Trane Trg Trc007 En Users Manual

TRG-TRC007-EN to the manual 61fece07-31c3-43b9-9bb0-681fc3820853

2015-01-21

: Trane Trane-Trg-Trc007-En-Users-Manual-236104 trane-trg-trc007-en-users-manual-236104 trane pdf

Open the PDF directly: View PDF .
Page Count: 82

Air Conditioning

Clinic

Fundamentals of HVAC

Acoustics

One of the Fundamental Series

TRG-TRC007-EN

NO POSTAGE

NECESSARY

IF MAILED

IN THE

UNITED STATES

BUSINESS REPLY MAIL

FIRST-CLASS MAIL PERMIT NO. 11 LA CROSSE, WI

POSTAGE WILL BE PAID BY ADDRESSEE

TRANE

Attn: Applications Engineering

3600 Pammel Creek Road

La Crosse WI 54601-9985

NO POSTAGE

NECESSARY

IF MAILED

IN THE

UNITED STATES

BUSINESS REPLY MAIL

FIRST-CLASS MAIL PERMIT NO. 11 LA CROSSE, WI

POSTAGE WILL BE PAID BY ADDRESSEE

TRANE

Attn: Applications Engineering

3600 Pammel Creek Road

La Crosse WI 54601-9985

Crop to width of 7.75”

Perforation 5.5” from bottom/top

Perforation 0.75” from edge

Response Card

We offer a variety of HVAC-related educational materials and technical references, as well as software tools

that simplify system design/analysis and equipment selection. To receive information about any of these

items, just complete this postage-paid card and drop it in the mail.

Education materials ❏Air Conditioning Clinic series About me…

❏Engineered Systems Clinic series Name _______________________________________________

❏Trane Air Conditioning Manual Title _______________________________________________

❏Trane Systems Manual Business type _______________________________________________

Software tools ❏Equipment Selection Phone/fax _____________________ ________________________

❏System design & analysis E-mail address _______________________________________________

Periodicals ❏Engineers Newsletter Company _______________________________________________

Other? ❏_____________________________ Address _______________________________________________

_______________________________________________

Thank you for your interest!

Trane

An American Standard Company

www.trane.com

For more information contact

your local district office or

e-mail us at comfort@trane.com

Comment Card

We want to ensure that our educational materials meet your ever-changing resource development needs.

Please take a moment to comment on the effectiveness of this Air Conditioning Clinic.

Level of detail (circle one) Too basic Just right Too difficult

One of the Fundamental Series

Rate this clinic from 1–Needs Improvement to 10–Excellent

…

TRG-TRC007-EN Content 12345678910

Booklet usefulness 12345678910

Slides/illustrations 12345678910

Presenter’s ability 12345678910

Training environment 12345678910

Other comments? _________________________________________________________

_______________________________________________________________________________

About me … Type of business _________________________________________________________

Job function _________________________________________________________

Optional: name _________________________________________________________

phone _________________________________________________________

address _________________________________________________________

Give the completed card to the

presenter or drop it in the mail.

Thank you!

Trane

An American Standard Company

www.trane.com

For more information contact

your local district office or

e-mail us at comfort@trane.com

Fundamentals of HVAC

Acoustics

Fundamentals of HVAC

Acoustics

One of the Fundamental Series

A publication of

Trane, an American

Standard Company

Fundamentals of HVAC

Preface

TRG-TRC007-EN

Trane believes that it is incumbent on manufacturers to serve the industry by

regularly disseminating information gathered through laboratory research,

testing programs, and field experience.

The Trane Air Conditioning Clinic series is one means of knowledge sharing.

It is intended to acquaint a nontechnical audience with various fundamental

aspects of heating, ventilating, and air conditioning (HVAC). We have taken

special care to make the clinic as uncommercial and straightforward as

possible. Illustrations of Trane products only appear in cases where they help

convey the message contained in the accompanying text.

This particular clinic introduces the reader to the fundamentals of HVAC

acoustics.

