holds true
for constant-volume systems. But for VAV systems, following this guideline can
result in low-frequency noise problems due to fan stall when operating at partial
load. Figure 2-50 compares the operation of two fans: a large one selected for optimum efficiency at 100% design airflow and a smaller fan that is optimized at about
80% of design airflow. The figure shows how operation of the system at about 40%
of design airflow (not unusual in VAV systems) can bring the system close to the
large fan’s stall region. Since the small fan would not operate near its stall region
at 40% of design airflow, it is a better selection even though its LW values may be
slightly higher at the full-capacity design operating point.

Figure 2-50 Basis for fan selection in a VAV system.

The method chosen for controlling a fan’s capacity can affect its noise. Each
type of capacity control device has its unique effect on fan noise. Inlet vanes, scroll
dampers, scroll curtains, and other similar devices result in higher fan noise
because they add turbulence to the airstream by obstructing the airflow. This is
shown in Figure 2-51, where a set of adjustable guide vanes are nested inside the
bellmouth inlet of an airfoil centrifugal fan. The airflow obstruction caused by the
vanes can raise a fan’s LW values by as much as 15 dB, depending on the octave
band of concern and the fan’s operating conditions. On the other hand, variablefrequency drive (VFD) motor speed controllers reduce a fan’s capacity by slowing
it down, which gives the dual benefits of lower noise and lower energy consumption. Figure 2-52 shows a VFD mounted on the side of a large air-handling unit. The
concept of system must be remembered, because it is the combined effect of the fan
and its control method that determines how much noise is generated.
VFD motor controllers are the most desirable method of capacity control.
However, even their use requires following certain system-related guidelines:
1.

The fan’s vibration isolators should be selected on the basis of the fan’s lowest
practical rpm rate; for a 1000 rpm fan in a typical commercial system, the lowest
practical rate might be as low as 500 rpm.

The VFD controller should have a feature called “critical frequency band jump.”
This feature allows a user to program the controller to “jump” over certain fan/
motor rpm settings that might excite sympathetic vibrations in the building structure. The more advanced VFDs permit “jumping” over several frequencies.

Control System
Some VAV system noise problems have been traced to control system problems. While most of the problems are due to poor installation, some are caused by
design team neglect. The mechanical engineer must be sure to specify high-quality
equipment that will operate in its optimal range and not near the edge of its specification range where tolerances can lead to inaccurate fan control. Also, the in-duct
static pressure sensor must be placed in a section of duct having the lowest possible
turbulence, i.e., at least three equivalent duct diameters from any elbow, takeoff,
transition, offset, damper, etc., to ensure that it transmits accurate static pressure
readings to the control system.
VAV system noise problems have often been traced to improper air balancing.
For example, downstream dampers are often set up to require higher-than-design
static pressure delivery from a fan. This causes the fan to operate at an inefficient
(and, therefore, noisy) point on its curve, perhaps even in its stall region. The air
balance specification should require that the contractor perform the air balance with
the specific requirement that the final system static pressure should be as low as
possible.

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A Practical Guide to Noise and Vibration Control for HVAC Systems

Airside Equipment

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Figure 2-51 Nested inlet vanes obstruct airflow.

Figure 2-52 Variable-frequency drive.

A Practical Guide to Noise and Vibration Control for HVAC Systems

RAISED FLOOR AIR DISTRIBUTION

If the supply air reaches the underfloor plenum via a shaft that is served by a rooftop air-handling unit or packaged unit, verify that the velocity of air that exits the
discharge openings at the bottom of the shaft does not exceed 7.5 m/s. This may
be difficult in installations where the underfloor plenum height is small, and it
may require multiple openings at the bottom of the shaft.

Office partitions in areas that use raised floor air distribution typically rest on top
of the raised floor and do not extend to the structural floor below. In such cases
the continuous underfloor plenum can be a sound leakage path between adjacent
rooms. One way to provide moderate speech privacy between adjacent rooms is
to install a short duct elbow with 50-mm-thick acoustical liner in the floor plenum
space under each floor diffuser, making sure that the elbow openings in adjacent
rooms are not facing each other.

Because the air velocities in an underfloor plenum and through floor diffusers are
very low, the system is often too quiet. Increasing the discharge air velocity of the
floor diffuser could raise the room sound level, but such velocities would create
undesirable drafts near the floor. Therefore, an electronic background noise
masking system may be needed to provide speech privacy in open plan and
private offices where a raised floor air distribution system is used.

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All of the guidelines presented above also apply to the design of raised floor
air distribution systems, with the addition of three very important additional
concerns:

Water-Side Equipment
CHILLERS
After fans and air-handling units, chillers are the most significant sources of
noise and vibration in HVAC systems. Their noise signatures include high levels
of broadband sound, but complaints of chiller noise and vibration are usually associated with the tones generated by their compressors, so treating these tones will
go a long way toward reducing the number of complaints.
Each compressor type—centrifugal, reciprocating, scroll, screw (rotary), and
magnetic bearing—has its own characteristic tonal structure that is related to the
speeds of its rotating components. Chilled-water systems that use absorption chillers also generate tones, but they are typically caused by the chiller’s associated
pumps. Like the blade passage frequency (BPF) tones in fans, chiller tones include
a fundamental tone and one or more harmonics, which are integer multiples of the
fundamental. The relative strengths of the fundamental and harmonic tones vary
with operating point.
Screw compressors have become very popular because of their low cost and
energy efficiency, but the mid-frequency tones (in the 250 to 1000 hertz octave
bands) produced by dual-screw compressors can be particularly troublesome
because the tones are very strong. Single-screw compressors generate tones that are
somewhat quieter than dual-screw compressor tones.
Factory noise-reduction options for chillers typically include heavy insulated
blankets that can be wrapped around the compressors and other noise-radiating
components. Figure 3-1 shows a photo of a water-cooled screw chiller with blankets wrapped around its compressor (foreground) and oil separator (background).
The blankets do not provide any significant low-frequency noise reduction, but they
reduce the strength of the compressor tones by about 3 dB. Some manufacturers

Figure 3-1

Water-cooled screw chiller with several noise and vibration
control treatments.

offer sheet metal compressor enclosures, which reduce the compressor tones by
about 5 to 7 dB. Additional noise reduction requires a complete chiller enclosure
like that shown in Figure 1-7.
Several factory noise control options are available for air-cooled chillers,
including compressor blankets, sheet metal compressor enclosures, and low-noise,
low-rpm condenser fans. The chart in Figure 3-2 shows the effectiveness of a
factory-option noise reduction package for an 875 kW air-cooled screw chiller. The
package includes acoustically-lined sheet metal compressor enclosures and lownoise condenser fans.
Compressor vibration is often as troublesome as compressor noise, so proper
vibration isolation of the chiller and its associated pumps and piping is typically necessary. Figure 3-1 shows spring floor mounts (background near bottom right corner) and
flexible pipe connectors at the condenser water connection (left foreground) and

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Water-Side Equipment

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 3-2

ARI-370 LW values for a 875 kW air-cooled chiller with and
without factory noise reduction options.

COOLING TOWERS · EVAPORATIVE COOLERS·
AIR-COOLED CONDENSING UNITS
The dominant noise source for this type of equipment is the fan, which can be
either a centrifugal fan in a forced-draft arrangement or a propeller fan in either a
forced-draft or induced-draft arrangement. Both fan types produce high levels of
low-frequency noise that can be difficult and costly to control. Since “add-on” noise
control treatments usually interfere with equipment efficiency, it is best to select the
quietest unit that conforms with the application’s space and cost parameters and
consider adding such acoustical treatments as barriers or close-coupled duct silencers only if absolutely necessary. The noise ratings of cooling towers, evaporative
coolers, and air-cooled condensers are obtained in accordance with the Cooling
Technology Institute’s Acceptance Test Code ATC-128. Consider the following
equipment selection and placement guidelines.

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rupture disk discharge piping (top background). Flexible pipe connectors should also
be used at the chilled water piping connections (under the insulation in the photo).
See the section entitled “Mechanical Equipment Rooms and Outdoor Equipment Areas” in Chapter 1 for guidelines related to the design of chiller rooms.

Water-Side Equipment

Guidelines for Cooling Towers, Evaporative Coolers,
and Air-Cooled Condensing Units
1.

Compare the ATC-128 acoustical data for various types of equipment that will
serve the required function. For example, Figure 3-3 compares the ATC-128
octave band sound levels at a distance of 15 m from the air inlet for three types
of 2800 kW cooling towers. Note that the tower with the forced-draft propeller
fan is much noisier than the other types.

Compare the ATC-128 acoustical data for equipment with standard-sized fans
and oversized fans (larger diameter). Increasing a fan diameter by one size typically yields a lower fan rpm and provides a reduction of about 3 dB at all frequencies. Figure 3-4 compares the octave band sound levels for fans of two different
diameters.

Compare the ATC-128 acoustical data for equipment with narrow blades and
wide-chord blades. The wide-chord blades usually result in lower fan RPM rates
and lower noise levels. Figures 3-5 and 3-6 show views of standard blades and

Figure 3-3

ATC-128 octave band LP values at 15 m from the air inlet side of
three types of 2800 kW cooling towers.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 3-4

ATC-128 octave band LP values for cooling towers of the same
fabrication series but with fans of different diameters.

Open towers and evaporative coolers also produce water noise (see Figure 3-8).
Some equipment manufacturers offer water basin silencers that interrupt the
waterfall a few centimeters above the basin. The remaining drop of only a few
centimeters can reduce the water noise by as much as 4 to 8 dBA. Figure 3-9
shows a view of water basin silencers installed in a tower.

Locate the equipment where a building or the natural topography (hillsides,
berms, etc.) will act as a noise barrier between the equipment and any noise-sensitive areas. Tall, solid barriers are often used for visual and acoustical screening of
outdoor equipment. For the greatest effect, such a barrier used for noise control
purposes should be installed as close as possible to the noise source, blocking
line-of-sight to the noise-sensitive area. However, with air-cooled equipment, be
sure that the barrier is not so close to the equipment that it reduces airflow or
causes air recirculation, either of which will reduce the equipment’s capacity.
Also, be sure that the side of the barrier facing the noise source has a soundabsorbing surface with weather protection; these barrier-absorber combinations

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wide-chord blades from within the cooling tower basins. Figure 3-7 compares the
octave band sound levels of two of these fans.

Water-Side Equipment

Figure 3-5

View of “standard” cooling tower induced-draft fan.

Figure 3-6

View of an induced-draft fan with wide-chord blades. This fan type
can be as much as 12 dBA quieter than a “standard” fan of the same
diameter.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 3-7

ATC-128 octave band LP values for cooling towers with “standard”
and wide-chord fan blades.

are also used for highway noise barriers. Figure 3-10 shows a sound-absorptive
barrier at a cooling tower installation. The photo in Figure 3-11 shows a close-up
of a sample barrier; note that the fibrous, sound-absorbing material is sandwiched
between a solid back-wall and a fluted, perforated front face. The flutes provide
rain protection for the fibrous material by keeping the perforations out of direct
contact with it.
6.

If the equipment has a “quiet” side (e.g., opposite the air inlet side of a forceddraft cooling tower), orient it toward any noise-sensitive areas. A piece of equipment’s “quiet” side can be determined by a review of its ATC-128 sound level
data sheet.

As the load on the equipment varies throughout the day, the intermittent starting
and stopping of the fans can cause additional noise complaints. This is especially
true of belt-drive equipment where “belt squeal” occurs when full motor torque
is applied instantly at start-up. Variable-speed drives or “soft-start” starters eliminate this problem (see discussion below).

Water-Side Equipment

Figure 3-8

Cooling tower basin with free-falling condenser water.

Figure 3-9

Honeycomb water basin silencers installed several centimeters
above the water surface to reduce the velocity of the falling water
before it hits the basin.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Photo courtesy of Empire Acoustical Systems.

Figure 3-10 Outdoor noise control barrier installation.

Figure 3-11 Close-up view of a sample of a sound-absorbing, outdoor noise
barrier panel. The perforated front layer faces the noise source.
Noise penetrates through the perforations and is absorbed by the
fibrous fill material. The solid back layer controls sound
transmission to the noise-sensitive region behind it.

For a multi-cell tower installation, the control sequence can be set up to minimize
equipment noise emissions to surrounding areas. The “typical” control sequence
for a two-cell cooling tower with a pair of single-speed fan motors would leave
Fan #1 on virtually full-time and bring Fan #2 on when required by the load.
Under this scenario the tower noise levels would remain constant over the entire
51% to 100% operating range and would decrease by only 3 dB at loads below
50% of full capacity.
Alternatively, if the two fans are controlled by VFD motor controllers, and if
the control sequence is set up correctly, the sound levels can be greatly reduced
at lower operating capacities. Figure 3-12 shows how the sound level would
vary for a proportional control sequence that does not let either fan speed up
beyond 50% of its maximum rpm until the second fan is also at the same operating point. Note that the sound level at 50% of full capacity under this control
sequence is about 10 dBA lower than the “full load” sound level. Similar noise
reduction can be achieved with a VFD controlling one fan, while the second
fan uses a two-speed motor.

BOILERS
Boiler manufacturers rarely measure their equipment’s noise, so calculations
for predicting boiler noise exposure at noise-sensitive locations are rarely done.
The combustion process in some boilers produces strong low-frequency noise that

Figure 3-12 Low-noise control sequence for a two-cell cooling tower.

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Water-Side Equipment

A Practical Guide to Noise and Vibration Control for HVAC Systems

is carried through the flue system and eventually radiates into the atmosphere. The
flue outlet should be placed away from any noise-sensitive areas so that the low
frequency rumble is not excessive near a building’s perimeter windows or ventilation openings.
Forced-draft boilers use high-pressure induced-draft fans for combustion air,
and octave band Lw values are available for these fans, so an acoustical analysis is
possible for this part of the system. Care should be taken to locate the combustion
air fan remote from noise-sensitive areas. Also, isolate flue pipes from the building
structure where they pass near vibration-sensitive areas.
PUMPS

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Pumps are rarely responsible for airborne noise problems. However, structureborne vibration transmission due to inadequate vibration isolation of the pump or
its attached piping or conduit can cause severe noise problems. Problems of this
type are usually heard as a strong tone at the pump’s impeller blade passage
frequency, which is generated at the clearance between the impeller tips and the

Figure 3-13 Pump impeller sizing guideline for minimizing the strength of the
blade passage frequency tone.

Water-Side Equipment

cutwater (see Figure 3-13). Specifying the impeller radius to be no more than about
85% of the cutwater radius (or about 90% of its untrimmed radius) will help minimize the problem.
Figures 3-14 and 3-15 show the proper installations for base-mounted and inline pumps. Note, in particular, that the base-mounted assembly shows the suction
elbow mounted on the isolated frame.

Figure 3-14 Proper installation of an end-suction pump.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 3-15 Proper installation of an inline pump.

PIPING SYSTEMS
There are five primary acoustical concerns with piping systems:
1.

Excessive fluid velocity flow across valves, strainers, and backflow preventers
(check valves) can cause high levels of broadband noise due to the strong turbulence.

Inadequate vibration isolation allows the pipes to serve as transmission paths for
noise and vibration generated by the attached rotating equipment.

Unsealed pipe penetrations through sound-rated walls and slabs can allow sound
leakage through the penetrations.

Penetrations with piping that is allowed to touch the penetrated wall or floor can
result in structure-borne transmission of piping system vibration into the wall or
floor, which, in turn, can transmit the vibration into nearby walls or floors.

Undersized valves or valve actuators are occasionally responsible for cavitation
noise, valve chatter, or whistle.

Water-Side Equipment

Unfortunately, acoustical testing that would help optimize the selection of
valves and strainers has not been done, so the general rule of thumb is to select these
items one size larger than the attached piping if they include a component that is
in the fluid stream. For example, a butterfly valve (disc in the waterflow) should be
upsized, whereas a gate valve permits unobstructed flow and does not need to be
oversized. An exception to the “one size larger” rule of thumb applies to throttling
control valves, which must be sized to provide proper control. Table 3-1 gives the
recommended maximum water velocities for pipes that are above lightweight
acoustical ceilings in typical office areas. If the piping system is enclosed in a
gypsum board shaft or horizontal enclosure, somewhat higher velocities may be
acceptable.
In general, all pumped pipe risers, mechanical room piping, and all piping
mains require vibration isolation in the form of either floor mounts or hangers. See
the chapter titled “Vibration Isolation” for more information.
Figure 3-16 shows how to isolate a pipe riser with neoprene pads under steel
load-distributing plates where minimal vibration isolation is needed. Greater vibration control would need spring isolators. Details for preserving sound isolation at
wall and slab penetrations while maintaining the necessary isolation between the
pipe and the structure are shown in Figures 3-17 and 3-18.
Table 3-1.

Maximum Recommended Waterflow Rates

Nominal Pipe
Size, mm

Maximum Velocity
in m/s

Approximate Max. L/s for
Schedule 40 Pipe

0.5

1.2

2.5

1.5

100

125

2.2

150

2.4

200

2.7

250

2.9

145

300 or more

215

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 3-16 Vibration isolation for piping riser. The use of neoprene pads
under the steel load-distributing plates indicates that the nearby
occupancy was not very noise-sensitive. For more critical cases,
steel spring isolators would be used.

Figure 3-17 Duct and pipe penetrations through walls.

Water-Side Equipment

Sketches courtesy of McGraw-Hill Book Company.

Figure 3-18 Sealing pipe penetrations for sound isolation.

Packaged and
Unitary Equipment
Note: For information on air-handling units, see the section titled “Air-Handling
Units and Fancoil Units” in Chapter 2.
ROOFTOP PACKAGE UNITS
(AIR-CONDITIONING AND AIR-HANDLING VERSIONS)
Rooftop units (RTUs) are responsible for some of the most frustrating and
expensive noise and vibration problems; frustrating because after building occupancy it is often very difficult to track down the specific cause(s) of the problem(s),
and expensive because the solution can require lifting the unit off the roof while
extensive changes are being made to the building’s roof and roof structure. For
example, a successful 1989 retrofit for a 210 kW unit cost $40,000.
The major noise sources in RTU systems are the fan(s) and compressors(s). In
package units larger than 70 kW, it is very important to design the duct system in
strict accordance with SMACNA guidelines for low static pressure losses because
many units of this type use forward-curved (FC) fans, which cannot accommodate
high duct system static pressures without generating high levels of low-frequency
noise. Most large RTU noise complaints occur where excessive duct system static
pressure causes an FC supply fan to operate in rotating stall.
Proper matching of the roof’s structural design, the placement of the RTU, and
the use of vibration isolators are critical in controlling vibration from the RTU’s
fan(s) and compressor(s). The design guidelines given below and in Figures 4-1
through 4-4 address all of these potential problems.

Packaged and Unitary Equipment

Figure 4-1

Very noisy rooftop unit installation.

Figure 4-2

Moderately noisy rooftop unit intallation.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 4-3

Moderately quiet rooftop unit installation.

Figure 4-4

Quietest rooftop unit installation.

Packaged and Unitary Equipment

Rooftop Unit Design Guidelines (All Sizes)
1.

The roof structure should be stiff enough so that the marginal deflection of the
structure due to the equipment load is no more than 6 mm. This may require either
reduced column spacing or auxiliary steel framing in the vicinity of the unit.

The unit should be at least 8 m from any private office or other noise-sensitive
area. When locating a unit, keep in mind that an unducted return air opening can
be as noisy as the ducted opening at the supply air end of the unit.

Mount the unit on a prefabricated vibration isolation roof curb. Factory-installed
internal vibration isolation is often ineffective. (See Chapter 5 for more information on vibration isolation.)

Make all ductwork, piping, and electrical connections to the unit with flexible
connectors.

For units at 70 kW and larger, consider the following additional steps:
1.

Avoid downblast units. Instead, select either a side-discharge unit or a downdischarge unit with a discharge plenum. Some manufacturers use a discharge
plenum as the heating section in a heating/cooling unit. Even if the heating capability is not required, specify this type of unit to take advantage of the acoustical
and aerodynamic benefits provided by the discharge plenum.

For installations over noise-sensitive areas, mount the unit on high-deflection (50
to 100 mm) spring isolators resting on an elevated steel frame that is supported
600 to 900 mm above the roof by extensions of the building columns.

Also, for installations over noise-sensitive areas, use a parallel arrangement of
circular supply ducts to distribute the conditioned air to the occupied space.
Circular ductwork radiates far less low-frequency noise and rumble than rectangular ductwork. The transition from the unit’s rectangular opening to the circular
ductwork should be heavy (1.6 or 2.0 mm thick steel) to help control breakout
noise through the flat portions of the transition.

Because the ducts attached to these units are often very wide, the ceiling and wall
subcontractors are often tempted to attach their framing to the ductwork or its
hangers. This must be avoided because contact with an oil-canning duct can
cause audible creaking of the attached wall or ceiling.

The roof construction under and around the unit should be dense concrete for
controlling downward noise transmission. Within the curb boundaries, do not cut
out more of the roof than is necessary for the duct penetrations. All roof penetrations for conduit and piping should be outside the curb.

Any noise control steps considered for the supply airside (duct silencers, duct
lining, etc.) should also be considered for the return airside. Return air plenums

A Practical Guide to Noise and Vibration Control for HVAC Systems

on larger units can often accommodate duct silencers inside the plenum directly
on top of the return air inlet. Another common noise control design for the return
air path splits the return air inlet into a lined “T” arrangement just below the unit.
This distributes the noise over a wider area at a somewhat lower level.
7.

Refer to the section titled “Duct System Components” in Chapter 2 for duct
system design guidelines.

WATER-SOURCE HEAT PUMPS
The noise and vibration generated by water-source heat pump units vary
widely among manufacturers, depending mainly on the size of the fan and the type
of compressor used. The extent of vibration isolation used to mount the compressor
and fan in the unit’s cabinet is also critical, e.g., spring isolation is much more effective than elastomeric pads, grommets or bushings, especially for horizontal heat
pumps that are suspended in ceiling plenums where the ceiling tile is a lightweight
mineral fiber or fiberglass. Guidelines for suspended horizontal and floor-mounted
vertical heat pumps are given below.
Guidelines for Suspended Water-Source Heat Pumps (See Figure 4-5)
1.

Suspend the unit as high as possible in the ceiling plenum using spring isolation
hangers.

Make condenser water connections to the unit with slack, flexible rubber or
neoprene hose connectors—many heat pump manufacturers offer these as standard accessories.

Condensate drain hookups may also require short flexible connectors.

Make electrical connections with slack, flexible conduit.

The discharge duct should be fiberglass-lined sheet metal, at least 1.5 m long, and
sized for a velocity of 5 m/s or less. An alternate is a 900 mm long duct silencer
that is selected for a static pressure drop of no more than 35 Pa.

Diffuser branch takeoffs from the unit’s discharge duct should be at least 1.2 m
downstream from the unit outlet.

A fiberglass-lined sheet metal elbow should be attached to the unit’s return air
inlet.

Guidelines for Floor-Mounted Water-Source Heat Pumps (See Figure 4-6)
1.

Locate the unit in a closet next to a corridor or other nonsensitive area. None of
the closet’s walls should be common with a bedroom or other noise-sensitive
area.

The closet should be large enough to allow adequate return airflow into the unit;
allow at least a 150 mm clearance all around the unit. In some cases, fiberglass

Packaged and Unitary Equipment

Figure 4-5

Guidelines for suspended heat pump units.

Figure 4-6

Guidelines for floor-mounted heat pumps.

A Practical Guide to Noise and Vibration Control for HVAC Systems

The closet door should be metal or solid core wood and have airtight, full perimeter jamb and head seals, and either a solid threshold sweep seal or an automatic
door bottom. Double doors need astragal closures to seal the gap between the
door panels. Provide a return air silencer at the return air opening into the closet.

Using a louvered closet door for return airflow is not recommended. However,
if one is used, install a sheet metal shield with 50 mm thick fiberglass plenum
liner behind the louver to reduce direct sound transmission from the unit to the
occupied space.

Mount the heat pump on vibration isolators. The type selected will depend on the
unit’s size and the noise-sensitivity of the occupancies near the unit.

Make condenser water connections to the unit with slack, flexible rubber or
neoprene hose connectors—many heat pump manufacturers offer these as standard accessories.

Condensate drain hookups may also require short flexible connectors.

Make electrical connections with slack, flexible conduit.

Size the discharge duct for a velocity of 5 m/s or less.

A wall-mounted, water-source heat pump should never be used in a room with
a criteria rating below NC/RC/NCB-50. This type of unit has very little internal
noise or vibration control, so the lack of a closet around it exposes the noise from
its fan and compressor to the occupied space at levels that are generally unacceptable in residences and offices. Also, because this type of unit is typically attached
directly to wall studs, its fan and compressor vibration are transmitted to the
surrounding walls, which radiate the vibration energy as audible hum.
WALL-MOUNTED PACKAGE UNITS
Units of this type in the capacity range of 7 to 18 kW are frequently used in
construction trailers and school classrooms. These units were originally designed
for use in telecommunications shacks, where equipment noise levels are not important. Their use is not recommended in rooms where speech communication is
important because they typically generate sound levels in the range of 50 to 65 dBA,
which is excessive for a classroom or any room where good speech intelligibility
is needed. Teachers, for example, turn these units off when they begin lecturing to
students.
A horizontal version of this type of unit in the 2.5 to 5 kW range is commonly
used in hotel guest rooms and produces sound levels that range from about 50 to
60 dBA, depending on the quality of the unit, its fan speed, and its compressor mode
setting. Sound ratings for these units are obtained in accordance with AHRI Standard 350, which gives the A-weighted LW value at each operating point in terms of

--``,,`,``,```,,`,,```````,,`,,,-`-`,,`,,`,`,,`---

plenum liner from 25 to 100 mm thick should be surface-applied to the closet
walls. Be sure to account for this when determining the required closet size.

Packaged and Unitary Equipment

bels (1 bel = 10 decibels). Specifying a maximum AHRI 350 rating of 5.0 bels will
result in a sound level of about 43 dBA in the middle of the room.
SMALL COMMERCIAL AND RESIDENTIAL SPLIT SYSTEMS
(UNDER 35 KW)
Two kinds of noise problems are common with split systems. The first is
condensing unit noise disturbing either a neighbor or the system’s user. Figure 4-7
shows how the problem can occur and how to avoid it with a solid fence of the proper
height. Be sure to follow the condensing unit manufacturer’s guidelines for placement of the fence so that it does not restrict condenser air circulation.
The second kind of problem involves the indoor fan/coil unit or furnace and the
location of its ductwork and grilles. Figure 4-8 shows guidelines for two kinds of
fan/coil unit installations.
Vibration problems most frequently associated with split systems are caused
by rigid contact between the building and either the outdoor condensing unit, the
indoor fan/coil unit, or the refrigerant piping. Figure 4-9 gives guidelines for avoiding this kind of problem.
Ductless split systems have been used successfully in Asia for many years and
are now common throughout the rest of the world. These systems’ indoor fancoil
units and outdoor condensing and heat pump units are quieter than the more traditional split system components. Figures 4-10 and 4-11 show photos of an indoor
evaporator section and an outdoor condensing section of this system type, respectively. Some versions of this unit use a variable-speed evaporator fan, so the indoor

Figure 4-7

Small condensing unit noise control.

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A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 4-8

Guidelines for fan coil unit installations.

Packaged and Unitary Equipment

Guidelines for vibration isolation of split systems.

Figure 4-10 Indoor fan coil section of a ductless split system. This type of
equipment with a variable-speed tangential fan can be very quiet.

--``,,`,``,```,,`,,```````,,`,,,-`-`,,`,,`,`,,`---

Figure 4-9

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 4-11 Outdoor condensing unit typically used with ductless split
systems; it can also be used with ducted fan coil units.
sound level for small capacities of this unit can be as low as the 35 to 45 dBA range,
depending on the thermostatically controlled fan speed. The condensing/heat
pump unit is also quieter that its traditional system counterpart.
STANDBY, EMERGENCY, AND
DISTRIBUTED ENERGY GENERATOR SETS
The skid-mounted radiators in engine-generator sets are often as loud as the
engines, and in indoor installations they are often installed directly behind a
discharge louver that directs both the radiator noise and the engine noise to the
outside, where a noise-sensitive area may be present. This potential problem can
be addressed by using a remote radiator (see Figure 4-12). The benefits of a remote
radiator are as follows:
1.

Since the hot radiator is outside the generator room, less ventilation air is needed
in the room, so the ventilation inlet and discharge louvers can be smaller. The
reduced louver size helps control the amount of engine-generator noise that is
transmitted to the outdoors.

The remote radiator can be located as needed to control the impact of its noise to
any surrounding noise-sensitive areas.

Remote radiators are available with low-noise, variable-speed fans that are as
much as 20 dBA quieter than skid-mounted fans.

--``,,`,``,```,,`,,```````,,`,,,-`-`,,`,,`,`,,`---

Packaged and Unitary Equipment

Photo courtesy of Young Touchstone, a Wabtec Company.

Figure 4-12 Remote radiator for engine-generator sets can be quiet with an
oversized, variable-speed cooling fan.

Where a remote radiator is not feasible, duct silencers can be used at the generator room’s air intake and discharge openings to reduce noise emissions. However,
the “silenced” openings must be oversized so that the static pressure losses through
the silencers are compatible with the radiator fan performance.

100

Vibration Isolation

The equipment type, drive-type (direct, gear or belt), rpm, and motor kW rating.

The mass and location of the vibrating component(s) within the equipment enclosure or on the equipment skid.

The nature of the equipment’s vibration—vertical or horizontal, rotating or reciprocating.

The location of the equipment relative to nearby noise and vibration-sensitive
areas.

The stiffness of the building structure supporting the isolated equipment (usually
related to the column spacing and beam depth).

The spacing between isolator mounting points.

These factors were all considered in the development of Table 5-1, “Vibration
Isolation Selection Guide,” which is based on Table 42 in Chapter 47, “Sound and
Vibration Control,” in the 2003 ASHRAE Handbook—Applications. Brief descriptions of the isolator and base types are given below.

101

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HVAC equipment that is rigidly attached to a slab, wall, or ceiling can transmit
the equipment vibration into the mounting surface and cause unacceptable levels
of structure-borne noise that can be heard in remote locations throughout the building. Likewise, piping, conduits, and ductwork can act as transmission paths of
structure-borne sound if they form rigid connection paths between the building and
the vibrating equipment. Isolating equipment vibration from building slabs, walls,
and ceilings is essential for controlling structure-borne sound transmission. There
are many types of vibration isolation systems for HVAC equipment, and the most
cost-effective selection for each piece of equipment depends on the following
factors:

Vibration Isolation Selection Guide
Equipment Location
Floor Span

Slab on Grade

Equipment Type

kW or
Other Rating

Up to 7 m

7 to 9 m

9 to 12 m

RPM

Base
Type

Isolator
Type

Min.
Defl.,
mm

Base
Type

Isolator
Type

Min
Defl.,
mm

Base
Type

Isolator
Type

Min
Defl.,
mm

Base
Type

Isolator
Type

Min
Defl.,
mm

Refrigerator Machines and Chillers

102

Reciprocating

All

Centrifugal, screw

All

Open Centrifugal

All

Absorption

All

Up to 7.5

All

Air Compressors and Vacuum Pumps
Tank-mounted horiz.

7.5 and up

All

Tank-mounted vert.

All

Base-mounted

All

Large Reciprocating

All

Pumps
Close-coupled

Large inline

Up to 5.5

All

5.5 and up

All

3 to 18.5

All

18.5 and up

All

Vibration Isolation

Table 5-1.

Vibration Isolation Selection Guide (continued)
Equipment Location
Floor Span

Slab on Grade

End suction
and split case

Cooling Towers

9 to 12 m

RPM

Base
Type

Isolator
Type

Min.
Defl.,
mm

Base
Type

Isolator
Type

Min
Defl.,
mm

Base
Type

Isolator
Type

Min
Defl.,
mm

Base
Type

Isolator
Type

Min
Defl.,
mm

Up to 18.5

All

18.5 to 90

All

90 and up

All

Up to 300

103
Boilers (Fire-tube)

7 to 9 m

All

301 to 500

501 and up

All

Axial Fans, Fan Heads, Cabinet Fans, Fan Sections
Up to 600 mm diameter
600 mm diameter and
up

All
Up to 500 Pa
s.p.

500 Pa s.p.
and up

Up to 300

300 to 500

501 and up

Up to 300

300 to 500

501 and up

A Practical Guide to Noise and Vibration Control for HVAC Systems

Equipment Type

kW or
Other
Rating

Up to 7 m

Vibration Isolation Selection Guide (continued)
Equipment Location
Floor Span

Slab on Grade

Equipment Type

kW or
Other
Rating

Up to 7 m

7 to 9 m

9 to 12 m

Min.
Defl.,
mm

Base
Type

Isolator
Type

Min
Defl.,
mm

Base
Type

Isolator
Type

Min
Defl.,
mm

Base
Type

Isolator
Type

Min
Defl.,
mm

RPM

Base
Type

Isolator
Type

All

Up to 30

Up to 300

Centrifugal Fans
Up to 600 mm diameter
600 mm diameter and
up

104
30 and up

300 to 500

501 and up

Up to 300

300 to 500

501 and up

Propeller Fans
Wall-mounted

All

Roof-mounted

All

Heat Pumps

All

A/D

Condensing Units

All

A/D

Vibration Isolation

Table 5-1.

Vibration Isolation Selection Guide (continued)
Equipment Location
Floor Span

Slab on Grade

7 to 9 m

9 to 12 m

RPM

Base
Type

Isolator
Type

Min.
Defl.,
mm

Base
Type

Isolator
Type

Min
Defl.,
mm

Base
Type

Isolator
Type

Min
Defl.,
mm

Base
Type

Isolator
Type

Min
Defl.,
mm

Up to 7.5

All

7.5 and up,
up to 1 kPa
s.p.

Up to 300

Packaged AH, AC, H and V Units
All

Up to 300

300 to 500

501 and up

All

A/D

Up to
300 L/s

All

300 L/s & up

All

105

300 to 500
501 and up

7.5 and up, 1
kPa s.p. and
up

Packaged Rooftop
Eqmt.

All

See Reference Note No: 17

Ducted Rotating Equipment
Small fans,
fan-powered boxes

Engine-Generators

A Practical Guide to Noise and Vibration Control for HVAC Systems

Equipment Type

kW or
Other
Rating

Up to 7 m

Vibration Isolation

ISOLATOR TYPES
Each of the isolators described below is available in different sizes to accommodate a wide range of load ratings. For example, Type 3 floor mounts are available
with load capacities that range from about 9 to 18,000 kg. The references to “static
deflection” in the descriptions below refer to the compression of the isolator’s
spring or neoprene element under the equipment load. It does not refer to the motion
of the isolator while the equipment is operating.
Type 1—Ribbed or Waffled Neoprene Pad
or Compressed Fiberglass Pad
These isolators are usually used where the vibration frequency of concern is
at least 100 hertz; and they are typically selected for a load of about 0.04 kg/mm2
and a static deflection of less than 1.2 mm. They are available in durometer ratings
from 30 to 70, with the higher ratings able to support heavier loads. Pads with lower
durometer ratings are more resilient, but carry less weight, than those with higher
durometer ratings. These pads can be stacked in sandwiches with metal loaddistributing plates between the pads for higher static deflections (but not higher
load capacities). Figure 5-1 shows four types of neoprene pads, including a metal/
neoprene assembly with a neoprene bushing that prevents metal-to-metal contact
where the isolator is bolted to a floor structure.

Products courtesy of Mason Industries.

Figure 5-1

Elastomeric pads. The elastomeric bushing inserted in the metal
load-distributing plate prevents metal-to-metal contact between
the plate and its thru-bolt.

106

A Practical Guide to Noise and Vibration Control for HVAC Systems

Type 2—Neoprene-in-Shear or
Compressed Fiberglass Floor Mount
These isolators have equipment mounting bolts and base plates for direct attachment to a building structure without the use of neoprene bushings. They are usually used
where the vibration frequency of concern is at least 50 hertz and are usually selected for
a static deflection up to 10 mm (see Figure 5-2). A seismic version (neoprene element
captive in a steel housing) of this isolator type is shown in Figure 5-3. Hanger versions
of this type are also available.
Type 3—Steel Spring Floor Mounts and Hangers
These isolators are used with a wide range of static deflections, from about 12
to 100 mm. The isolation elements in these units are coiled compression springs
that typically rest on neoprene cups or pads. The spring elements should be selected
to accommodate a load that is 50% higher than the design load before “bottoming
out.” For proper horizontal stability, the spring’s loaded (compressed) height
should be approximately equal to its coil diameter. Figure 5-4 shows two sizes of
floor-mount spring isolators. The neoprene pad under the spring baseplate is an
essential part of the isolator because it reduces the strength of the high-frequency
vibration energy that “leaks” through the spring coils.

Products courtesy of Mason Industries and Kinetics Noise Control.

Figure 5-2

Elastomeric or compressed fiberglass isolation mounts are used
where a static deflection less than 10 mm is needed.

107

Vibration Isolation

Products courtesy of Mason Industries.