Fundamentals of HVAC Acoustics

A Trane Air Conditioning Clinic

Figure 1

TRG-TRC007-EN iii

Contents

period one Fundamentals of Sound ..................................... 1

What is Sound? ....................................................... 2

Octave Bands ......................................................... 6

Sound Power and Sound Pressure ........................... 9

period two Sound Perception and Rating Methods ...... 15

Human Ear Response ............................................ 15

Single-Number Rating Methods ............................. 17

Octave-Band Rating Method .................................. 28

period three Acoustical Analysis ............................................ 29

Setting a Design Goal ............................................ 30

Source–Path–Receiver Analysis ............................. 32

Sound-Path Modeling ............................................ 37

Terms Used in Sound-Path Modeling ..................... 41

period four Equipment Sound Rating ................................ 47

Fields of Measurement ......................................... 48

HVAC Equipment Sound Rating ............................. 54

period five Review ................................................................... 63

Quiz ......................................................................... 68

Answers ................................................................ 70

Glossary ................................................................ 71

iv TRG-TRC007-EN

TRG-TRC007-EN 1

notes

period one

Fundamentals of Sound

People have become increasingly conscious of acoustics as a component of a

comfortable environment. Sound levels, both indoor and outdoor, can be

affected to varying degrees by HVAC equipment and systems.

The degree to which the HVAC system affects the sound at a particular location

depends on the strength of the sound source and the environmental effects on

the sound as it travels from that source to the listener.

period one

Fundamentals of Sound

Fundamentals of HVAC Acoustics

Figure 2

2TRG-TRC007-EN

notes

period one

Fundamentals of Sound

What is Sound?

Sound is the audible emissions resulting from the vibration of molecules

within an elastic medium. It is generated by either a vibrating surface or the

movement of a fluid. In the context of building HVAC systems, this elastic

medium can be either air or the building structure. For structurally-borne sound

to become audible, however, it must first become airborne.

Noise is different than sound. Sound is always present, but is not always

obtrusive. Noise is defined as unwanted sound. Generally, people object to

sound when it interferes with speech, concentration, or sleep.

What is Sound?

IAudible emissions resulting from vibration of

molecules within an elastic medium

IGenerated by vibrating surface or movement

of a fluid

IIn buildings, it may be airborne or structure-

borne

INoise is unwanted sound

Figure 3

TRG-TRC007-EN 3

period one

Fundamentals of Sound

notes

Airborne sound is transmitted away from a vibrating body through the transfer

of energy from one air molecule to the next. The vibrating body alternately

compresses and rarefies (expands) the air molecules. The pressure fluctuations

that result from the displacement of these air molecules take the form of a

harmonic, or sine, wave. The amplitude of the wave depicts pressure. The

higher the amplitude, the louder the sound.

This transfer of energy takes time. Each complete sequence of motion

(compression and rarefaction) constitutes a cycle, and the time required to

complete one cycle is the cycle period. The frequency of the periodic motion is

the number of cycles that occur in a second. The unit of measure for frequency

is the hertz (Hz). One hertz is equal to one cycle per second.

The terms pitch and frequency are often (incorrectly) used interchangeably.

Frequency is an objective quantity that is independent of sound-pressure level.

Pitch, however, is a subjective quantity that is primarily based on frequency,

but is also dependent on sound-pressure level and composition. Pitch is not

measured, but is described with terms like bass, tenor, and soprano.

Sound Wave and Frequency

amplitude

one cycle

time

cycles

seconds

frequency, Hz

frequency, Hz =

Figure 4

frequency, Hz cycles

seconds

-----------------------

4TRG-TRC007-EN

notes

period one

Fundamentals of Sound

The wavelength of the sound is the linear measurement of one complete

cycle. The wavelength and frequency of a sound are related by using the

following equation:

The speed of sound transmission is a physical property of the medium. For air,

the speed varies slightly with temperature change. Because the temperature

range encountered in the study of HVAC acoustics is relatively small, the speed

of sound can be considered a constant 1,127 ft/s (344 m/s). For example, sound

traveling through the air at a frequency of 200 Hz has a wavelength of 5.6 ft

(1.7 m).