Figure 5-3

Seismically rated elastomeric mounts. The mount on the left (75 mm
tall) has a nominal load rating of about 40 kg. The mount on the right
(165 mm tall) has a nominal load rating of as much as 700 kg.

Products courtesy of Mason Industries.

Figure 5-4

Two types of spring floor mounts. The mount on the left is for low
loads and static deflections of less than 25 mm. The mount on the
right is for larger loads and static deflections greater than 25 mm.

108

A Practical Guide to Noise and Vibration Control for HVAC Systems

Isolation hangers contain a spring element within a hanger box. As in the floor
mount, a hanger’s spring should rest in a neoprene cup to control high-frequency
vibration. The load of the suspended equipment or pipe is transferred to the top of
the spring via a length of all-thread rod. To minimize the potential for “shortcircuiting” due to misalignments in the field, specify hangers that allow the allthread rods to be misaligned as much as 15 degrees from vertical without touching
the hanger box. Figure 5-5 shows properly installed spring hangers in a trapeze
arrangement.
Type 4—Restrained Spring Isolator
Type 4 is similar to Type 3, with the addition of a restraining assembly that
limits equipment movement during start-up, earthquakes, or high winds. This isolator is also often used with waterside equipment (e.g., cooling towers, chillers,
piping, etc.) to limit uplift when the equipment is drained for service or maintenance. The photo in Figure 5-6 shows two versions of this isolator type.
Base Types
•

Type A: No base; isolator attached directly to the equipment frame or
mounting leg.

Figure 5-5

Spring hanger installation with proper spring compression and
hanger rod centered through hole at bottom of hanger box.

109

Vibration Isolation

Figure 5-6

•
•

Two types of spring floor mounts with seismic/wind-loading
standby restraints. The mount on the left is for low loads and static
deflections under 25 mm. The mount on the right is for larger
loads and static deflections greater than 25 mm.

Type B: Structural steel rails or frame (channel or wide flange, depending on
the load and the frame size) with height-saving brackets, as required.
Type C: Concrete inertia base whose weight is equal to that of the isolated
equipment.

Other Isolator Types
Pneumatic isolators (air springs) generally provide the highest degree of vibration isolation and are typically used where very large fans, chillers, or electrical
transformers are installed near noise-sensitive or vibration-sensitive areas. The
isolator is usually a neoprene bellows with equipment and floor-base mounting
plates. Pneumatic isolators can be used as free-standing units or in restraint housings. Figure 5-7 shows pneumatic isolators in restraint housings supporting an aircooled chiller. As with automobile tires, the pneumatic bellows in these isolators
need occasional refilling; this usually requires the presence of a permanent 700 kPa
pneumatic system that includes an air compressor and load-leveling valves that
maintain the isolated equipment at the proper operating height.
Cast metal floor mounts are still available but are not recommended because
of their potential for “short-circuiting.” See Figure 5-8.
Thrust restraints are required where a medium- or high-pressure fan (e.g.,
inline, plenum, etc.) is installed with vibration isolators. Their purpose is to prevent
metal-to-metal contact between the fan and any part of the air distribution system
by counteracting the fan’s thrust force. Figure 5-9 shows a thrust restraint inserted
between a plenum fan and its inlet bulkhead wall. A similar restraint is installed on

110

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Products courtesy of Mason Industries.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 5-7

Pneumatic isolators (“air bags”) supporting a rooftop air-cooled
chiller. Thin metal tubing connects 700 kPa compressed air to each
isolator.

Figure 5-8

Cast-metal floor mount is prone to “short-circuiting” and should
never be used. Instead, use a floor mount like that shown in
Figure 5-6.

111

Vibration Isolation

Photo courtesy of Governair Inc.

Figure 5-9

Thrust restraint at mid-height of fan inlet panel prevents contact
between the panel and the equipment housing framework. An
identical restraint is installed on the opposite side of the fan.

the opposite side of the fan. The restraints’ springs work in compression to resist
the fan’s thrust toward the bulkhead wall during fan operation.
Flexible pipe (pump) connectors are used to reduce the transmission of waterside equipment vibration to a building structure. A flexible, watertight material
(usually neoprene, rubber, or a more advanced elastomer) with either flanged or
threaded ends is installed in a pipe as close as possible to the vibrating equipment.
Best performance occurs when the connector is installed in a pipe that is parallel
to the equipment shaft. Flexible connectors should never be used to accommodate
equipment/piping misalignment because the static offset stresses the elastomer and
shortens its service life. The pipes should be properly aligned before installing the
flexible connector. The photo in Figure 5-10 shows flanged and threaded flexible
connectors.
“Flexible” metal hoses, whether braided or unbraided, are not as effective as
elastomeric pipe connectors, but they may be the only option available for refrigerant relief piping and piping that carries natural gas or corrosive or very hot fluids.
In these cases, select the metal hose for a length to be at least ten times the nominal
pipe diameter. Figure 5-11 shows short connectors of this type on an end suction
pump.
Frames and Bases: Lightweight, flexible equipment frames can result in excessive equipment vibration being transmitted into a supporting building structure. A stiff,

112

A Practical Guide to Noise and Vibration Control for HVAC Systems

Products courtesy of Mason Industries.

Figure 5-10 Flanged and threaded flexible pipe (pump) connectors.

Figure 5-11 Braided metal pump connector is not an effective vibration
isolator.

113

Vibration Isolation

Figure 5-12 Floor mount spring isolator under a height-saving bracket with a
separate seismic restraint.

auxiliary steel frame or concrete-filled inertia base is sometimes required to mitigate
the vibration transmission. The auxiliary frame or base stabilizes the equipment,
lowers its center of gravity, and stiffens its isolator attachment points. Figure 5-12
shows a frame with height-saving brackets that lowers the center of gravity that is seen
by the isolator. Base-mounted pumps frequently require inertia bases that serve as stiff
platforms to help keep the motor and pump shafts aligned.
Equipment vibration can “leak” past an inertia base due to the entrapped air
between the bottom of the base and its housekeeping pad or floor slab. Also,
construction debris can be unknowingly trapped beneath a base and cause a “shortcircuit.” Minimize the possibility of these problems by specifying that the inertia
base float as least 50 mm above the housekeeping pad or slab. If the inertia base’s
smaller footprint dimension is more than 1 m, leave one or more air relief openings
near the center of the inertia base.
Frames and bases should be designed to minimize the number of isolators
required to carry the load. Proper isolator loading is more difficult to achieve as the
number of isolators increases. Three isolators is the optimal number, although four
are most frequently used.
Special Concerns with Seismic and Wind-Loading Restraints
Vibration-isolated HVAC system components that are installed in seismic
areas or outdoors above occupied spaces may need standby restraints (sometimes
called “snubbers”) to prevent the components from being shaken or blown loose

114

A Practical Guide to Noise and Vibration Control for HVAC Systems

from their building attachments. Individual snubbers can be installed around an
equipment frame after the equipment has been set in place on its isolators. Alternatively, the snubbers can be integrated into the isolators themselves. Figure 5-12
shows a vibration-isolated equipment frame with a snubber installed just to the
right of the isolator. Figure 5-13 shows a pump mounted on spring isolators with
integral snubber restraints.
Isolated equipment is prone to horizontal offset if pulled or pushed by the
attachment of a misaligned pipe. If separate isolators and seismic snubbers are
used, small offsets may not cause a “short-circuiting” problem. However, when
a combination isolator/snubber is used, the offset can easily cause a “shortcircuit” that will let equipment vibration bypass the isolator. The ASHRAE
publication, A Practical Guide to Seismic Restraint, and the new FEMA
Manual 412, Installing Seismic Restraints for Mechanical Equipment, show
how restraints should be installed to prevent “short-circuiting” of the vibration
isolators.
There are literally dozens of other variations of isolators, frames, and bases
for both general and specific uses. A qualified isolation equipment vendor will
have an engineering staff that can design and deliver an isolator or complete isolation system for a specific purpose.

Figure 5-13 Pump mounted on combination isolator/restraint under heightsaving bracket. This type of isolator is not recommended because
it is prone to short-circuiting.

115

Vibration Isolation

The Importance of the Supporting Structure

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Vibration isolation systems work as expected only if the supporting structure
is much stiffer than the isolator. As a worst case, a supporting structure with approximately the same static deflection as an isolator resting on it can amplify the equipment vibration that is transmitted into the structure. This problem can be avoided
by working with the structural engineer to ensure that all structures that accommodate either supported or suspended equipment on isolators have a static deflection
of no more than 20% of the isolator’s static deflection. For example, the structure
that supports a piece of equipment on 25 mm deflection isolators should have a
static deflection of no more than 5 mm. The sketches in Figure 1-16 in Chapter 1
show the effect qualitatively.

116

Specifications

The Construction Specification Institute (CSI) has reserved section 23.04.48
for vibration control products in its new six-digit numbering format. The more
familiar section 15074 is the reserved section in the old five-digit format. Neither
the old nor the new CSI format reserves a section specifically for noise control
products. Some specifiers prefer to include the acoustical performance specifications for all system components in a single section titled “Noise Control.” For
example, the acoustical specifications for fans, air-handling units, terminal units,
diffusers, chillers, cooling towers, duct silencers, duct liners, etc., would all be in
the same specification section. Unfortunately, an acoustical specification in this
location could be overlooked by an equipment vendor who is focusing only on the
overall specification section for his/her product. The acoustical specification is less
likely to be overlooked if it is included in that components’s project specification
section. The section below gives some example acoustical specifications that can
be included in the various products’ specification sections.
SAMPLE SPECIFICATIONS
Fans
Acoustical specifications for fans should specify the maximum sound power
level (Lw) values according to the latest revision of AMCA Standard 300. A sample
specification is shown below.

117

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Construction drawings are the primary tools for presenting design requirements to a contractor. However, they do not do the whole job because they cannot
fully define the quality of an HVAC system's materials and workmanship. Drawings also cannot always define special installation procedures. Those tasks are
often done with specifications.

Specifications

When tested in complete accordance with the latest revision of AMCA 300,
“Reverberant Room Method for Sound Testing of Fans,” in a laboratory registered
by AMCA to perform the test, the tested LW values shall not exceed the values scheduled in the following table when operating at the scheduled operating conditions.
Maximum LW values in dB re 1 pW
Octave Band Center Frequency in Hz

Airflow in TSP Inlet/
Mark
L/s in Pa Outlet

125

250

500

SF-1

1000

1125

Outlet

SF-2

8400

525

Outlet

SF-2

8400

475

Inlet

1000 2000 4000 8000

Air-Handling Units and Fancoil Units
Acoustical specifications for AHUs should specify the maximum LW values
according to the latest revision of AHRI Standard 260. A sample specification is
shown below.
All AHUs listed in the schedule below shall be tested in complete accordance
with the latest revision of AHRI Standard 260, “Sound Rating of Ducted Air Moving
and Conditioning Equipment.” When operating at the maximum design capacities,
the tested LW values shall not exceed the values scheduled in the following table.
Maximum LW values in dB re 1 pW
Octave Band Center Frequency in Hz
Mark

Location

125

250

500

1000

2000

4000

8000

AH-1

SA discharge

AH-1

RA inlet

AH-1

ExA discharge

AH-1

OA inlet

AH-1

Case-radiated

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A Practical Guide to Noise and Vibration Control for HVAC Systems

Chillers
Some chillers produce more noise at partial load than at full load. Therefore,
specify the maximum LP values under all operating conditions, as follows.
Chiller LP values shall be obtained in complete accordance with the latest revision of AHRI Standard 575, “Method of Measuring Machinery Sound Within an
Equipment Space.” The representative octave band LP values, as defined in the
standard, shall not exceed the values scheduled below for chillers operating at any
operating condition between 25% and 100% of the maximum design capacity.
Maximum LP at 1 m in dB re 20 μPa
Octave Band Center Frequency in Hz
Mark

125

250

500

1000

2000

4000

8000

CH-1

CH-2

Duct Silencers
Duct silencers attenuate the noise that travels through them, but they also
create “self-noise” due to the turbulent airflow that they generate. Silencers, therefore, must be specified with two sets of acoustical performance limits: a set of minimum insertion loss values and a set of maximum “self-noise” values. Also, because
the acoustical and aerodynamic performances of duct silencers are so closely
linked, it is customary to specify a maximum pressure drop in the acoustical specification. A sample specification is shown below.
When tested in complete accordance with the latest revision of ASTM E477,
“Standard Method of Testing Duct Liner Materials and Prefabricated Silencers for
Acoustical and Airflow Performance” in a laboratory that is NVLAP-accredited to
conduct the test, the acoustical and aerodynamic performance of all duct silencers
shall conform to the values given in the following schedules.

Mark Type

Minimum Dynamic Insertion Loss in dB
Octave Band Center Frequency in Hz

Airflow
in L/s

Velocity
in m/s

Max. P.D.
in Pa

125

250

500

1000 2000

4000

ST-1

+9400

+5.8

ST-2

+8400

+4.9

ST-3

8400

7.5

Type Legend: D = Dissipative, F = Film-lined, R = Reactive

119

Specifications

Maximum Self-Noise Sound Power Level
in dB re 1 pW
(0.37 m2 face area)
Octave Band Center Frequency in Hz

Airflow
in L/s

Velocity
in m/s

Max. P.D.
in Pa

125

250

500

1000

2000

4000

+9400

+5.8

ST-2

+8400

+4.9

ST-3

8400

7.5

Mark Type
ST-1

Type Legend: D = Dissipative, F = Film-lined, R = Reactive

Positive (+) values for airflow and velocity indicate the airflow and noise travel
in the same direction, whereas negative (–) values indicate that airflow and noise
travel in opposite directions. Good design practice recommends selecting silencers
with static pressure drop of no more than 63 Pa.
Vibration Isolators
Specifications for vibration isolators should include detailed descriptions of
their fabrication materials and methods. The sample specification below is a very
abbreviated example that includes a vibration isolator/frame selection schedule.
All vibration isolators shall be selected to have a minimum static deflection as
scheduled below and a reserve load capacity of at least 50% of the design load. All
spring isolators shall have a loaded height/diameter ratio in the range of 0.8 to 1.2. To
provide adequate frame stiffness all frame and base members shall have a height that
is at least 10% of the spacing between adjacent isolators. All spring hangers shall function properly with a hanger rod misalignment of no more than 15 degrees from vertical
in any direction. Seismic/wind-loading restraints and cables shall not affect the isolation system effectiveness during normal operating conditions. All vibration isolators
shall be selected in complete conformance with the schedule below.
Min. Static
Deflection in
mm

Isolator Type

Frame Type

Mark

RPM

Motor
kW

CH-1

3600

125

Spring mount

SF-1-12

1150

18.5

Spring mount

channel

P-1,2

1750

Spring mount

EF-1

750

Hanger

channel

B-1

—

Pad mount

—

CHW piping

—

---

Spring mount

—

CW piping

—

Spring mount

—

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A Practical Guide to Noise and Vibration Control for HVAC Systems

The acoustical specification for duct and plenum liners should include their
locations and acoustical performance. Because the relevant ASTM testing standard
permits mounting the tested samples in one of several mounting arrangements per
ASTM E795, it is important to specify the mounting type. A sample is shown
below.
Duct and plenum liners are rigid or semi-rigid foam or mineral fiber boards
with coatings to prevent fiber erosion under all expected airflow conditions. Duct
liner of the specified thickness shall be located as shown on the drawings and as
listed below.
a. All supply air ducts up to 8 m from fan discharge—50 mm thick.
b. All other supply air ductwork—25 mm thick.
Plenum liner of the specified thickness shall be located as shown in the drawings and as listed below.
a. Supply fan discharge plenum—100 mm thick.
b. Supply fan intake plenum—50 mm thick.
The sound absorption coefficients of all duct and plenum liners shall be determined in complete accordance with the latest revision of ASTM C423, “Standard
Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method,” in a laboratory that is NVLAP-accredited to conduct the
test. The sound absorption coefficient values of all duct and plenum liners shall
conform to the schedule below. Samples shall be tested with a Type A mounting in
complete accordance with the latest revision of ASTM E795, “Standard Practices
for Mounting Test Specimens during Sound Absorption Tests.”
Minimum Sound Absorption Coefficient
Octave Band Center Frequency in Hz

Service

Thickness,
mm

125

250

500

1000

2000

4000

Duct

0.13

0.50

0.70

0.95

0.98

0.99

Duct

0.25

0.70

0.96

0.99

Plenum

0.25

0.70

0.96

0.99

Plenum

100

0.72

0.95

0.99

121

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Duct and Plenum Liners

Specifications

TERMINAL UNITS
While terminal units are sometimes specified in terms of their cataloged NC
ratings, a complete acoustical specification should include more detailed information. A sample specification is shown below. Note the distinction between
discharge and radiated noise.
All terminal units shall be tested in complete accordance with the latest revision of AHRI 880, “Standard for Air Terminals,” in a laboratory that is AHRI-certified to conduct the test. When operating at the design capacities scheduled below,
the tested LW values shall not exceed the scheduled values. Units used in a pressure-independent system shall be tested with their inlet velocity sensors installed.
Maximum Discharge LW in dB re 1 pW
Octave Band Center Frequency in Hz

Mark

Design
Aiflow
in L/s

Static
Pressure
in Pa

125

250

500

1000

2000

4000

VAV-1

700

250

VAV-2

470

250

VAV-3

280

250

Maximum Radiated LW in dB re 1 picowatt
Octave Band Center frequency in Hz

Mark

Design
Aiflow in Static PresL/s
sure in Pa

125

250

500

1000

2000

4000

VAV-1

700

250

VAV-2

470

250

VAV-3

280

250

EXAMPLES OF HOW “NOT” TO SPECIFY
Because specifications offer the opportunity to precisely define acoustical
requirements, do not waste the opportunity and detail them with imprecise statements, such as:

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A Practical Guide to Noise and Vibration Control for HVAC Systems

“The HVAC equipment shall not produce objectionable noise or vibration.”

This statement misses the mark on two counts. First, it refers to only the HVAC
equipment, not the entire system. One of the main points of this guide is that most
HVAC noise problems are caused by how the system is put together. Secondly, the
statement does not define the phrase objectionable noise or vibration. Subjective
statements such as this do not belong in contract documents.
“All HVAC system noise and vibration shall be completely eliminated.”

This statement recognizes that the system performance is important, but it mistakenly requires that all “noise and vibration shall be completely eliminated.” In fact,
some system noise is desirable for masking other annoying building sounds, such
as conversations in nearby work areas or transformer and lighting ballast hum.
“HVAC system noise shall not exceed NC-40.”