Wavelength

one cycle

frequency

wavelength

wavelength =

= speed of sound

speed of sound

Figure 5

wavelength speed of sound

frequency

-------------------------------------------

wavelength 1,127 ft/s

200 Hz

-------------------------

=5.6 ft=

wavelength 344 m/s

200 Hz

----------------------1.7 m==

èø

æö

TRG-TRC007-EN 5

period one

Fundamentals of Sound

notes

The wave form shown in Figure 5 represents sound occurring at a single

frequency. This is called a pure tone.

A pure sinusoidal wave form, however, is very rare in HVAC acoustics. Typically,

sounds are of a broadband nature, meaning that the sound is composed of

several frequencies and amplitudes, all generated at the same time. Figure 6

represents the components of broadband sound.

Alternatively, plotting the amplitude (vertical axis) of each sound wave at each

frequency (horizontal axis) results in a graphic of the broadband sound that

looks like this. As you can see from this example, the sound energy is greater at

some frequencies than at others.

Broadband Sound

amplitude

time

time Figure 6

Broadband Sound and Tones

amplitude

frequency

tone

logarithmic scale

Figure 7

6TRG-TRC007-EN

notes

period one

Fundamentals of Sound

Again, a pure tone has a single frequency. If a sound in a narrow band of

frequencies is significantly greater than the sound at adjacent frequencies, it

would be similar to a tone. Tones that stand out enough from the background

sound can be objectionable. Many of the sounds generated by HVAC

equipment and systems include both broadband and tonal characteristics.

Octave Bands

Because sound occurs over a range of frequencies, it is considerably more

difficult to measure than temperature or pressure. The sound must be

measured at each frequency in order to understand how it will be perceived in a

particular environment. The human ear can perceive sounds at frequencies

ranging from 20 to 16,000 Hz, whereas, HVAC system designers generally focus

on sounds in the frequencies between 45 and 11,200 Hz. Despite this reduced

range, measuring a sound at each frequency would result in 11,156 data points.

For some types of analyses, it is advantageous to measure and display the

sound at each frequency over the entire range of frequencies being studied.

This is called a full-spectrum analysis and is displayed like the example shown

in Figure 7.

To make the amount of data more manageable, this range of frequencies is

typically divided into smaller ranges called octave bands. Each octave band is

defined such that the highest frequency in the band is two times the lowest

frequency. The octave band is identified by its center frequency, which is

calculated by taking the square root of the product of the lowest and highest

frequencies in the band.

The result is that this frequency range (45 to 11,200 Hz) is separated into eight

octave bands with center frequencies of 63, 125, 250, 500, 1,000, 2,000, 4,000,

and 8,000 Hz. For example, sounds that occur at the frequencies between 90 Hz

and 180 Hz are grouped together in the 125 Hz octave band.

Octave Bands

octave

band

octave

band

center

frequency (Hz)

125

250

500

1,000

2,000

4,000

8,000

center

frequency (Hz)

125

250

500

1,000

2,000

4,000

8,000

frequency

range (Hz)

45 to 90

90 to 180

180 to 355

355 to 710

710 to 1,400

1,400 to 2,800

2,800 to 5,600

5,600 to 11,200

frequency

range (Hz)

45 to 90

90 to 180

180 to 355

355 to 710

710 to 1,400

1,400 to 2,800

2,800 to 5,600

5,600 to 11,200

Figure 8

center frequency lowest frequency × highest frequency =

TRG-TRC007-EN 7

period one

Fundamentals of Sound

notes

Octave bands compress the range of frequencies between the upper and lower

ends of the band into a single value. Sound measured in an octave band is the

logarithmic sum of the sound level at each of the frequencies within the band.

Unfortunately, octave bands do not indicate that the human ear hears a

difference between an octave that contains a tone and one that does not, even

when the overall magnitude of both octaves is identical. Therefore, the process

of logarithmically summing sound measurements into octave bands, though

practical, sacrifices valuable information about the “character” of the sound.

Octave Bands

1,000

125

125 500

500

250

250 2,000

2,000 4,000

4,000 8,000

8,000

logarithmic

sums

amplitude

frequency, Hz

frequency, Hz Figure 9

8TRG-TRC007-EN

notes

period one

Fundamentals of Sound

Middle ground between octave-band analysis and full-spectrum analysis is

provided by one-third octave-band analysis. One-third octave bands divide

the full octaves into thirds. The upper cutoff frequency of each third octave is

greater than the lower cutoff frequency by a factor of the cube root of two

(approximately 1.2599). If tones are contained in the broadband sound, they will

be more readily apparent in the third octaves.

The use of octave bands is usually sufficient for rating the acoustical

environment in a given space. One-third octave bands are, however, more

useful for product development and troubleshooting acoustical problems.

One-Third Octave Bands

amplitude

frequency, Hz

1,000

125

125 500

500

250

250 2,000

2,000 4,000

4,000 8,000

8,000

Figure 10

TRG-TRC007-EN 9

period one

Fundamentals of Sound

notes

Sound Power and Sound Pressure

Sound power and sound pressure are two distinct and commonly confused

characteristics of sound. Both are generally described using the term decibel

(dB), and the term “sound level” is commonly substituted for each. To

understand how to measure and specify sound, however, one must first

understand the difference between these two properties.

Sound power is the acoustical energy emitted by the sound source, and is

expressed in terms of watts (W). It is not affected by the environment.

Sound pressure is a pressure disturbance in the atmosphere, expressed in

terms of pascals (Pa), that can be measured directly. Sound pressure magnitude

is influenced not only by the strength of the source, but also by the

surroundings and the distance from the source to the listener. Sound pressure

is what our ears hear and what sound meters measure.

While sound-producing pressure variations within the atmosphere can be

measured directly, sound power cannot. It must be calculated from sound

pressure, knowing both the character of the source and the modifying

influences of the environment.

Sound Power and Sound Pressure

ISound power

KAcoustical energy emitted by the sound source

KUnaffected by the environment

ISound pressure

KPressure disturbance in the atmosphere

KAffected by strength of source, surroundings, and

distance between source and receiver

Figure 11

10 TRG-TRC007-EN

notes

period one

Fundamentals of Sound

The following comparison of sound and light may help illustrate the distinction

between these two properties. Think of sound power as the wattage rating of a

light bulb. Both measure a fixed amount of energy. Whether you put a 100-watt

light bulb outdoors or in a closet, it is always 100-watt light bulb and always

gives off the same amount of light.

Sound pressure corresponds to the brightness, from the light emitted by the

light bulb, in a particular location in the room. Both sound pressure and

brightness can be measured with a meter, and the immediate surroundings

influence the magnitude of each. In the case of light, brightness depends on

more than the wattage of the bulb. It also depends on how far the observer is

from the light bulb, the color of the room, how reflective the wall surfaces are,

and whether the light bulb is covered with a shade. These other factors affect

how much light reaches the receiver, but do not affect the wattage of the light

bulb.

Similarly, sound pressure depends not only on the sound power emitted by the

source, but also on the characteristics of the surrounding environment. These

might include the distance between the sound source and the listener, whether

the room is carpeted or tiled, and whether the room is furnished or bare. Just as

with light, environmental factors like these affect how much sound reaches the

listener.

An Analogy

ISound power

KCorrelates to bulb wattage

ISound pressure

KCorrelates to brightness

Figure 12

TRG-TRC007-EN 11

period one

Fundamentals of Sound

notes

Sounds encompass a wide range of volumes, or levels. The loudest sound the

human ear can hear without damage due to prolonged exposure is about

1,000,000,000 times greater than the quietest perceptible sound. A range of this

magnitude makes using an arithmetic scale cumbersome, so a logarithmic

scale is used instead.