By requiring the contractor to meet a certain NC (Noise Criteria) design goal, this
statement assumes that the contractor will perform an acoustical analysis. Unless
the project is design/build, the contractor probably will not have the capability and
should not have that responsibility. The statement also specifies only one NC
rating, presumably for the entire building. A review of Table B-1 in Appendix B
shows that an NC, RC, or NCB rating should be selected for each type of building
occupancy.
“Duct silencers shall attenuate fan noise by 25 dB.” or “Fan noise shall not
exceed 85 dB.”

These statements specify only one acoustical characteristic of a duct silencer and
a fan with no frequency qualification. The sample specifications at the beginning
of this chapter give recommended specification characteristics at specific octave
bands for various HVAC system components.
“All vibration isolators shall be 90% efficient.”

This statement gives an unreasonable, incomplete requirement that is difficult and
expensive to field-verify. First of all, it is hard to justify the same isolation efficiency at
a roof-mounted, 700 kW reciprocating chiller and a smooth-running, 4 kW, 3600 rpm
pump mounted on grade. Secondly, the concept of isolation efficiency only applies to
a perfect isolator with an infinitely stiff base attachment. A proper vibration isolation
specification details certain fabrication requirements for the isolators and their bases
and refers to a schedule on the drawings where the isolators and base construction for
each piece of equipment are listed.
“All equipment shall be mounted on the manufacturer's standard isolation
base, subject to approval.”

The requirement for a manufacturer’s “standard isolation base” can come back to

123

Specifications

haunt you if you aren't completely familiar with all of the isolation packages
offered by each manufacturer. Manufacturers are continually re-engineering their
products so that yesterday's “standard isolation package” may not resemble
today's. More importantly, a “standard isolation package” cannot be matched to the
structural conditions in every building. Every isolator should be selected on the
basis of the equipment’s vibration characteristics, the noise/vibration sensitivity of
the surrounding areas, and the mounting location's structural stiffness.
“When field conditions result in an interference between ductwork and
another building service, the contractor shall install ductwork offsets and
transitions as necessary to avoid the interference.”

This sentence is supposed to save time when an interference is discovered on
a job by instructing the contractor to take care of the problem without bothering the
design team. Unfortunately, when an offset or transition occurs in a medium- or
high-velocity duct, especially near a fan or takeoff, an unexpected result can be
excessive airflow noise, fan noise, or duct rumble. The contractor should never be
allowed to change the duct system design near a fan or takeoff without a review by
the mechanical engineer. A more proper statement would be:
“When field conditions result in an interference at any medium- or
high-velocity duct, the contractor shall submit a revised duct layout for
review by the mechanical engineer.”

124

Construction Phase Tasks
VALUE ENGINEERING (COST-CUTTING)
Many projects have a “value engineering” phase in which the contractor presents cost-saving alternatives to the HVAC system design. The contractor and other
project team members can bring valuable experience to a project and propose
changes that could allow the deletion or reduction of some acoustical treatments.
However, since most contractors are unaware of the acoustical impacts of their costsaving proposals, it is important that the design team review them for their noise and
vibration impacts. This is especially important because most cost-saving proposals
involve either reducing the size of equipment, ductwork, and mechanical rooms or
substituting cheaper equipment in place of what was specified. Table 7-1 lists some
common value engineering proposals and their potential acoustical impacts.
SUBMITTALS AND SHOP DRAWING REVIEWS
Great care must be taken in reviewing submittals, especially those that include
manufacturer’s sound data. To be reliable, the submitted sound data must be
collected and reported exactly as required in the appropriate test standard. Incorrect
factory data can easily lead to unexpected noise problems on the job site. A more
thorough discussion of manufacturers’ sound data is included in the ASHRAE
publication, Application of Manufacturers’ Sound Data.
Fans, Air-Handling Units (AHU), and Fancoil Units (FCU)
Pay particular attention when reviewing sound power level (LW) submittals.
Some manufacturers submit octave band sound power levels that are calculated by
the obsolete ASHRAE calculation method, while others submit A-weighted octave

125

Construction Phase Tasks

Value Engineering Proposal

Possible Acoustical Impact

Reduce fan size and speed up the fan

Higher noise level due to
fan inefficiency and higher outlet velocity

Reduce duct sizes

Higher noise level and increasing chance of
rumble due to higher airflow velocities

Reduce duct wall thickness
(increase gauge number)

Greater chance of rumble due to flexible duct
walls; lighter weight allows more breakout sound
transmission through duct walls

Reduce equipment room size

Higher noise level in adjacent spaces due to
proximity of equipment to wall(s)

Delete masonry walls, use drywall
partitions at mechanical rooms

Higher levels of low-frequency noise
and rumble in adjacent spaces

Replace VFD motor controllers
with inlet vanes for fan capacity control

Higher noise levels at all operating capacities
due to addition of inlet vanes at fan inlet

Reduce sizes of terminal units or grilles

Increased noise due to higher airflow velocities

Fiberglass ductboard instead
of sheet metal ductwork

More breakout sound transmission
through duct walls

Use neoprene mounts instead of
steel spring isolators

The lower static deflection of neoprene
will reduce vibration isolation effectiveness

Delete duct silencers

Higher noise levels throughout the duct system

Reduce the number of VAV zones

Higher noise levels near the
larger VAV terminal units

Change from “chilled water” system
to large rooftop package units

Higher noise levels on the top floor directly
beneath the rooftop unit and unacceptable shaftwall vibration—both impacts due to excessive air
turbulence in the ductwork near the unit and poor
vibration isolation inside the unit

126

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Table 7-1. Common Value Engineering Proposals
and Their Potential Acoustical Impacts

A Practical Guide to Noise and Vibration Control for HVAC Systems

band LW values that at first glance make a unit appear to be very quiet at low
frequencies. Since virtually all fan and AHU acoustical specifications specify
unweighted octave band LW values, it is important that any submitted A-weighted
LW values be converted to unweighted values using the simple procedure given in
Table 7-2. The example represented in the table shows how correction factors are
added to the submitted A-weighted LW values to obtain the unweighted values for
a typical forward-curved centrifugal fan. The A-weighting adjustments can be used
for any octave band LP or LW spectrum.

For equipment used in variable air volume systems, verify that the submitted
LW data include the effects of whatever capacity control device is used. Ensure that
the specified fan type, diameter, speed, and performance curve are submitted. All
of these parameters will affect the noise level.
An acoustical performance submittal for a fan, AHU, or FCU should include
at least the following:
1.

The name and location of the laboratory conducting the test.

The date of the test.

A complete identification of the equipment, e.g., series name, model number, etc.
Table 7-2. Procedure for Converting from
A-Weighted LW Values to Unweighted LW Values
Octave Band Center Frequency in Hertz
63

125

250

500

1000

2000

4000

8000

Submitted A-weighted LW
values in dB re 1 pW

Adjustment factor to
remove A-weighting

+26

+16

–1

Resulting unweighted LW
values in dB re 1 pW

127

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Fans, AHUs, and FCUs produce different sound power levels at their intakes
and discharges, yet most fan manufacturers mistakenly submit the same LW data
for both locations. Verify that the submittal is for the correct side of the equipment.
This kind of equipment also produces case-radiated noise, which can be significant
in some applications.

Construction Phase Tasks

The equipment operating conditions, e.g., airflow, total static pressure, rpm, and
motor kW rating.

A statement confirming that the measurements were obtained in complete
conformance with the applicable test standard (AMCA 300 for fans, AHRI 260
for AHUs and FCUs).

The unweighted octave band sound power levels and a note as to whether the
values are for discharge, inlet, or case-radiated noise.

Terminal Units and Lab Air Valves
Terminal unit and lab air valve submittals sometimes include only the unit’s
single-number NC rating at various combinations of airflow and differential pressure operating conditions. These NC ratings can be used to compare the relative
loudness of different terminal units, but there is no way to ensure that the published
NC rating will occur in a field installation because the published values are based
on atypical system designs and room conditions. The only way to be sure of a terminal unit’s field performance is to use the published discharge and radiated octave
band LW values in a set of calculations that are based on the actual design conditions, e.g., unit location and operating point, room dimensions and surface finishes,
ceiling type and height, etc.
An acoustical performance submittal for any kind of air terminal unit or lab air
valve should include at least the following:
1.

The name and location of the laboratory conducting the test.

The complete equipment model number.

The equipment operating conditions airflow and static pressure drop.

A statement that the measurements were obtained in complete conformance with
the latest revision of test standard AHRI 880.

For terminal units, the octave band LW values for discharge and case-radiated
noise.

For lab air valves, the octave band LW values for discharge, exhaust, and caseradiated noise.

Air Devices
Test standard AHRI 890 is used to determine the octave band LW values for air
devices at various airflows. A standard adjustment is applied to the measured data
at each airflow to determine estimates of the device’s flow-dependent NC ratings
in a large room. An acoustical performance submittal for any kind of air device
should include at least the following:

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A Practical Guide to Noise and Vibration Control for HVAC Systems

The name and location of the laboratory conducting the test.

The device’s complete model number and dimensions.

The airflow quantities used during the test.

The presence of any appurtenances, e.g., dampers, plenums, etc.

A statement that the measurements were obtained in complete conformance with
the latest revision of test standard AHRI 890.

The octave band LW values and standardized NC rating at each airflow.
Be very careful to note the following in the submittal:

If there will be a damper attached to the device’s inlet (thereby converting it to a
register) verify that the submitted NC rating was determined with the damper in
place. The presence of a damper can add from 5 to 15 points to a device’s NC
rating.

Any device with adjustable deflectors or vanes will produce different sound
levels depending on the deflector or vane settings. Therefore, verify that the
submitted NC rating is based on the vane or deflector positions that will be used
in the field installation.

Slot diffusers are typically tested for airflow and sound under two conditions:
a.

Installed in a very large plenum, and

Installed in a small, close-fitting sheet metal boot with a round or oval duct
connection.

Verify that the submitted acoustical data are for the appropriate plenum configuration for the project application.
Water-Source Heat Pumps
There are no acoustical test codes specifically for water-source heat pumps.
However, since these units typically have ducted discharges, their discharge LW
values should be determined in accordance with AHRI 260, “Sound Rating of
Ducted Air Moving and Conditioning Equipment.” And since the inlet of this
equipment is typically unducted, the proper test procedure for determining inlet LW
values is AHRI 350, “Sound Rating of Non-Ducted Indoor Air-Conditioning
Equipment,” which is usually used to determine the combination of inlet and caseradiated sound power level in each octave band from 63 to 4000 hertz.
Since this equipment can operate in “fan only,” “cooling,” and “heating” modes,
the submittal should include octave band LW values for all three modes of operation.
An acoustical performance submittal for a water-source heat pump should
include at least the following:

129

Construction Phase Tasks

The name and location of the laboratory conducting the test.

The test date.

A complete identification of the equipment, e.g., series name, model number, etc.

The equipment operation conditions, e.g., operating mode, airflow, TSP, rpm,
and motor kW rating.

A statement confirming that the measurements were made in complete conformance with the applicable test standard (AHRI 260, AHRI 350, etc.). Any variations from strict compliance with the test standards should be fully explained.
The inclusion of any noise reduction products or packages should be noted in the
submittal.

The unweighted LW values in the octave bands from at least 63 to 4000 hertz for
each operating mode (fan only, cooling, and heating) and a note as to whether the
values are for discharge, inlet, or case-radiated noise. All three should typically
be reported.

Cooling Towers and Evaporative Coolers
Factory noise data for cooling tower, evaporative coolers, and similar equipment are measured in accordance with Cooling Technology Institute’s Acceptance
Test Code ATC-128. An acoustical performance submittal for this type of equipment should include at least the following:
1.

The name and location of the laboratory conducting the test.

A complete identification of the equipment, e.g., type, series name, model
number, etc.

The equipment’s operating conditions; for equipment with variable-speed fans,
the test should be run at 50%, 75%, and 100% of full fan speed.

A statement confirming that the measurements were obtained in complete
conformance with the latest revision of test standard ATC-128 and that the
measured data include fan and water noise (if applicable).

The octave band LP values for each operating condition at distances of 1.5
and 15 m from all four sides of the equipment, as well as above the fan
discharge.

Water-Cooled Chillers
The noise of indoor water-cooled chillers is measured in accordance with
AHRI 575. An acoustical performance submittal for an indoor water-cooled chiller
should include at least the following:

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A Practical Guide to Noise and Vibration Control for HVAC Systems

The name and location of the laboratory conducting the test.

A complete identification of the equipment, e.g., series name, model number, etc.

The equipment’s operating conditions; for variable capacity equipment, the test
should be run at 50%, 75%, and 100% of full capacity.

A statement confirming that the measurements were obtained in complete
conformance with the latest revision of test standard AHRI 575.

The octave band LP values for each operating condition at a distance of 1 m from
all four sides of the equipment.

Air-Cooled Chillers
The noise of air-cooled chillers is measured in accordance with AHRI 370. An
acoustical performance submittal for an air-cooled chiller should include at least
the following:
1.

The name and location of the laboratory conducting the test.

A complete identification of the equipment, e.g., series name, model number, etc.

The equipment’s operating conditions; for variable capacity equipment, the test
should be run at 50%, 75%, and 100% of full capacity.

A statement confirming that the measurements were obtained in complete
conformance with the latest revision of test standard AHRI 370.

The octave band LW values for each operating condition.

SHOP DRAWINGS
In the early construction phase coordination that often precedes the start of
construction in a large project, the ductwork subcontractor will prepare a set of shop
drawings that will help identify field interferences with other trades (e.g., plumbing, electrical, and structural). These shop drawings inevitably include ductwork
layouts that differ from those in the design drawings. The shop drawing reviewer
must, therefore, be careful to verify that any proposed revised duct routings do not
result in noise problems.
For ductwork shop drawings:
1.

Verify that the proposed duct runs, especially the trunk ducts, have not been
moved near noise-sensitive areas.

Verify that the proposed duct sizes conform to the specified maximum air
velocities.

131

Construction Phase Tasks

3.
4.

Verify that there are no dampers or duct silencers shown within three equivalent
duct diameters of duct fittings or fan discharge openings.
Pay particular attention to offsets and transitions that may have been added in
medium- and high-pressure duct systems, especially near equipment rooms. An
improperly located offset or transition can cause excessive regenerated noise due
to the extra turbulence. It can also cause the creation of excessive static pressure
drop that can force the fan to operate in rotating stall or surge. Figure 7-1 shows
an example where an unexpected offset caused a very troublesome noise problem
at the discharge of an air-handling unit.

Verify that all duct penetrations through sound-rated slabs and partitions (usually
those with insulation batts in their stud cavities) are sleeved and caulked airtight.

6.
7.

Verify that all of the specified duct and plenum liners, duct silencers, and acoustical louvers are shown on the shop drawings.
Verify that all duct silencers are in the right locations and that their attenuating
baffles are oriented properly. For duct silencers near elbows, the baffles should
be oriented in the plane of the turn.

If a special duct gauge is specified, verify that it is shown on the drawings.

Figure 7-1

Overhead plan views of AHU rooms showing the effects of a duct
offset.

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A Practical Guide to Noise and Vibration Control for HVAC Systems

For piping system shop drawings:
1.

Verify that pipe runs have not been located above ceilings or in shafts that are
adjacent to noise-sensitive areas.

Verify that no pipe runs will interfere with the design drawing duct locations; any
interference should be resolved by moving the pipe, not by adding an offset in the
ductwork.

Be sure that the pipe sizes agree with the specified maximum waterflow velocities.

Verify that all of the specified flexible pipe connectors are shown at pumps, chillers, boilers, cooling towers, and other waterside equipment.

Verify that all pipe penetrations through sound-rated partitions and slabs are
sleeved and sealed airtight without making contact with the wall or slab.

Verify that the total fan pressure, fan speed, operating kW rating, and airflow are
consistent with the performance curve for each fan.

Excessive duct leakage will force the balancing technician to increase the fan rpm
(and noise) to achieve the design airflows. Compare traverse readings with air
device totals to determine if duct leakage is occurring.

Compare the measured airflow for each grille and register with the design values
in the contract documents.

Verify that the in-duct static pressure just upstream of the terminal unit that is
farthest from the fan does not significantly exceed the minimum required to operate the terminal unit.

SITE INSPECTIONS
A building’s design team takes great care in developing guidelines and details
that are intended to minimize the possibility of noise and vibration problems. If the
guidelines and details are not followed, not only is the design time wasted, but also
the problems that the details were intended to avoid can occur.
The items in the checklists below can be applied to site inspections of buildings
under construction or to troubleshoot situations where visual observation is the first
step in solving the problem. If the answer to any question is “No,” the possibility
of a noise or vibration problem exists.
Site Inspection Checklist for Fan and Air-Handling Unit Installations
1.

Is the installed equipment exactly as submitted and approved?

For unducted inlets, is the inlet at least 1 full fan wheel diameter from the nearest
obstruction?

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For air balance reports:

Construction Phase Tasks

For ducted inlets, is the nearest elbow, offset, or transition at least 3 equivalent
duct diameters from the unit inlet?

For unducted outlets, is the nearest airflow obstruction at least 1.5 fan wheel
diameters from the unit discharge?

For ducted outlets, does the outlet transition have an expansion angle of 15
degrees or less?

For ducted outlets, is the first elbow, offset, damper, backflow preventer, or duct
silencer at least 3 equivalent duct diameters downstream of the unit discharge
opening?

Does the electrical conduit serving the unit have a flexible loop to prevent vibration transmission via the conduit? Can the conduit be easily moved?

Is the duct system attached to the equipment with a flexible canvas connector?

Does the equipment float freely on its vibration isolators? It should rock freely
when pushed or jumped on.

11. Are there any unexpected elbows, offsets, transitions, etc.?
12. In systems with airflow of 900 L/s and higher, is all medium- and high-pressure
ductwork made of sheet metal (instead of fiberglass ductboard)?
13. Is the fan rotation correct?
Site Inspection Checklist for Terminal Units
1.

Is every unit exactly as submitted and approved?

Is every unit installed in accordance with the approved shop drawings and located
away from noise-sensitive areas?

Is every unit free of contact with nearby pipes, walls, ceilings, etc.?

Is the nearest upstream duct elbow or offset at least 3 equivalent duct diameters
from the unit inlet? See Figure 7-2 for a proper unit installation.

Is the duct leading to the box inlet made of metal, either aluminum or steel? Flexible inlet ducts permit inlet noise breakout into the ceiling plenum.

Is the low-pressure discharge duct made of sheet metal with a minimum 25 mm
thick internal acoustical liner?

In a system using the ceiling plenum for return air, is the nearest return air grille
at least 1.5 m from the nearest terminal unit?

In troubleshooting situations, is the complete ceiling in place and is the air system
balanced?