The measurement of sound level is expressed in terms of decibels (dB), a

dimensionless quantity. A decibel is a calculated value based on the ratio of two

quantities. It is defined as ten times the logarithm to the base ten (log10) of the

measured quantity divided by the reference quantity. The reference quantity

must be specified to prevent confusion regarding the magnitude of the ratio.

Decibel

measured value

reference value

dB = 10 log10

Figure 13

dB 10 log10

measured value

reference value

---------------------------------------------

12 TRG-TRC007-EN

notes

period one

Fundamentals of Sound

A logarithm is the exponent power of the base. In this case, the base is ten. For

example, the log10 of 10 (or 101) equals 1, the log10 of 100 (or 102) equals 2, and

the log10 of 1,000,000,000 (or 109) equals 9.

As mentioned earlier, the loudest sound the human ear can hear without

damage due to prolonged exposure is about 1,000,000,000 times greater than

the quietest perceptible sound. If we use the quietest perceptible sound as the

reference value, this ratio would range from 1 to 1,000,000,000. Converting this

arithmetic range to a log10 scale yields a range of 0 to 9. This unitless result is

described in terms of bels. Multiplying by ten results in the more-commonly

used broader range of 0 to 90 decibels (dB).

Logarithmic Scale

ratio

100

1,000

10,000

100,000

1,000,000

10,000,000

100,000,000

1,000,000,000

ratio

100

1,000

10,000

100,000

1,000,000

10,000,000

100,000,000

1,000,000,000

log10

10 ´

´´

´log10

10 ´

´´

´log10

90 Figure 14

TRG-TRC007-EN 13

period one

Fundamentals of Sound

notes

When a reference value is established and placed in the denominator of the

ratio, the dB can be calculated for any value entered into the numerator.

The reference value used for calculating sound-power level is 1 picowatt (pW),

or 10-12 watts. Therefore, sound-power level (Lw) in dB is calculated using the

following equation:

The reference value used for calculating sound-pressure level is 20

micropascals (µPa), or 2 ×10-5 Pa. Therefore, sound-pressure level (Lp) in dB,

is calculated using the following equation:

Equation for Sound Power

sound power, W

10-12 W

Lw= 10 log10

Figure 15

LW10 log10

sound power, watts

10-12watts

--------------------------------------------------------

Equation for Sound Pressure

sound pressure, m

mPa

20 m

mPa

Lp= 20 log10

Figure 16

14 TRG-TRC007-EN

notes

period one

Fundamentals of Sound

Again, these reference values can be considered the threshold of hearing. The

multiplier 20 is used in the sound-pressure level equation instead of 10 because

sound power is proportional to the square of sound pressure.

Measuring sound using a logarithmic scale means that decibel values cannot

be added arithmetically. Instead, logarithmic addition must be used to add two

or more sound levels. This involves converting the decibel values into ratios of

sound intensity, adding these ratios, and then converting the sum back into

decibels. The mathematics become rather involved—the graph in Figure 17 has

been developed to simplify the procedure.

To demonstrate the use of this figure, consider the example of adding a 50 dB

sound to a 44 dB sound. The difference between these two sounds is 6 dB.

Therefore, 1 dB is added to the higher of the two sounds (50 plus 1) to arrive

at the logarithmic sum of 51 dB.

Also, notice that the logarithmic sum of two sounds of equal magnitude (0 dB

difference) results in a 3 dB increase. Therefore, adding two 50 dB sounds

would result in a combined sound level of 53 dB.

Lp20 log10

sound pressure, µPa

20 Pa

----------------------------------------------------------or 10 log10

sound pressure, µPa

20 Pa

----------------------------------------------------------

Logarithmic Addition of Decibels

510

10 15

dB difference between values being added

add to the higher dB value

50 dB + 44 dB = 51 dB

Figure 17

TRG-TRC007-EN 15

notes

The study of acoustics is affected by the response of the human ear to sound

pressure. Unlike electronic sound-measuring equipment, which provides a

repeatable, unbiased analysis of sound pressure, the sensitivity of the human

ear varies by frequency and magnitude. Our ears are also attached to a highly

arbitrary evaluation device, the brain.

Human Ear Response

The ear acts like a microphone. Sound waves enter the auditory canal and

impinge upon the ear drum, causing it to vibrate. These vibrations are

ultimately transformed into impulses that travel along the auditory nerve to

the brain, where they are perceived as sound. The brain then analyzes and

evaluates the signal.

period two

Sound Perception and Rating

Methods

Fundamentals of HVAC Acoustics

Figure 18

The Human Ear

outer

ear

outer

ear

auditory

canal

auditory

canal

eardrumeardrum

eardrum

middle

ear

middle

ear

nerves

to brain

nerves

to brain

Figure 19

period two

Sound Perception and

Rating Methods

16 TRG-TRC007-EN

notes

period two

Sound Perception and

Rating Methods

The sensation of loudness is principally a function of sound pressure, however,

it also depends upon frequency. As a selective sensory organ, the human ear

is more sensitive to high frequencies than to low frequencies. Also, the ear’s

sensitivity at a particular frequency changes with sound-pressure level. Figure

20 illustrates these traits using a set of contours. Each contour approximates an

equal loudness level across the frequency range shown.

For example, a 60 dB sound at a frequency of 100 Hz is perceived by the human

ear to have loudness equal to a 50 dB sound at a frequency of 1,000 Hz. Also,

notice that the contours slant downward as the frequency increases from 20

to 200 Hz, indicating that our ears are less sensitive to low-frequency sounds.

The contours are flatter at higher decibels (> 90 dB), indicating a more uniform

response to “loud” sounds across this range of frequencies.

As you can see, the human ear does not respond in a linear manner to pressure

and frequency.

frequency, Hz

50 100

100 200

200 500

500 1,000

1,000 2,000

2,000 5,000

5,000

20 10,000

10,000

120

100

sound pressure, dB ref 20

sound pressure, dB ref 20 m

mPa

Loudness Contours

Figure 20

TRG-TRC007-EN 17

period two

Sound Perception and

Rating Methods

notes

Additionally, tones evoke a particularly strong response. Recall that a tone is

a sound that occurs at a single frequency. Chalk squeaking on a blackboard,

for example, produces a tone that is extremely irritating to many people.

Single-Number Rating Methods

The human ear interprets sound in terms of loudness and pitch, while electronic

sound-measuring equipment interprets sound in terms of pressure and

frequency. As a result, considerable research has been done in an attempt to

equate sound pressure and frequency to sound levels as they are perceived

by the human ear. The goal has been to develop a system of single-number

descriptors to express both the intensity and quality of a sound.

With such a system, sound targets can be established for different

environments. These targets aid building designers in specifying appropriate

acoustical requirements that can be substantiated through measurement. For

example, a designer can specify that “the background sound level in the theater

Response to Tones

Figure 21

Single-Number Rating Methods

IA-, B-, and C-weighting

INoise criteria (NC) curves

IRoom criteria (RC) curves

ISones

IPhons

Figure 22

18 TRG-TRC007-EN

notes

period two

Sound Perception and

Rating Methods

shall be X,” where X is a single-number descriptor conveying the desired

quality of sound.

The most frequently used single-number descriptors are the A-weighting

network, noise criteria (NC), and room criteria (RC). All three share a common

problem, however: they unavoidably lose valuable information about the

character, or quality, of sound. Each of these descriptors is based on octave-

band sound data which, as noted earlier, may already mask tones. Further,

the process of converting from eight octave bands to a single number overlooks

even more sound data.

Despite this shortcoming, the single-number descriptors summarized in this

clinic are valuable tools for defining sound levels in a space, and are widely

used to specify the acoustical requirement of a space.

One simple method for combining octave-band sound data into a single-

number descriptor is A-, B-, or C-weighting. The weighting curves shown in

Figure 23 compensate for the varying sensitivity of the human ear to different

frequencies.