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10. Is the ductwork installed in accordance with the approved shop drawings and
SMACNA guidelines?

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 7-2

Properly installed dual-duct variable air volume unit. The unit is
installed high in the plenum cavity and the two inlet ducts are
straight for several duct diameters upstream of the unit. Airflow is
from right to left.

Is all ductwork free of visible oil-canning or other movement?

Is all ductwork free of contact with other trades’ work?

Do ducts that penetrate walls avoid contact with the wall? Are the gaps around the
ducts sealed with a silicone-based sealant?

Is the high/medium-pressure duct system free of steep-angle transitions or offsets
that may have been installed to avoid field interferences?

Do all high- or medium-velocity rectangular elbows have turning vanes? Check
for turning vanes by looking for the weld spots or sheet metal screws at the appropriate places on the elbow cheeks. The photo in Figure 7-3 shows the screws that
were used to install turning vanes inside a large elbow.

Check duct gauge labels and reinforcements against SMACNA requirements.

Site Inspection Checklist for Vibration Isolators
1.

Is all rotating equipment and its attached piping either mounted on or suspended
from vibration isolators?

Are the installed isolation systems exactly as submitted and approved? Check
isolator labels against the approved shop drawings.

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Site Inspection Checklist for Ductwork

Construction Phase Tasks

Figure 7-3

Check of large duct elbow verifying screw attachment of turning
vanes.

Are all vibration isolation hangers attached directly to the stiffest parts of the
structure above (i.e., beams)? They should not be installed in the middle of a floor
diaphragm or at the bottom or in the midspan of the hanger rod.

Are all floor-mounted vibration isolators installed directly over or very near stiff
structural elements, e.g., beams or beam intersections?

Are the frames of all vibration-isolated equipment free from distortion or
sagging?

Are all inertia bases fully filled with the specified amount of concrete?

Can all isolated equipment move freely on its isolators? Check this by rocking or
jumping on the equipment while it is operating. A reasonably healthy person can
rock a properly isolated 1750 kW chiller or cooling tower.

Is the area under an isolated frame/base free from tools or construction debris that
might “short-circuit” the isolation? See Figure 7-4 for an example where a small
piece of electrical conduit is “short-circuiting” an isolated equipment frame.

Are the isolators properly loaded to give the specified deflection? This can be
checked by comparing each isolator's loaded height with its free height (given on

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Figure 7-4

Conduit debris short-circuiting isolator effectiveness. The small
piece of flexible conduit forms a rigid contact path between the
equipment frame and the slab, allowing equipment vibration to
bypass the spring. Also note that the spring is overloaded and
should be replaced with one that can accommodate a heavier load.

its submittal sheet). Figure 7-5 shows an overloaded spring hanger. Figure 7-6
shows an overloaded spring floor mount.
10. Have the shipping shims been removed from the isolators and seismic/windloading restraints? Figure 7-7 shows a “short-circuited” isolation floor mount
whose shipping shims have not been removed.
11. Does every hanger rod freely penetrate the hole in the bottom of the hanger box?
Figure 7-8 shows an installation where the hanger rod is touching the hanger box,
thereby “short-circuiting” the spring isolator.
12. Are thrust restraints keeping the fans centered on their isolators while they are
operating?
13. Is the floor/roof free of feelable vibration? Check this in areas surrounding the
equipment.
14. Do the seismic or wind-loading restraints remain “out-of-contact” during normal
equipment operation?
Site Inspection Checklist for Central Plants Near Occupied Spaces
1.

Is all of the installed equipment exactly as submitted and approved?

Is all floor or roof-mounted equipment and piping supported on vibration
isolators?

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Construction Phase Tasks

Figure 7-5

Overloaded spring hanger.

Figure 7-6

Overloaded free-standing floor mount. The overloading allowed
the gussets welded to the side of the equipment frame to rest on
top of the isolator baseplate. The lower right corner of the gusset
was burned off to eliminate the gusset/baseplate contact. The
proper action would have been to replace the overloaded spring
with a stiffer one able to carry the heavy load.

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Figure 7-7

Short-circuited floor mount isolator whose shipping shims have
not been removed.

Figure 7-8

Faulty spring hanger installation with hanger rod touching the
hanger box. This allows pipe vibration to bypass the spring and
enter the building structure above.

139

Is all suspended equipment and piping hung from the slab above with vibration
isolation hangers that are attached directly to stiff portions of the structure above?

Is there any debris lodged beneath vibration isolation frames or concrete inertia
bases that might short-circuit the isolators? Check this by visual inspection and
by rocking or jumping on the equipment. Figure 7-4 shows a small piece of
conduit debris that is “short-circuiting” a floor-mounted spring isolator.

Does the electrical conduit serving the equipment have a flexible loop to prevent
vibration transmission via the conduit?

Are all pipe penetrations sleeved through walls and slabs to avoid contact with the
wall or slab?

Are the walls constructed according to the specifications?

Site Inspection Checklist for Cooling Towers,
Air-Cooled Chillers, and Air-Cooled Condensers
1.

Is all of the equipment installed exactly as submitted and approved?

Is the “noisy” side of the equipment aimed away from the most noise-sensitive
area?

For rooftop installations, is the unit mounted on high-deflection spring isolators
mounted on piers or grillage beams that are directly supported by column extensions or major roof beams? Mounting arrangements that transfer the equipment
load to the roof might encounter problems due to slab flexibility.

Does the equipment float freely on its isolators? Figure 7-9 shows an isolated
cooling tower whose taut electrical conduit forms a vibration transmission path
between the tower and the roof structure.

Is the equipment free from visible vibration?

Is all attached piping isolated from the building? Rigidly attached piping or riser
clamps resting directly on a slab can carry vibration from the equipment into the
building structure. Figure 7-10 shows three pipe risers whose clamp ears are resting directly on the floor slab. The noise and vibration sensitivity of the nearby
occupancies will determine whether the clamps should rest on neoprene pads or
spring floor mounts.

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Construction Phase Tasks

A Practical Guide to Noise and Vibration Control for HVAC Systems

Taut outdoor “flexible” conduit forms a vibration “short-circuit” at
cooling tower. The grillage under the cooling tower is resting on
spring isolators, but the short conduit between the disconnect and
the roof penetration transmits tower vibration into the roof.

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Figure 7-9

Figure 7-10 Pipe risers without vibration isolation. The pipe clamps transmit
pipe vibration to the slab.

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Vibration Complaints
It has been the author’s experience that the airside noise and vibration
complaints that are the most difficult to mitigate are those associated with excessive
duct system static pressure drops and airflow turbulence that cause the fan to operate in either stall or surge. The large majority of these problems can be avoided by
following the various guidelines given in this book, the chapter titled “Sound and
Vibration Control” in the most recent ASHRAE Handbook—Applications, and in
the most recent editions of the SMACNA publications titled HVAC Systems—Duct
Design and HVAC Duct Construction Standards—Metal and Flexible.
The most troublesome waterside noise and vibration problems are usually
associated with inadequate vibration isolation at equipment and piping and
untreated equipment room openings, e.g., either ventilation openings or pipe penetrations. Keep these potential problems in mind when investigating a noise or
vibration complaint.
GENERAL APPROACH
A. Describe the complaint as completely as possible.
1.

Is the complaint one of noise, vibration, or both?

Is the problem heard, felt, or seen?

What word(s) best describes the complaint—rumble, roar, hiss, ripples in a
cup of coffee, etc.?

Is the problem continuous or repetitive on a certain schedule?

Is the problem at its worst at any particular time of day or time of year?

Can the problem be associated with the operation of a specific piece of
equipment?

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The search for the answers to the above questions may lead not only to the
specific source of the problem but also to its solution without the assistance of an
acoustical professional. Whether or not an acoustical professional is retained, all of
the questions should be answered before beginning an investigation. Some of the
most frequent noise complaints are noted later in this chapter. Figure 8-1 gives
some of the complaint names and the frequency ranges usually associated with the
complaint.
B. Determine the source of the complaint.
This is usually the most difficult step because there are often many possible
sources, including some that are not even part of the HVAC system. An example
is rumble at the top floor of a building that could be caused by the HVAC system
but could also be due to ineffective vibration isolation at the elevators’ motor-generator sets or at least a half-dozen other reasons.
Determining the source is often done by turning off individual pieces of equipment and asking the complainant if the problem has gone away. This procedure
works well as long as each piece of equipment can be turned off individually.

Figure 8-1

Frequency ranges of the most likely sources of common acoustical
complaints.

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Unfortunately, this procedure sometimes cannot be used to find a problem associated with a chilled water pump, for example, because its associated chiller, which
could be source of the complaint, should also be turned off to prevent chiller
damage. If the noise/vibration problem disappears when both machines are turned
off, then a more detailed analysis by an acoustical professional may be required to
isolate the exact cause(s) and transmission path(s).
C. Investigate the source and the transmission path.
Find out what it is about the source, its appurtenances, its operation, or any
attached system components that might be responsible for the complaints. First,
look at as much of the system as possible. For an airside system, are the fans and
ductwork all installed according to the contract documents, the guidelines given in
this book, and the SMACNA publication titled “HVAC Systems—Duct Design”?
For a waterside system, are the required vibration isolators installed on all equipment and piping? If the source can operate at variable capacities, operate it at various setpoints to determine if the problem occurs in a particular operating range. An
example of this is where the part-load rotation rate of a variable-speed fan resonates
with one of a building’s structural resonances. The solution depends on the
frequency and strength of the resonance.
If there are no obvious visible causes for the complaint, a test and balance technician should be called in to verify that the equipment is operating properly. Field
investigations of airside noise complaints often find that the fan is operating in a
stall condition (i.e., toward the left side of the fan curve), which can cause very high
levels of low-frequency rumble. The problem is often caused by excessive static
pressure in the fan’s associated ductwork due to an installation that does not comply
with the ASHRAE and SMACNA guidelines.
Remember that a single source may be causing more than one problem; that
is, excessive noise or vibration may be transmitted from a source to a complaint area
by more than one path. The air-handling unit room described in the introduction of
this guide is a good example. Turbulent discharge airflow conditions, a noisy fan,
an undersized mechanical room, and ineffective vibration isolation could all be
causes of complaints. Solving only one of the problems may not be satisfactory.
Conversely, a problem may only occur when two different system components
are operating a particular way. For example, a built-up system using parallel supply
fans with slightly different rpm rates will often exhibit a surge or throb. The surge/
throb is caused by either turbulent airflow conditions through the fan system or the
unequal operation of the fans themselves. The throb/surge rate is proportional to the
rpm difference between the two fans.
D. Select the best retrofit.
After analyzing the problem, review the possible solutions for their probable
acoustical effectiveness and their impact on system operation. The effectiveness of

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a retrofit depends on many factors and is frequently limited by the space available
and the system's flexibility to accept a change. For example, adding a duct silencer
to solve a fan noise problem could worsen the problem if space constraints require
locating the silencer too close to the fan discharge. The turbulence caused by this
close placement could result in excessive silencer self-noise and excessive pressure
drop that could put the fan into a stall condition, causing an increase in the noise
level.
SPECIFIC COMPLAINTS
Characterizations of noise complaints are listed in bold; possible causes are
then listed with mitigation strategies noted below it in italics.
Buzz

Ineffective or missing vibration isolation at a high-speed machine, e.g., a chiller.
— Make sure the equipment floats freely on its isolators.
— Install isolation where necessary to permit equipment to float freely without
contacting the building or other building services.
Ineffective or missing vibration isolation at a variable-frequency motor speed
controller.
— Make sure that the controller and all attached conduits are isolated from the
building using neoprene or spring isolators and flexible conduits.
Click—Intermittent
Loose accessory or debris in ductwork.
— Visually inspect ductwork. Tighten loose connections or remove debris.
Ceiling wires vibrating against ductwork, metal grid, or light fixtures.
— Inspect ceiling hanger wires in the vicinity of the complaint and move them
as necessary to avoid contact with other metal objects.
Drumming: “It sounds like a drum roll or a machine gun”
Airborne reciprocating compressor noise transmitting through a lightweight wall.
— Make an airtight seal at all wall penetrations into the equipment room, and
— Build an airtight noise enclosure around the equipment, or
— Furr out an additional gypsum board assembly to “double-up” the existing
wall, or

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Misaligned fan—rotating wheel hitting against its housing.
— Check bearings, realign and balance fan.

A Practical Guide to Noise and Vibration Control for HVAC Systems

— Replace the existing gypsum board assembly with masonry.
(The acoustical performance of the floor slab and the effectiveness of the
vibration isolators sometimes limit the improvements.)
Airborne reciprocating compressor noise transmitting through a lightweight slab.
— Make an airtight seal at all slab penetrations into the equipment room, and
— Build an airtight enclosure around the equipment if the complaint is from the
floor above, or
— Install a vibration-isolated gypsum board ceiling assembly between the
equipment and the complaint area to create a “double-barrier” assembly.
Structure-borne vibration from a reciprocating compressor transmitted via ineffective or short-circuited isolators or via contact between piping/conduit and a
mechanical room wall/slab.
— Verify that the equipment floats freely on its vibration isolators, and
— Verify that all attached piping is isolated from the building via spring hangers, floor mounts, and sealed penetrations, and
— Verify that all attached electrical conduits are slack and flexible.
Hiss
Air leak in duct or AHU cabinet.
— Visually inspect duct and seal all leaks. If done safely, a lit candle can be
used to find an air leak.
Excessive airflow in VAV box or grille.
— Reduce airflow through the air distribution device, or
— Move grille damper upstream of flexible duct, or
— Replace with quieter equipment selection (e.g., usually a larger size to
reduce air velocity).
Sound leak through door undercut or return air grille.
— Verify proper door alignment in its frame, and
— Install frame-mounted door seals and automatic door bottom to ensure
airtight door closure.
— Attach lined boot over return air grille.
Air leak in pneumatic control system.
— Trace piping to leak and repair or replace faulty component.

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Airflow turbulence noise transmitted through thin, circular ductwork or pipe.
— Rarely a problem; an airtight enclosure or lagging spaced from 50 to
100 mm away from the pipe or duct usually helps. Install fiberglass batting
in the gap between the enclosure and the pipe/duct.
— Leak in steam piping system.
Hum
Structure-borne vibration due to ineffective or missing vibration isolation at equipment or piping.
— Verify that the equipment floats freely on isolators during operation.
— Verify that isolators are sufficiently deflected when loaded.
Structure-borne vibration due to rigid contact between pipe or conduit and the
building.
— Detach pipe or conduit from building and reattach with isolator or resilient
clamp.
Common non-HVAC source; structure-borne vibration due to ineffective or missing transformer vibration isolation.
— Insert neoprene or spring/neoprene isolators under transformer.
— Make all conduit connections with flexible conduits.
Structure-borne vibration from internally isolated equipment whose shipping
bolts, blocks, or shims have not yet been released.
— Release shipping bolts, blocks, or shims to allow the equipment to float
freely on its internal isolators.
Roar: “It's as noisy as when I'm driving on the freeway”
Excessive duct velocity and airflow turbulence.
— Reduce airflow velocity by increasing duct size or adding another duct to
share the airflow.
— Replace high-pressure-drop fitting with one having better aerodynamic
performance and lower pressure drop.
Excessive fan noise due to poor fan selection.
— Replace fan with a more efficient selection.
Excessive fan roar due to lack of duct silencer or duct liner.
— Replace a duct section with a duct silencer, or
— Replace an unlined duct section with lined ductwork.

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Lightweight ceiling near a mechanical room with a plenum return air opening.
— Install a duct silencer or thickly lined duct elbow (a lined boot) at the equipment room return air opening.
Lightweight ceiling under a high velocity duct.
— Replace duct with one of thicker duct gauge, or
— Laminate and strap gypsum board directly to duct walls, or
— Enclose duct in a gypsum board enclosure, or
— Replace ceiling with solid gypsum board, or
— Lay gypsum board panels on top of ceiling.
Medium- or high-velocity duct with closely spaced fittings.
— Reconfigure ductwork to allow 3 equivalent duct diameters between adjacent fittings.

Fan operating in rotating stall because of excessive duct system static pressure.
— This usually happens when a duct system presents an unexpectedly high
pressure drop and the fan rpm rate is increased to deliver the necessary
airflow without regard to fan pressure. The first step is to inspect all control
system components to be sure that no damper is improperly restricting
airflow. Then check to be sure that there is no debris in the ductwork that
seriously obstructs airflow. Then check to be sure that no system component
was field-modified in a manner that would increase its pressure drop. The
solution for any of these problems is to reconfigure the duct and control
systems for the lowest possible pressure drop. A variable volume fan will
then automatically reduce its speed for the lower pressure drop. A constant
volume fan will need to be manually adjusted for the lower system static
pressure.
Excessively turbulent inlet airflow conditions at fan.
— Move fan or obstruction to provide adequate inlet clearance.
Excessively turbulent discharge airflow condition at fan.
— Reconfigure discharge duct according to SMACNA recommendations.
Figures 8-2 and 8-3 show examples of extremely turbulent fan/duct
discharge airflow conditions that caused noise complaints.

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Rumble: “I can see the walls shaking and can almost hear/feel the air
vibrating around me”

Troubleshooting Noise and Vibration Complaints

Figure 8-2

Example of poor fan discharge duct design. The mitered elbow
with turning vanes and steep transition downstream of the fan
outlet cause fan instability and accompanying noise and rumble. A
better installation would have used a horizontal discharge fan with
fewer downstream fittings.

Excessive static pressure in duct system due to an improper air balance.
— Rebalance system by opening dampers to reduce system pressure, and
— Reduce fan rpm to match fan performance to the new system curve.

Oil-canning of fan housing.
— Check belt and sheave alignments,
— Add reinforcing stiffeners to housing panels.
Oil-canning ductwork caused by turbulent airflow.
— Reduce airflow velocity and/or reconfigure ductwork according to
SMACNA requirements for smoother airflow and lower pressure drop.

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Ceiling grid suspended from ductwork.
— Remove ceiling hangers from ductwork and reattach to building structure
using a different suspension method that does not touch the ductwork.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 8-3

Fan installation with poor discharge duct system aerodynamics.
The close-coupled discharge damper and reverse-direction elbow
create extremely high turbulence that creates rotating fan stall,
performance reduction, and rumble.

— Add duct reinforcing in accord with SMACNA guidelines—helps only if the
original installation was not installed or reinforced according to SMACNA
guidelines.
— Replace ductwork with that of thicker duct gauge or with spiral-wound
circular ductwork.
— Lag ductwork with additional reinforcements and gypsum board between
the reinforcements.
Excessive fan vibration due to imbalance.
— Balance fan.
Excessive fan vibration because of loose adjustable sheave.
— Replace the adjustable sheave with a fixed one of the proper pitch.

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Excessive fan vibration because of worn shaft or bearings or misaligned shaft.
— Realign or replace shaft and bearings as needed.
Fan surging because of loose belts.
— Tighten or replace belts.
Duct or pipe touching wall or ceiling assembly.
— Break contact between the pipe/duct and the wall/ceiling assembly.
Fan, cooling tower, pump, or piping not properly vibration-isolated from the building.
— Install vibration isolation mounts or hangers at all equipment attachments
to the building. Install vibration isolators at all connected piping, electrical
conduits.
— Verify that ducts are attached with flexible duct connectors.
Common non-HVAC source, unisolated elevator motor-generator sets.
— Install vibration isolation mounts under the motor-generator sets.
— Verify that the attached electrical conduit is flexible and not rigidly attached
to the building.
Squeal
Belt slippage during fan start-up.
— This is normal for high kW, constant-speed fans.
— Convert to a variable-speed drive or a “soft start” motor starter.
Belt slippage during normal operation.
— Tighten belts, check sheave alignment.
Surge: “The roar or hum comes and goes about once every second or so”
Fan drive instability due to fan belt mismatch.
—Replace with a matched set of new belts and realign the sheaves.
Fan instability due to poor airflow conditions at inlet
— Improve airflow conditions by either increasing clearance to the fan inlet or
by installing a velocity-smoothing grid at the fan inlet.
Fan instability due to poor airflow conditions at discharge
— Improve discharge airflow conditions by reconfiguring the discharge ductwork to move all duct fittings and appurtenances at least 3 equivalent duct
diameters downstream of the fan discharge opening.

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Fan instability due to operation at the left side of the fan curve.
— Reconfigure ductwork and/or dampers to reduce system static pressure
drop, and
— Reduce fan rpm to move operation to the right side of the curve, or
— Replace fan with one that can accommodate the higher static pressure.
Pump instability because of worn rotor bearings.
— Replace rotor bearings.
Parallel fans operating at slightly different rpm rates.
— Usually occurs with paired belt-drive fans.
— Convert to direct-drive fans on a single or master-slave VFD arrangement.
Chiller control system misadjusted.
— Readjust control system to load the compressor properly.
Tap
See Click
Throb
See Surge or Rumble.
Whine
Blade passage tone of vane-axial fan, inadequate noise control at or near fan.
— Verify proper fan selection for the operating setpoint.
— Verify smooth airflow conditions at fan inlet and outlet.
— Verify the presence of close-coupled duct silencers on one or both sides of
the fan.
Blade passage tone of vane-axial fan, inadequate vibration isolation at fan.
— Verify that the fan floats freely on its vibration isolators.
— Verify the presence of neoprene pads at the base of the spring isolators.
— Verify that the duct(s) is (are) connected to the fan via flexible connectors.
— Verify the presence of flexible conduit connections to the motor.
Pump impeller blade tone, inadequate vibration isolation at pump or piping.
— Make sure the pump floats freely on its vibration isolators and the piping is
not rigidly connected to any part of the building.

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Airborne “pumping tone” from an indoor, water-cooled screw (rotary) chiller
— Verify that the problem is not due to structure-borne vibration caused by
rigid attachment of chiller, its piping, or its conduits to building.
— If the problem is verified as airborne, enclose the entire chiller in an airtight
enclosure (enclosing only the compressor is usually ineffective because
there is typically significant radiation from the oil separator and heat
exchangers), or
— Install a vibration-isolated gypsum board ceiling assembly between the
equipment and the complaint area to create a “double-barrier” assembly.
— Adding 50 mm thick sound-absorbing panels to the equipment room ceiling
and walls will be effective if only a small amount of noise reduction (about
3 to 5 dBA) is needed.
Structure-borne “pumping tone” from a water-cooled, indoor screw (rotary)
chiller.
— Verify that the chiller is mounted on high-deflection spring isolators that
have neoprene inserts under the springs; in extreme cases, pneumatic isolators are needed.
— Verify that all piping and conduits are resiliently attached to the chiller.
— Verify that all piping is attached to the building using spring hangers that
have neoprene inserts.
— Verify that all isolators are properly adjusted to preclude “short-circuiting.”
Figure 8-4 shows a “short-circuited” restrained spring mount.
Airborne “pumping tone” from an outdoor, air-cooled screw (rotary) chiller.
— Install an airtight enclosure around the compressors and oil separator (and
the heat exchangers, if feasible).
— Install a barrier to block the line-of-sight between the chiller and the
complaint area (verify that the barrier does not restrict airflow to the
chiller’s condenser airflow).
Whistle
Water valve instability.
— Inspect valve to be sure the control mechanism is functioning properly.
Excessive airflow through an air outlet or inlet.
— Reduce airflow through the device.

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A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 8-4

Restrained spring isolator with a “short circuit” between its
baseplate and equipment mounting plate. This type of isolator has
become obsolete and is not recommended for any type of HVAC
equipment.

Uneven airflow through a grille or diffuser.
— Adjust the upstream ductwork to straighten the airflow into the device’s inlet
collar.
Damper or extractor too close to grille or diffuser inlet.
— Remove damper or extractor or move it upstream at least 3 duct widths from
the device’s inlet collar.
Raw sheet metal edge exposed to airflow.
— Trim edge to be as short as possible or turn edge to be blunt.
SITE INSPECTION PHOTOGRAPHS
Figures 8-5 through 8-23 show proper and improper installations of various
HVAC system components. In many cases, direct comparisons are made to highlight the most common problems in the areas of vibration isolation, ductwork, and
equipment installation.

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Troubleshooting Noise and Vibration Complaints

Figure 8-5

View into the fan section of a rooftop unit; the tight cabinet
clearance and airflow obstructions at the fan inlet cause excessive
turbulence, pressure drop, and noise.

Figure 8-6

Excellent rooftop package unit installation; the unit is resting on
spring isolators and an elevated frame, thereby avoiding contact
with the roof.

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Figure 8-7

Improperly installed fan-powered variable-air-volume unit. The
unit is resting on the ceiling grid. This allows unit vibration to
transmit directly into the partition below. The partition then acts
as a sounding board, radiating noise into the occupied space.

Figure 8-8

Proper installation of an indoor self-contained packaged HVAC
unit. Note the dual radius supply duct split on top of the unit; the
return air sound traps to the left of the supply duct, the black
plenum liner on the mechanical room wall, and the spring isolator
under the unit.

157

Figure 8-9

Closely spaced circular duct fittings produce turbulence and noise.
This duct run does not follow the guideline recommending 3 duct
diameters between adjacent fittings. It has five fittings within 8 m.

Figure 8-10 Closely spaced rectangular duct fittings produce turbulence and
noise. A better offset around the pipes could have been made with
a duct section on a 45° slope between the vertical and horizontal
sections.

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Troubleshooting Noise and Vibration Complaints

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Figure 8-11 Improper duct transition at fan inlet. The high-velocity airflow in
the short, abrupt transition between the fan outlet and the vertical
duct causes turbulence and rumble.

Figure 8-12 Vane-axial fan intakes too close to wall. Airflow through the
restricted area between the fans and the wall causes turbulence,
surging, and rumble.

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Troubleshooting Noise and Vibration Complaints

Figure 8-13 Duct split using radius elbows. This design is preferred because it
produces less pressure drop and noise than a bullhead tee.

Figure 8-14 Faulty installation of a large equipment isolator with stanchion
restraints. The spring top plate is touching the near stanchion and
the equipment mounting plate is resting on the stanchions. Both
conditions allow equipment vibrations to bypass the spring and
enter the building structure through the isolator baseplate.

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Figure 8-15 Proper installation of a large equipment isolator with stanchion
restraints. Note that the springs are centered between the
stanchions and are properly compressed and that the equipment
mounting plate is floating about 6 mm above the stanchion top
plates.

Figure 8-16 Braided metal pump connectors do not provide significant
vibration isolation.

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Troubleshooting Noise and Vibration Complaints

Figure 8-17 Neoprene pump connectors provide better isolation of pump
vibration from attached piping.

Figure 8-18 Incomplete vibration isolation at cooling tower. Even though the
tower is isolated, the condenser water pipe supports are mounted
rigidly to the roof. This allows some tower vibration to enter the
roof structure.

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Figure 8-19 Taut “flexible” conduit forms a vibration short-circuit at vane-axial
fan. Spring isolators under the vane-axial fan perform as expected,
but the tight conduit arc transmits fan vibration into the
disconnect, which is rigidly supported on the floor. A longer, more
slack conduit would provide better isolation.

Figure 8-20 Correctly installed flexible conduit between electrical disconnect
and motor. The flexible installation reduces fan and motor
vibration in the floor slab.

163

Figure 8-21 View of pipe penetration from below roof. The pipe on the right
is contacting the left edge of its penetration. This installation
allowed pipe vibration to excite the roof slab, which radiated the
energy as noise.

Figure 8-22 Non-isolated pipe penetration. Rigid contact allows pipe vibration
to enter the wall, which radiates the energy as noise.

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A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure 8-23 Improperly placed neoprene hanger. The drywall assembly forms
a vibration transmission bridge across the isolator. This type of
interference can be identified and corrected in pre-construction
coordination meetings.

165

Appendix A

Some Basics of
HVAC Acoustics
Sound and vibration are unavoidable features of all HVAC systems. Turbulent
airflow in fans, ductwork, and plenums generates sound that can intrude into occupied spaces. Waterflow through pumps, central plant equipment, and piping
systems also generate noise that may require control. Additionally, the operation
of rotating and reciprocating equipment, such as fans, pumps, and chillers, creates
noise and vibration that can be transmitted throughout building structures and
annoy building occupants or be emitted into the environment and cause noise
complaints at nearby properties.
AIRBORNE AND STRUCTUREBORNE SOUND
HVAC system noise reaches building occupants by airborne and structureborne sound transmission paths. The principles of the two types of sound transmission are shown in Figure A-1. When the hammer in the figure hits the bell,
airborne sound transmission carries the ringing sound throughout the room and
through the walls to adjacent rooms. The transmission is considered to be airborne
because the source does not touch the building. Conversely, the impact of the
hammer on a nail that is embedded in a wall causes structureborne transmission
that carries the impulse throughout the entire building structure, so that even occupants in rooms on other floors can hear it. The contact of the source to the building
makes this a structureborne sound transmission path.
Figure A-2 shows how airborne and structureborne transmission carry sound
and vibration from a piece of equipment throughout a building. Airborne transmission carries sound directly from the equipment to the surrounding rooms. Even
though the equipment in the figure is shown to be on vibration isolators, it still
transmits vibration energy to the building structure. The energy enters the walls

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Some Basics of HVAC Acoustics

and ceilings, which radiate it as sound throughout the building. The isolators
reduce the structureborne energy to acceptable levels, but they do not eliminate it
completely.
Good acoustical design addresses the potential for both airborne and structureborne transmission through the effective use of space planning, structural
design, equipment selection, duct system design, and “add-on” noise and vibration
control products and systems, where necessary.

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Figure A-1 Airborne and structure-borne sound transmission.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure A-2 Airborne and
equipment.

structure-borne

sound

transmission

from

BASIC TERMINOLOGY
The acoustical terms most frequently encountered in HVAC system design are
decibels, frequency, octave bands, sound pressure level, and sound power level.
These terms are explained below. Definitions of other acoustical terms are given
in the glossary in Appendix E.
Decibel
The decibel (dB) is the basic unit for acoustical calculations and measurements. It applies to sound pressure levels (LP), sound power levels (LW), insertion
loss (IL), transmission loss (TL), and other acoustical properties of HVAC equipment and construction materials.

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Because the decibel scale is logarithmic, the combined sound level of two or
more sound sources is not found by arithmetically adding their individual sound
levels. For example, two identical sources producing 50 dB each generate a total
of 53 dB, not 100 dB. Conversely, if two identical sources produce a combined
sound level of 80 dB, shutting one off drops the level to 77 dB, not 40 dB. The chart
in Figure A-3 can be used for adding decibels. For the addition of more than two
noise sources, first perform the calculation for the two highest LP values, then
perform the calculation again using the previous sum plus the next highest LP value,
etc. This calculation procedure applies to either LP values or LW values.
Frequency
What we hear as sound is the back-and-forth oscillatory motion of the air molecules next to our eardrums. This oscillatory disturbance of the air around us is
caused by a corresponding oscillatory motion of the air molecules near one or more
sound sources. Each back-and-forth motion is called a cycle. The frequency of the
sound is the number of times the air molecules vibrate back and forth each second
and is expressed in terms of cycles per second (cps) or hertz (Hz).

Figure A-3 Chart for adding decibel values.

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Audible sound occurs in the frequency spectrum that spans from about 20 to
20,000 Hz. Figure A-4 shows the approximate frequency ranges of some everyday
sounds.
Octave Bands
For convenience, the frequency spectrum is divided into octave bands, each of
which is a continuous range of frequencies identified by its logarithmic center
frequency. The highest frequency of each octave band is twice its lowest frequency.
Figure A-5 shows the center frequencies of the octave bands that are most important
in HVAC acoustics, as well as the octave band distribution of noise from various kinds
of HVAC equipment. Acoustical analyses at narrower frequency ranges (1/3 octave
wide or even narrower) are frequently used for troubleshooting noise complaints and
product development research.
Table A-1 shows the lower and upper frequency limits for each of the eight
octave bands that are typically encountered in HVAC noise analyses.
Sound Pressure Level
Sound pressure level, abbreviated LP or SPL, is expressed in dB and is the
most widely used acoustical descriptor. It indicates the level of sound at a specific
location at a specific time for a specific set of conditions. Each octave band (or

Figure A-4 Everyday sound sources—their frequencies and wavelengths.

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Figure A-5 Frequencies at which various types of HVAC equipment generally
control their sound spectra.

Table A-1. Octave Band Center Frequencies
and Their Frequency Ranges
Octave Band
Center Frequency
In Hertz

Lower
Frequency Limit
in Hertz

Upper
Frequency Limit
in Hertz

125

177

250

177

354

500

354

707

1000

707

1414

2000

1414

2828

4000

2828

5656

8000

5656

11,312

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1/3-octave band) of a sound spectrum can have its own LP value. For most HVAC
installations, the LP can range from less than 10 dB in a very quiet recording
studio or concert hall to as much as 125 dB in a very noisy fan or chiller room.
Figure A-6 shows the approximate sound pressure levels for common everyday
activities.
Each person hears changes in sound level differently, but the average perceptions of sound level changes are fairly well understood and are summarized in
Table A-2. Note that the 3 dB drop that results from shutting down one of two iden-

Figure A-6 Sound pressure levels of some everyday activities.

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Some Basics of HVAC Acoustics

Table A-2.

Subjective Impressions of Sound Level Differences

Subjective Impression

63 Hz
Octave Band

125 Hz Octave
Band

250+ Hz
Octave Bands

Half as Loud

–6 dB

–8 dB

–10 dB

Significantly Quieter

–3 dB

–4 dB

–5 dB

Just Barely Quieter

–1 dB

–1.5 dB

–2 dB

Just Barely Louder

+1 dB

+1.5 dB

+2 dB

Significantly Louder

+3 dB

+4 dB

+5 dB

Twice as Loud

+6 dB

+8 dB

+10 dB

tical sound sources yields a “just perceptible” reduction in loudness. Try this using
two radios tuned to the same station at the same volume level. Notice that the loudness after turning off one of the radios is only a little quieter than before, not half
as loud.
Sound Power Level
Sound power level, abbreviated LW, PWL, or SWL, also expressed in dB, is
used to indicate the amount of sound that a source produces. The sound power
levels that are generated by an HVAC system component depend on its operating
conditions; in general, operation at higher motor kW rating for a given piece of
equipment will usually be louder than at a lower kW rating. Sound power levels are
used as the starting points for most acoustical calculations. The effects of the equipment’s location and the attenuation due to any nearby architectural elements and
HVAC system components are accounted for in the calculation process.
The LW values for each type of HVAC component are determined in specially
designed acoustical laboratories through the use of nationally recognized test standards. LW values cannot be measured directly but are calculated from LP or sound
intensity measurements. Each octave band or 1/3-octave band of a sound spectrum
can have its own LW value.
Because LP and LW are both expressed in dB and describe levels of sound, they
are often confused with each other. In fact, LP and LW are easy to understand when
compared with joules (J) and the temperature in degrees. Just as the number of

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joules can be used to indicate the amount of heat released by a piece of equipment,
the LW value indicates the amount of sound it radiates. The temperature in degrees
at some location away from a heat source depends on the distance from the source
and the environment’s thermal characteristics. Similarly, the LP value at a given
listener’s location away from a sound source depends on the distance from the
source and the acoustical characteristics of the environment between the source and
the listener.
Acoustical professionals and other trained engineers use equipment LW values
to calculate the expected LP values in a finished installation. The calculations are
typically done in octave bands, although A-weighted calculations (see
Appendix B, “Acoustical Rating Systems and Criteria”) are sufficient in some
instances. The chapter titled “Sound and Vibration Control” in the quadrennial
ASHRAE Handbook—Applications (2007, 2011, etc.) is one of many references
that explain the calculation process.
CHARACTERISTICS OF HVAC NOISE
The noise of HVAC system components can be characterized as either broadband or tonal. Broadband noise contains sound energy of similar strength over a
wide range of contiguous frequencies. Rain and wind create broadband noise.
HVAC system examples are airflow through ductwork and diffusers and waterflow
through pipes.
Tonal noise contains sound energy at discrete frequencies and is usually more
annoying than broadband noise of equal sound level.
Many kinds of HVAC equipment produce both broadband and tonal noise. For
instance, fans with fewer than 20 blades typically generate strong tones at the blade
passage frequency along with broadband noise caused by the turbulent airflow
through the fan. Likewise, a screw chiller’s compressor creates very strong “pumping tones” at frequencies that depend on the rotation rate of its screws, while the
turbulent flow of water and refrigerant through the chiller package piping create
broadband noise. In many cases, equipment that produces strong tones also
produces significant vibration at the tonal frequencies, so careful attention to both
noise and vibration control are needed.