A-weighting, which is most appropriately used for low-volume (or quiet) sound

levels, best approximates human response to sound in the range where no

hearing protection is needed. B-weighting is used for medium-volume sound

levels. C-weighting is used for high-volume (or loud) sound levels where the

response of the ear is relatively flat.

relative response, dB

frequency, Hz

50 100

100 200

200 500

500 1,000

1,000 2,000

2,000 5,000

5,000

20 10,000

10,000

-40

-30

-20

-10

frequency responses for sound

meter weighting characteristics

A–B–C Weighting

Figure 23

TRG-TRC007-EN 19

period two

Sound Perception and

Rating Methods

notes

The following steps describe how to calculate an A-weighted value.

1Starting with the actual sound-pressure levels for the eight octave bands,

add or subtract the decibel values represented by the A-weighting curve

shown in Figure 23. These weighting factors are also listed in the table in

Figure 24. Subtract 26 dB from the 63 Hz sound-pressure level, 16 dB from

the 125 Hz level, 9 dB from the 250 Hz level, and 3 dB from the 500 Hz level.

Then, add 1 dB each to the sound-pressure levels in the 2,000 Hz and

4,000 Hz octave bands.

2Logarithmically sum all eight octave bands together to arrive at an overall

A-weighted sound-pressure level. This value is then expressed using the

units of dBA.

For the sound-pressure data in this example, the A-weighted sound-pressure

level is 42 dBA.

Most sound meters can automatically calculate and display the A-weighted

sound-pressure level, providing a simple and objective means of verifying

acoustical performance. However, as mentioned earlier, one of the drawbacks

of a single-number descriptor is that data about the relative magnitude of each

octave band is lost when the eight octave bands are combined into one value.

Therefore, even if the target dBA level is achieved, an objectionable tonal

quality or spectrum imbalance may exist.

A–Weighting Example

octave

band

octave

band

center

frequency (Hz)

125

250

500

1,000

2,000

4,000

8,000

center

frequency (Hz)

125

250

500

1,000

2,000

4,000

8,000

A-weighting

factor (dB)

–26

–16

–9

–3

+ 0

+ 1

+ 0

A-weighting

factor (dB)

–26

–16

–9

–3

+ 0

+ 1

+ 0

actual sound

pressure (dB)

actual sound

pressure (dB)

A-weighted

sound pressure (dB)

42 dBA

A-weighted

sound pressure (dB)

42 dBA Figure 24

20 TRG-TRC007-EN

notes

period two

Sound Perception and

Rating Methods

A-weighting is often used to define sound in outdoor environments. For

example, local sound ordinances typically regulate dBA levels at property lines.

Hearing-related safety standards, written by organizations such as the

Occupational Safety and Health Administration (OSHA), also commonly refer to

A-weighted sound-pressure levels when determining whether hearing

protection is required in a certain environment.

To avoid confusion, we recommend that A-weighting be applied only to octave-

band sound-pressure data, not to sound-power data. Also, A-weighting should

be limited to expressing a single-number descriptor. Displaying sound data in

all eight octave bands in terms of A-weighted sound pressures should be

avoided.

A–Weighting

IUsed for outdoor sound

ordinances and indoor,

hearing-related safety

standards (OSHA)

IUse with sound pressure

data only, not sound power

IExpress as a single number

descriptor only, not as

octave-band data

Figure 25

TRG-TRC007-EN 21

period two

Sound Perception and

Rating Methods

notes

Noise criteria (NC) curves are probably the most common single-number

descriptor used to rate sound-pressure levels in indoor environments. Like the

equal-loudness contours on which they are based, the loudness along each NC

curve is about the same. Each NC curve slopes downward to reflect the

increasing sensitivity of the ear to higher frequencies.

It should also be noted that NC charts do not include the 16 Hz and 31.5 Hz

octave bands. Although HVAC equipment manufacturers typically do not

provide data in these bands (because it is very difficult to obtain reliably), these

octave bands do effect the acoustical comfort of the occupied space.

Nevertheless, these octave bands can be measured in a space that is already

built and may provide useful diagnostic information.