175

Appendix B

Acoustical Rating
Systems and Criteria
Because the human response to noise depends on so many factors (e.g., level,
frequency content, modulation rates, etc.), it is difficult to describe an acceptable
noise environment. Several single-number rating methods, each with its own set of
assumptions and limiting conditions, have been developed to help establish acceptable sound exposure criteria in and around buildings, but no single method has been
universally accepted. The methods that are most frequently used to rate HVAC
system noise are the A-weighted sound level (dBA), C-weighted sound level
(dBC), loudness level (sones), Room Criteria Mark II (RC Mk II), Noise Criteria
(NC), and Balanced Noise Criteria (NCB) rating systems. A brief description of
each rating system is given below.
A-WEIGHTED SOUND LEVEL (dBA)
The A-weighted sound level (LPA), expressed in dBA, is often used to rate
HVAC noise where either hearing conservation or outdoor noise exposure is of
concern. The dBA method has gained popularity because an acoustical environment’s A-weighted sound level can be used to roughly assess its subjective loudness and because it can be easily measured with an inexpensive, handheld sound
level meter. (More information on sound level meters is given in Appendix C.) A
sound level meter’s A-weighting electrical circuit filters the measured noise in
accordance with the lowest curve shown in Figure B-1. The effect of the curve is
to filter out most of the sound’s low-frequency content in a manner that is similar
to the filtering that occurs in the human ear.
Measuring the A-weighted sound level is rarely valuable when investigating
a noise complaint because it provides no information on the frequency content of
the noise. The A-weighted sound level is most useful when comparing the relative
loudness of one acoustical environment with another similar environment. For

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Acoustical Rating Systems and Criteria

Figure B-1 Frequency weighting curves. Effects of linear, A-weighting, and Cweighting filters.

example, in the case of diffuser noise, measuring the A-weighted sound level at
various airflow quantities could help determine if lowering the air quantity to the
diffuser would be an effective noise reduction treatment.
C-WEIGHTED SOUND LEVEL (dBC)
The turbulence caused by wind blowing over a sound level meter’s microphone
can create very high levels of self-generated noise at low frequencies. If the meter
is set to its “linear” (unfiltered) mode, then the unfiltered sound energy at all
frequencies, including the turbulence-generated self-noise, will be included in the
LP value that is displayed by the meter. The dashed line in Figure B-1 shows the

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“linear” filter response. The C-weighting filter was designed to filter out most of
the turbulence-generated self-noise with little or no filtering effect at the frequencies of sound that are typically produced by HVAC equipment.
The C-weighted sound level (LPC), expressed in dBC, is often used in conjunction with a microphone windscreen to measure unweighted sound levels outdoors
in the presence of low-to-moderate winds. In the absence of octave band data,
comparing an acoustical environment’s LPA and LPC values can provide a rough
estimate of frequency balance but usually will not provide enough information to
develop noise reduction treatments when troubleshooting a noise complaint.
LOUDNESS LEVEL (SONES)
The sone rating method was developed to give system designers a linear scale
for rating the approximate loudness of non-ducted fans, such as the small cabinet
fans used in residential bathrooms, kitchens, and utility rooms, and for propeller
panel fans used in general ventilation applications. Unlike the various decibelbased ratings, for which a doubling of the loudness requires an approximate 10 dB
increase in the sound level, the sone method is linear, such that a doubling of the
sone rating is approximately a doubling of the loudness. For example, a rating of
1 sone is the approximate loudness of sound environment with an A-weighted
sound level in the range of 35 to 40 dBA; a 2 sone rating would apply to an environment where the sound level is in the 45 to 50 dBA range, and a 4 sone rating
would correlate with a sound level in the 55 to 60 dBA range.
Unfortunately, there are two different methods for calculating a sone rating,
and they always result in different values. Therefore, since a sone rating is a single
number that does not provide any information on the frequency content of the sound
spectrum, and since there is no universally accepted method for calculating a sone
rating, its usefulness should be limited to comparing the approximate loudness of
fans of the same type from the same manufacturer.
ROOM CRITERIA (RC) AND RC MARK II
The Room Criteria (RC) rating method and its successor method, RC Mark II,
were intended to help rate the acoustical acceptability of measured octave band LP
spectra in finished buildings. They were never intended for rating calculated LP
values obtained during the system design phase; nonetheless, they are sometimes
used erroneously for this purpose. Both of the RC methods compare a set of fieldmeasured, octave band LP values with a set of reference curves. Figure B-2 shows
the original RC reference curves, and Figure B-3 shows the RC Mark II reference
curves.
Both of the RC rating methods have two factors: (1) a numerical value that
rates the ease of voice communication in the measured acoustical environment and

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Acoustical Rating Systems and Criteria

Figure B-2 Blank RC chart.

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Figure B-3 Blank RC mark II chart.

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Acoustical Rating Systems and Criteria

(2) an alphabetical suffix that indicates the subjective quality of the noise. The RC
Mark II method also includes a quality assessment index (QAI) value that indicates
the relative balance between the measured spectrum’s low, middle, and high
frequency octave band sound levels. Details on the use of both methods are given
in Chapter 47 of the 2003 ASHRAE Handbook—Applications. Proponents of the
RC-based methods recommended using only the RC Mark II method as a diagnostic tool when investigating noise complaints.

The NC rating method was the first widely used method for rating octave band
LP spectra. Like the RC method, it can be used to compare field-measured octave
band LP values with a set of reference curves (see Figure B-4). However, since it
is also a design tool, the NC method can also be used to assess the results of design
phase acoustical calculations, for example, to determine whether or not a system
needs a quieter fan or an “add-on” noise reduction treatment, such as a duct silencer
or acoustical louver.
The NC rating of an octave band LP spectrum is determined by finding the
highest reference curve that is tangent to the octave band spectrum. Figure B-5
shows an example of a spectrum that is rated at NC-45 because its 125 Hz sound
level is tangent to the NC-45 curve and no other octave band level is tangent to a
higher NC reference curve. Because NC ratings are defined only on five-point
increments, there is no precisely defined rating such as NC-42. Spectra that fall
between NC reference curves are described by their proximity to the nearest curve.
The NC rating method is still the most popular rating method; however, it
should be used with caution when rating field-measured LP values because the NC
reference curves do not extend to the 16 and 31 Hz octave bands, regions where
some of the most troublesome HVAC noise and vibration control problems occur.
The NCB method, a successor to the NC method, attempts to resolve this shortcoming by defining reference values in the 16 and 31 Hz octave bands while also
adjusting the slopes of the reference curves at the higher frequency bands (see
Figure B-6). Like RC Mark II, the complete NCB method rates spectral balance.
A complete description of the use of both methods can be found in ANSI Standard
S12.2-1995, Standard Criteria for Evaluating Room Noise.
WHICH RATING METHOD IS BEST?
None of the octave band rating methods has received unequivocal acceptance
by the entire acoustical consulting community because none of the methods has all
of the attributes necessary for properly rating the wide range of HVAC noise environments; a very complicated method based on 1/3-octave LP values would be

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NOISE CRITERIA (NC) AND
BALANCED NOISE CRITERIA (NCB)

A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure B-4 Blank NC chart.

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Acoustical Rating Systems and Criteria

Figure B-5 Octave band spectrum rated at NC-45.

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Figure B-6 Blank NCB chart.

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Acoustical Rating Systems and Criteria

required. Since virtually all HVAC equipment noise data are published in octave
bands, ASHRAE Technical Committee TC 2.6 is currently attempting to combine
the best attributes of all four octave band methods to develop a single rating method
that is optimal for the majority of design and commissioning uses. Updated criteria
recommendations will appear in either the “Sound and Vibration Fundamentals”
chapter of the latest quadrennial ASHRAE Handbook—Fundamentals (2005,
2009, etc.) or the “Sound and Vibration Control” chapter of the latest quadrennial
ASHRAE Handbook—Applications (2007, 2011, etc.).
Since the RC and RC Mark II methods were developed specifically for diagnosing noise complaints, they should not be used to establish design phase criteria
or for commissioning purposes. The NC and NCB methods were intended to assess
system designs, and it is the author’s opinion that the NCB system is more relevant
if the 16 and 31 hertz octave band levels are ignored; acoustical performance data
for HVAC system components are not available in these two octave bands, so calculations in these bands are irrelevant. The author, therefore, recommends using the
NCB reference curves in Figure B-6 (ignoring the 16 and 31 hertz octave bands)
as maximum limit curves for assessing system designs. In other words, if NCB-35
is the chosen criterion for a given room, none of the octave band LP values from 63
to 8000 hertz may exceed the NCB-35 curve.
ACCEPTABILITY CRITERIA
Indoor Sound Criteria
In general, acceptable HVAC system noise has the following five qualities:
•
•
•
•
•

It is not too loud.
Its level does not fluctuate significantly over short time periods.
It does not have audible tones.
It is spectrally balanced (i.e., not too much rumble, roar, or hiss).
It is not too quiet.

The first three qualities apply to HVAC system noise in virtually all spaces.
The last two qualities apply in rooms where sound quality is important and where
the HVAC system noise provides masking to reduce the distracting effects of
intruding sounds generated outside of the room.
Table B-1 gives ranges of recommended numerical design goals for various
building occupancies. The design goals given in the table are the result of a consensus and, in some instances, may be considered conservative. Depending on
economic considerations, they may be increased slightly (up to five points) where
all five of the qualities listed above exist.

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Table B-1.

Recommended Indoor Sound Criteria
Recommended Design Goal
NC, RC or NCB
(See Notes a & b below)

Type of Area
Single & multi-family residences

25 to 35

Hotels/Motels
Guest rooms & suites
Meeting/banquet rooms
Hallways, corridors, lobbies, service & support areas

25 to 35
25 to 35
35 to 45

Offices
Private offices & conference rooms
Rooms with frequent teleconferencing
Open-plan office areas
Corridors, hallways & other public areas

25 to 35
20 to 25
35 to 40
35 to 45

Hospitals & clinics
Private rooms, Exam rooms
Semi-private rooms, wards & waiting rooms
Operating rooms
Corridors, hallways & waiting rooms

25 to 35
35 to 40
25 to 35
35 to 45

Performing Arts Spaces
Recording studios & sound stages
Concert & recital halls
Drama theaters
Music teaching rooms
Solo & ensemble practice rooms

(See Note c)
(See Note c)
20 to 25
25 to 30
30 to 35

Sanctuaries, Temples & Mosques (See Note c)

25 to 35

Movie theaters

30 to 35

Schools
Classrooms (See Note d)
Lecture halls with speech amplification
Lecture halls without speech amplification

25 to 30
30 to 35
25

Libraries

35 to 40

Courtrooms
Unamplified speech
Amplified speech

25 to 35
30 to 35

Recording studios, sound stages and associated control rooms
Arenas & Gymnasiums
Gyms & natatoriums
Large arenas with amplified speech

(See Note c)
40 to 50
45 to 55

Values and ranges are based on judgment and experience, not on qualitative evaluation of human reactions.
They represent general limits of acceptability for typical building occupancies. Higher or lower values may
be appropriate and should be based on a careful analysis of economics, space usage and user needs.
b
When spectral balance is important specify criteria in terms of RC-XX(N). If spectral balance is not as important as sound level specify criteria in terms of NC or NCB.
c
Retain an experienced acoustical consultant for guidance with spaces that have critical acoustical needs.
d
The recommended design goal conforms with the 35 dBA & 55 dBC limits specified in ANSI Standard
S12.60-2002.

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Acoustical Rating Systems and Criteria

At industrial sites or buildings where hearing conservation is more important
than acoustical quality, the A-weighted sound level rating is usually adequate.
Table B-2 gives the Occupationa1 Safety and Health Administration (OSHA)
limits that require action by employers (such as periodic hearing tests and/or other
hearing protection strategies) for a range of daily noise exposures.
Outdoor Sound Criteria
Outdoor LP limits are usually established by municipal codes in terms of maximum acceptable LPA values, although several municipalities now also mandate
octave band limits. Some municipal codes specify the limits in terms of “ambient
plus 5 dBA,” whereas specific LPA limits are specified in other codes. Table B-3
gives examples of municipal code LPA limits for various land uses.
Speech Communication in a Noisy Environment
In some situations the quality of speech communication in an indoor or
outdoor environment may be critical, for example, where an emergency notification system must be audible or where reliable speech communication between two
people is necessary. The audibility of such speech can be estimated using the chart
in Figure B-7, where the distance between speakers, the voice level, and the background sound level in dBA are known.
Industrial Noise Levels Requiring Employer Action

Average Daily
Exposure Time

Average Sound Level above Which
Employer Action Is Required

8 hours

85 dBA

4 hours

90 dBA

2 hours

95 dBA

1 hour

100 dBA

30 minutes

105 dBA

15 minutes

110 dBA

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Table B-2.

A Practical Guide to Noise and Vibration Control for HVAC Systems

Table B-3.

Sample Municipal Code Limits*
Maximum Sound Level in dBA

Property
Zoning

Day
(7 a.m.-10 p.m.)

Night
(10 p.m.-7 a.m.)

Single Family Residential

Multi-Family Residential

Commercial

Industrial

* Note: Some municipal codes mandate the limits at property lines, while others place the limit at any habitable location on the property. Some municipalities use different time periods for day and night.

Figure B-7 Quality of speech communication in background noise.

189

Appendix C

Measuring HVAC
System Noise
Interested readers with access to sound-measuring equipment may want to
make their own sound level measurements to either rate an HVAC noise environment using one of the rating systems described in Appendix B or to investigate an
HVAC system noise complaint. In either case, meaningful results require selecting
the proper instrumentation and following the proper measurement procedure.
INSTRUMENTATION
The great majority of HVAC sound level measurements are made in the field
using a battery-operated, handheld sound level meter such as that shown in Figure
C-1. The meter uses a microphone, preamplifier, internal weighting filters, and a
visual readout to measure and display the LP values. The black ball on top of the
meter is a low-density foam windscreen that is specifically designed for use with
the meter. Sound level meters with octave band or 1/3-octave band filters are called
spectrum analyzers, and those that are able to measure all of the frequency bands
simultaneously are called realtime spectrum analyzers. The meter in Figure C-1 is
a real-time spectrum analyzer; the vertical bars in the display window represent the
Lp values for the various octave bands.
The most useful spectrum analyzers have high (Type 1) or moderate (Type 2)
precision microphone; linear, A-weighting, and C-weighting filter networks;
octave band filters with center frequencies from 31 to 8000 Hz; and an LCD
display. Sound level meters without octave band filters are much less useful
because they cannot be used to determine the NC, NCB, or RC rating in a room,
and, in a troubleshooting situation, they provide very little information on the
frequency content of the offending noise; knowing the frequency of an offending
noise is required to determine the most cost-effective noise mitigation.

191

Measuring HVAC System Noise

Figure C-1 Sound level meter with a foam windscreen protecting its
microphone.
MEASUREMENT PLAN
Meaningful sound level measurements require a comprehensive measurement
plan, such as the one shown in Figure C-2. Troubleshooting a noise complaint
almost always requires measuring the LP values in the octave bands from 31 to 4000
Hz. Frequently, the LP values for the 1/3-octave bands from 16 to 4000 Hz are
needed when the complaint is related to a strong tone.
A spectrum analyzer’s readout will often show a fluctuating value, especially
in the lower octave bands. This is normal because low-frequency sound levels are
rarely constant. If the displayed value fluctuates over a range of less than 4 dB,
record its “eyeball” average. If the fluctuation is 5 dB or more, record the highest
and lowest displayed values. Modern spectrum analyzers are able to store the maximum, minimum, and average LP values in each frequency band over a selected
measurement period.
Figure C-3 shows a sound measurement data sheet that can be used to record
the results of an octave band sound measurement. An example of a completed sheet
is shown in Figure C-4.

192

A Practical Guide to Noise and Vibration Control for HVAC Systems

1. Before arriving at the measurement site, verify the battery life and calibration of the sound
level meter.
2. Before making any measurements at the site, walk slowly through the measurement area and
listen for sound level or sound quality variations. This will help to choose the best measurement location(s). Fill out a Sound Measurement Data Sheet (Figure C-4) with the details of the
measurement.
3. If the measurement is made in a windy area or in a room with airflow, a specially-designed
open-cell foam windscreen should be fitted over the microphone. Even with a windscreen, LP
measurements in the presence of a 9+ m/s breeze may be disrupted by excessive airflow. The
windscreen should be supplied by the sound level meter manufacturer specifically for use with
the meter. Even in areas with low wind it’s a good idea to always use a windscreen to protect
the delicate microphone from possible damage; the small amount of high frequency attenuation inherent in the windscreen is usually insignificant for HVAC noise measurements.
4. Check the meter's instruction manual to determine which way to point the microphone. If
the manual is not available orient the microphone at a 45° angle from the predominant sound
source.
5. In almost all cases the sound level meter's “Fast/Slow” or “Fast/Slow/Impulse” switch
should be set to the “Slow” position. This makes it easier to determine the readout value.
6. Hold the sound level meter at arm's length or attach it to a tripod.
7. If possible, make all measurements with the microphone at least 1 m from any surface. This
applies to sound sources, room boundaries, people and room furnishings.
8. When measuring sound in a specific area - for example, to troubleshoot a noise complaint move the sound level meter very slowly so the microphone scans throughout a 600 to 900 mm
region near the complaint area.
9. For outdoor measurements at a property line, for instance, move the meter slowly so that the
microphone samples sound over that part of the properly line that has the greatest exposure
from the noise source(s) being tested.
10. Record the measurement results on the Sound Measurement Data Sheet (Figure C-4) and
the blank worksheet for the desired rating method (See Figures B-2, B-3, B-4, or B-6).
11. If measuring the sound levels of a specific source, make a separate “ambient” measurement
with the source turned off and adjust the “source-on” results per the procedure shown in
Figure C-5 per the procedure described in Table C-1.
12. Compare the measured results with the selected criteria determined using Table B-4.

Figure C-2 Sound measurement plan.

193

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Measuring HVAC System Noise

Figure C-3 Blank sound measurement data sheet.

194

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A Practical Guide to Noise and Vibration Control for HVAC Systems

Figure C-4 Completed sound measurement data sheet.

195

Measuring HVAC System Noise

It may sometimes be necessary to measure the noise due to a particular piece
of HVAC equipment in the presence of background noise from other sources that
cannot be turned off, such as automobile traffic or certain types of office machines.
Determining the LP value due to the selected equipment alone requires making two
measurements, one with the equipment operating with the background noise and
another with only the background noise (with the equipment turned off). This situation might occur, for example, when determining if the property line noise exposure from a cooling tower meets a local noise ordinance. The adjustment values
given in Table C-1 will help to determine the LP value of a particular machine in
a sound field with background noise.
When reporting an HVAC system LP measurement, always also report the
microphone location, the time, equipment operating conditions, and ambient noise
conditions.
VIBRATION MEASUREMENTS
While the control of HVAC system noise and vibration are of equal importance,
measuring vibration is not usually necessary for determining the sources or transmission paths of disturbing noise. Also, the techniques and instrumentation used for
vibration measurement and analysis are specialized and are beyond the scope of this
guide. Therefore, the reader should consult other sources, such as The Vibration
Institute, for descriptions of methods of vibration measurement and analysis.
Table C-1.

Adjustment Values for Determining Equipment Sound Levels
in the Presence of Constant Background Noise

Difference between
Measurement Result #1 and
Measurement Result #2

Adjustment* to
Measurement Result #1 to
Obtain Equipment Sound Level

10 dB or more

0 dB

6-9 dB

–1 dB

4 or 5 dB

–2 dB

3 dB

–3 dB

2 dB

–4 dB

0 or 1 dB

Equipment sound level cannot be accurately
determined, but is at least
6 dB below Measurement Result #1
Measurement #1 = Equipment Being Tested Plus Background Noise
Measurement #2 = Background Noise Only
* Adjustment applies to all types of frequency weightings and filters.

196

Appendix D

In response to demands by engineers and architects for accurate sound data,
industry standards have been developed for measuring and rating the acoustical
performance of various types of HVAC equipment. The published ratings based on
the standards have proven to be very helpful. However, misunderstanding and/or
misapplication of the published data often lead to installations that are noisier than
expected. For this reason, ASHRAE has published a companion publication to this
guide titled Application of Manufacturers’ Sound Data to help system designers
use and interpret factory-supplied acoustical data. The publication also includes
application guidelines similar to those found in this guide.
Although most of the test standards mentioned below require performance
data in octave bands, many laboratories acquire the data in 1/3-octave bands, so
such data might be available when trying to assess the likelihood of a strong tone
in the noise spectrum of a piece of equipment.
When reviewing manufacturers’ submitted acoustical data, be sure to verify
the following:
•

•

Fans—All acoustical data should be expressed in terms of unweighted
octave band LW values that are measured in complete accordance with the
latest revision of AMCA Standard 300. Data should be supplied for the fan’s
discharge opening, inlet opening, as well as its case-radiated noise.
Air-Handling Units and Fan Coil Units—All acoustical data should be
expressed in terms of unweighted octave band LW values that are measured in
complete accordance with the latest revision of AHRI Standard 260. The data
should be supplied for all openings, whether ducted or unducted, and caseradiated noise. Verify that the “end reflection loss” has been applied properly

197

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Using Manufacturers’
Sound Data

•

for the submitted data, as this could create an error of as much as 10 dB or
more in the 63 hertz octave band.
Terminal Units—Ignore published NC ratings. Accept only octave band LW
values that are measured in complete accordance with the latest revision of
AHRI Standard 880. The data should be supplied for both the discharge and
case-radiated noise.
Laboratory Air Valves—All acoustical data should be expressed in terms of
octave band LW values that are measured in complete accordance with the
latest revision of AHRI Standard 880. The data should be supplied for the
discharge (in the direction of the airflow), exhaust (in the direction opposite
to the airflow), and radiated noise.
Duct Silencers—All acoustical performance data should be expressed in
terms of the octave band dynamic insertion loss and self-noise LW values
that are measured in complete accordance with the latest revision of ASTM
E477. Static pressure drop values should also be included for various positive and negative airflow velocities.
Grilles and Diffusers—All acoustical data should be expressed in terms of
octave band LW values that are measured in complete accordance with the
latest revision of AHRI Standard 890. Lacking such data, assume that the
field NC rating of each device will be at least five points higher than the cataloged rating.
Linear (Slot) Diffusers—These diffusers are factory-tested with two types
of plenums. The first is a very large plenum in which the airflow approaching the diffuser is at a very low velocity. The second test is done with a very
small, close-fitting plenum that delivers relatively high-velocity, turbulent
airflow to the diffuser inlet. The close-fitting plenum arrangement can add as
much as 20 NC rating points to diffuser’s “low-velocity plenum” NC rating.
Verify which plenum for any submitted acoustical data.
Registers—Registers are rarely tested for noise generation, but informal factory tests have shown that a wide-open damper attached directly to a grille or
diffuser inlet collar can add as much as 5 NC rating points to a device’s “no
damper” NC rating. Closing the damper can add as much as 20 NC rating
points, depending on the damper setting and the upstream airflow conditions.
Chillers—All acoustical data should be expressed in terms of the
unweighted octave band LP values measured in complete accordance
with the latest revision of AHRI Standard 575. Note that field-measured
LP values will usually be higher than the AHRI-575 values because of
sound energy buildup in the equipment room. The increase can be as
much as 15 dB in a small room.

198

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Using Manufacturers’ Sound Data

A Practical Guide to Noise and Vibration Control for HVAC Systems

•

Cooling Towers—All acoustical data should be expressed in terms of the
unweighted octave band LP values measured at the ten locations specified in
CTI Test Code ATC-128. Verify that the submitted data include fan and
water noise.
Unducted Package Units (e.g., PTACs, ductless split-system fan-coil units,
etc.)—The indoor noise of these units should be expressed in terms of the
unweighted octave band LW values determined in complete accordance with
the latest revision of AHRI Standard 350. The outdoor noise should be
expressed in terms of the unweighted octave band LW values determined in
complete accordance with the latest revision of AHRI Standard 270.
Small Outdoor Equipment (e.g., residential condensing units)—All acoustical data should be expressed in terms of the unweighted octave band LW values determined in complete accordance with the latest revision of AHRI
Standard 270.
Large Outdoor Packaged Equipment (all types)—All acoustical data should
be expressed in terms of the unweighted octave band LW values determined
in complete accordance with the latest revision of ARI Standard 370.

199

Appendix E

Definitions and
Abbreviations
DEFINITIONS
absorption coefficient: The fraction of incident sound that is absorbed by a given
material, usually given in octave bands.
acoustical louver: A specially built louver using sound-attenuating baffles instead
of single-thickness bent steel or extruded aluminum blades.
aerodynamic noise: Noise due to turbulent airflow. See self-noise and self-generated noise.
ambient sound: The combination of all near and far sounds, none of which is
particularly dominant.
attenuation: A general term that describes the reduction of sound energy by any
of several mechanisms, including divergence, diffusion, absorption, scattering,
transmission loss, insertion loss, etc.
A-weighted sound level: The sound level measured using the A-weighting network
of a sound level meter. For broadband sounds, the A-weighted sound level is
loosely correlated with the perception of loudness. It is expressed in dBA.
background sound: All sounds, except sound from a particular source of interest.
balanced noise criteria (NCB): A single-number criteria rating system developed
to rate steady-state HVAC system noise.

201

Definitions and Abbreviations

bel: One bel = 10 decibels (dB)
blade passage frequency (BPF): The frequency of a tone produced by the rotation
of the blades on a fan wheel or pump impeller (equal to RPM times number of
blades/60).
breakout: The transmission of sound through a duct wall (from inside to outside).
broadband noise: Noise of random character with no discernable tones (e.g., rain
or wind.)
decibel (dB): The primary unit of sound measurement; used to quantify sound
pressure level, sound power level, as well as several types of attenuation mechanisms.
duct silencer: A specially built section of ductwork incorporating internal soundattenuating baffles; especially effective in the middle frequency range. Also called
sound traps, duct attenuators, or mufflers.
dynamic insertion loss: The sound insertion loss of a duct silencer while handling
airflow. It is expressed in dB.
equivalent duct diameter: For circular ducts, this is the actual diameter. For rectangular and flat-oval ducts, this is approximately the square root of the product of
the height times the width.
flanking path: An indirect sound transmission path, such as the structure-borne
path between two adjacent rooms.
floating floor: A floor assembly in which an array of fiberglass, neoprene, or
spring isolators supports a topping slab that floats above a structural slab; useful for
noise control, but not vibration control.
frequency: The number of oscillations per second, generally expressed in hertz
(Hz) or cycles per second (cps).
fundamental frequency: The lowest resonance frequency of a vibrating object.
insertion loss: The sound level reduction at a given location due to the insertion of
a noise control device, such as a duct silencer or acoustical louver. It is expressed
in dB. See also dynamic insertion loss.

202

A Practical Guide to Noise and Vibration Control for HVAC Systems

masking: The process through which the audibility of one sound is reduced
because of the presence of another.
noise: Undesirable sound that interferes with rest, sleep, mental concentration, or
speech communication.
noise criteria (NC): A single-number noise rating system developed to rate
steady-state continuous noise in a room from all types of equipment, including
fans, mixing boxes, diffusers, etc.
noise reduction coefficient (NRC): A single-number rating system used to
compare the approximate sound-absorbing characteristics of building materials.
octave: A continuous range of frequencies whose upper frequency limit is twice
that of its lower frequency limit. Typical center frequencies for HVAC analyses are
62.5, 125, 250, 500, 1000, 2000, 4000, and 8000 hertz, although the 8000 hertz
band is often ignored and the 31.25 hertz band is sometimes considered when
investigating noise and vibration complaints. The lower frequency limit for each
octave band is 70.7% of the center frequency, while the upper frequency limit is
141.4% of the center frequency. For example, the 1000-hertz octave band contains
noise energy at all frequencies from 707 to 1414 hertz.
octave band level (OBL): The sound pressure level of the sound energy within a
single octave band. It is expressed in dB.
random noise: See broadband noise.
regenerated noise: The noise caused by turbulent flow in air and water systems,
usually expressed as LW values in octave bands.
resonance: The natural oscillation of a construction assembly, resilient element, or
air column that persists after the shutoff of an outside excitation. The ringing
sounds that you hear after hitting a bell or plucking a guitar string are examples of
resonance.
resonance frequency: A frequency at which resonance occurs.
room criteria (RC): A single-number noise rating system developed to diagnose
and rate the HVAC noise exposure in a room. This rating system should not be used
for design analysis or building commissioning.

203

Definitions and Abbreviations

seismic restraints: Auxiliary restraints that hold a piece of vibration-isolated
equipment in place in the event of an earthquake. They must be designed and
installed very carefully to avoid “short-circuiting” the vibration isolators. These
same restraints are often used to resist wind-loading on outdoor equipment.
self-noise: The regenerated noise occurring in duct silencers or duct fittings.
self-generated noise: See self-noise.
short-circuiting: The process by which the improper adjustment of a seismic or
wind-loading restraint lets equipment vibration bypass an isolator.
sone: A linear scale that is used to rate the approximate loudness of an unducted
fan.
sound level meter: A meter, usually handheld, that is used to measure sound pressure levels.
sound power level (LW or PWL): The total acoustic energy radiated per unit time.
It is usually given in octave bands (dB re 1 pW) and, in general, is not affected by
the acoustic environment.
sound pressure level (LP or SPL): The calculated or measured level of sound pressure, expressed in dB (re 20 μPa), at a specific location under a specific set of
equipment operating conditions. The frequency range of the measurement or
calculation must be indicated along with the LP value in dB. The LP value of any
source varies with the distance from the source and depends upon the acoustic environment between the source and the listener.
sound transmission class (STC): A single-number rating system used to compare
the sound-isolating characteristics of partitions, doors, or windows used to separate
occupied spaces. It should not be used to select mechanical equipment room partitions or slabs.
sound trap: See duct silencer.
structure-borne sound: Sound that radiates from a construction assembly after
traveling through a building's structure in the form of vibration.
vibration isolation: The control of equipment vibration by the insertion of resilient
elements (usually a steel spring or elastomeric element) between the equipment
and a supporting structure. Also applies to the resilient attachment of service
connections (conduits, pipes, ducts, etc.) to the equipment.

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A Practical Guide to Noise and Vibration Control for HVAC Systems

ABBREVIATIONS
ADC
AHU
AMCA
ANSI
AHRI
ASHRAE
ASTM
CAV
CTI
dB
dBA
dBC
HVI
Hz
kg/m2
kg/m3
kW
Lp
LPA
LPC
LW
LWA
LWC
L/s
m/s
NC
NCB
NFPA
NR
NRC
NVLAP
OBL
Pa

Air Diffusion Council
air-handling unit
Air Movement and Control Association
American National Standards Institute
Air Conditioning and Refrigeration Institute
American Society of Heating, Refrigerating and Air-Conditioning
Engineers
American Society for Testing and Materials
constant air volume (a type of air terminal)
Cooling Technology Institute
decibel
A-weighted decibel
C-weighted decibel
Heating and Ventilating Institute
hertz (cycles per second)
kilograms per square metre (surface density)
kilograms per cubic metre (volume density)
motor power rating in kilowatts
sound pressure level
A-weighted sound pressure level
C-weighted sound pressure level
sound power level
A-weighted sound power level
C-weighted sound power level
liters per second
metres per second
noise criteria
balanced noise criteria
National Fire Prevention Association
noise reduction
noise reduction coefficient
National Voluntary Laboratory Accreditation Program
octave band sound pressure level
Pascals (pressure)

205

Definitions and Abbreviations

packaged terminal air conditioner
sound power level
room criteria
revolutions per minute
Sheet Metal and Air Conditioning Contractor's National Association
static pressure
sound pressure level
sound transmission class
total pressure
total static pressure
transmission loss
variable air volume
variable-frequency drive (motor speed controller)

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PTAC
PWL
RC
rpm
SMACNA
SP
SPL
STC
TP
TSP
TL
VAV
VFD

206

Addresses of Agencies
and Associations
Air Diffusion Council (ADC)
1901 N. Roselle Road, Suite 800
Schaumburg, Illinois 60195
http://www.flexibleduct.org
Air Conditioning, Heating, and Refrigeration Institute (AHRI)
2111 Wilson Blvd., Suite 500
Arlington, VA 22201
http://www.ahrinet.org
Air Movement and Control Association (AMCA)
30 West University Drive
Arlington Heights, IL 60004-1893
http://www.amca.org
American Society of Heating, Refrigerating, and Air-Conditioning Engineers
(ASHRAE)
1791 Tullie Circle N.E.
Atlanta, GA 30329
http://www.ashrae.org
ASTM International (ASTM)
100 Barr Harbor Drive
West Conshohocken, PA 19428-2959
http://www.astm.org

207

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Appendix F

Addresses of Agencies and Associations

American National Standards Institute (ANSI)
25 West 43rd Street, 4th Floor
New York, NY 10036
http://www.ansi.org
Cooling Technology Institute (CTI)
PO Box 73383
Houston, TX 77273-3383
http://www.cti.org
Sheet Metal and Air Conditioning Contractor's National Association
(SMACNA)
4201 Lafayette Center Drive
Chantilly, VA 20151-1219
http://www.smacna.org
The Vibration Institute
6262 S. Kingery Highway
Suite 212
Willowbrook, IL 60527
http://www.vibinst.org

208

Appendix G

Bibliography and
Selected References
BOOKS
Beranek, L.L. (ed.) 1988. Noise and Vibration Control—Revised edition. New
York: McGraw-Hill.
Egan, M.D. 1972. Concepts in Architectural Acoustics. New York: McGraw-Hill.
Egan, M.D. 1988. Architectural Acoustics. New York: McGraw-Hill.
Harris, C.M. (ed.) 1991. Handbook of Acoustical Measurements and Noise Control, 3rd ed. New York: McGraw-Hill.
Jones, R.S. 1984. Noise and Vibration Control in Buildings. New York: McGrawHill.
Miller, L.N. 1971. Lecture Notes on Noise and Vibration Control for Mechanical
and Electrical Equipment in Buildings. Cambridge, MA: Bolt, Beranek, &
Newman, Inc.
Nevins, R.G. 1976. Air Diffusion Dynamics. Chicago, IL: Air Diffusion Council.
Sharland, I. 1972. Woods Practical Guide to Noise Control. Colchester, UK:
Woods of Colchester, Ltd.
Woods, R.I. (ed.) 1972. Noise Control in Mechanical Services. Colchester, UK:
Sound Research Laboratories.
Fry, A.T. (ed.) 1988. Noise Control in Building Services. Colchester, UK: Sound
Research Laboratories.
FEMA. 2002. FEMA 412-Installing Seismic Restraints for Mechanical Equipment. Washington, DC: Federal Emergency Management Administration.
ASHRAE. 2000. A Practical Guide to Seismic Restraint. Atlanta, GA: American
Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

209

Bibliography and Selected References

INDUSTRY MANUALS, HANDBOOKS, AND BULLETINS
AMCA. 2002. Publication 201-02, Fans and systems. Arlington Heights, IL: Air
Movement and Control Association, Inc.
AMCA. 1998. Publication 202-98—Troubleshooting. Arlington Heights, IL: Air
Movement and Control Association, Inc.
ASHRAE. 2003. ASHRAE Handbook—HVAC Applications, Chapter 47. Atlanta,
GA: American Society of Heating, Refrigerating and Air-Conditioning
Engineers, Inc. (Revised every four years.)
ASHRAE. 2005. ASHRAE Handbook—Fundamentals, Chapter 7. Atlanta, GA:
American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. (Revised every four years.)
SMACNA. 1990. HVAC system—Duct design. Vienna, VA: Sheet Metal and
Air-Conditioning Contractor's Association.
SMACNA. 1995. HVAC duct construction standards—Metal and flexible.
Vienna, VA: Sheet Metal and Air-Conditioning Contractor's Association.
SMACNA. 1998. Seismic restraint manual: Guidelines for mechanical systems.
Vienna, VA: Sheet Metal and Air-Conditioning Contractor's Association.
Graham, J.B. 1959. Fan noise. Bulletin F-105. Buffalo, NY: Buffalo Forge Company.
Hassal, J.R., and K. Zaveri. 1979. Acoustic noise measurements, 4th ed. Naerum,
Denmark: Bruel & Kjaer.
Hirschorn, M. 1989. Noise Control Reference Handbook. Bronx, NY: Industrial
Acoustics Company.
Peterson, A.P.G., and E.E. Gross. 1972. Handbook of Noise Measurement, 7th ed.
Concord, MA: General Radio Company.
Schultz, T.J. 1986. Acoustical uses of perforated metal. Milwaukee, WI: Industrial Perforators Association.
Baltimore Aircoil Co. 2005. Product and Application Handbook, pp. J20-J54.
Baltimore, MD: Baltimore Aircoil Company.
Owens-Corning Fiberglas Company. 1987. Noise Control Design Guide. Publ.
No. 5-BL-11691-E. Toledo, OH: Owens-Corning Fiberglas Company.
Trane Company. 1982. Fans and their application in air conditioning. La Crosse,
WI: The Trane Company.
Trane Company. 1986. Acoustics in air conditioning. Application Manual
FNDAM-5. La Crosse, WI: The Trane Company.
Trane Company. 2002. Indoor sound—Sound advice from Trane for sound sensitive large rooftop installations. Bulletin RT-EB-80. La Crosse, WI: The
Trane Company.

210

A Practical Guide to Noise and Vibration Control for HVAC Systems

INDUSTRY PERIODICALS
Heating/Piping/Air Conditioning
Sound and Vibration
Specifying Engineer
JOURNALS
ASHRAE Journal
Journal of the Acoustical Society of America
Journal of Sound and Vibration
Noise Control Engineering Journal

211

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