NC-

-15

NC-

-20

NC-

-25

NC-

-30

NC-

-35

NC-

-40

NC-

-45

NC-

-50

NC-

-55

NC-

-60

NC-

-65

sound pressure, dB ref 20

sound pressure, dB ref 20 m

mPa

octave

octave-

-band frequency, Hz

band frequency, Hz

63 125

125 250

250 500

500 1,000

1,000 2,000

2,000 4,000

4,000 8,000

8,000

Noise Criteria (NC) Curves

approximate threshold for

hearing continuous noise

Figure 26

22 TRG-TRC007-EN

notes

period two

Sound Perception and

Rating Methods

The following steps describe how to calculate an NC rating.

1Plot the octave-band sound-pressure levels on the NC chart.

2The highest curve crossed by the plotted data determines the NC rating.

Although the NC curves are popular and easy to use, they do have a few

shortcomings. Specifically, they do not account for the tonal nature and relative

magnitude of each octave band. Figure 27 shows octave-band data measured in

an open-plan office space and plotted on an NC chart. The resulting value, NC-

39, is generally considered to be acceptable for this type of environment. Notice

that this NC value is set by the 63 Hz octave band and the sound drops off

quickly in the higher octave bands.

In this particular example, sound generated by the air-handling unit travels

through the ductwork, breaks out through the duct walls, and radiates into the

office area. To achieve the desired NC level, two layers of sheet rock were added

to the exterior surface of the duct to block the low-frequency sound.

Unfortunately, because high-frequency sounds are much more easily

attenuated than low-frequency sounds, the upper octave bands are now over-

attenuated.

Although an objective analysis deems the resulting NC-39 sound level

acceptable in this type of open-plan office space, most listeners in the space

would probably perceive this unbalanced spectrum as having an annoying

rumble.

NC-

-15

NC-

-20

NC-

-25

NC-

-30

NC-

-35

NC-

-40

NC-

-45

NC-

-50

NC-

-55

NC-

-60

NC-

-65

sound pressure, dB ref 20

sound pressure, dB ref 20 m

mPa

octave

octave-

-band frequency, Hz

band frequency, Hz

63 125

125 250

250 500

500 1,000

1,000 2,000

2,000 4,000

4,000 8,000

8,000

Noise Criteria (NC) Curves

NC-

-39

Figure 27

TRG-TRC007-EN 23

period two

Sound Perception and

Rating Methods

notes

Room criteria (RC) curves are similar to NC curves in that they are used to

provide a rating for sound-pressure levels in indoor environments. The major

difference is that RC curves give an additional indication of sound character.

As discussed in the previous example, sound spectrums can be unbalanced in

ways that result in poor acoustical quality. Too much low-frequency sound

results in a rumble, and too much high-frequency sound produces a hiss.

RC curves provide a means of identifying these imbalances. An RC rating

consists of two descriptors. The first descriptor is a number representing the

speech interference level (SIL) of the sound. The second descriptor is a letter

denoting the character of the sound as a subjective observer might describe it.

nN identifies a neutral or balanced spectrum

nR indicates a “rumble”

nH represents a “hiss”

nRV denotes perceptible vibration

Calculating an RC value from octave-band sound-pressure data is not quite as

easy as determining an NC value, but it is still fairly simple. The RC value is

based on sound-pressure data from the eight octave bands between 31.5 Hz

and 4,000 Hz. Note that these are different than the octave bands included on

the NC chart.

Room Criteria (RC) Curves

approximate threshold for

hearing continuous noise

sound pressure, dB ref 20

sound pressure, dB ref 20 m

mPa

octave

octave-

-band frequency, Hz

band frequency, Hz

31.5

31.5 63

63 125

125 250

250 500

500 1,000

1,000 2,000

2,000 4,000

4,000

RC-

-25

RC-

-30

RC-

-35

RC-

-40

RC-

-45

RC-

-50

Figure 28

24 TRG-TRC007-EN

notes

period two

Sound Perception and

Rating Methods

The following steps describe how to determine an RC rating.

1Plot the octave-band sound-pressure levels on the RC chart.

2Determine the SIL by calculating the arithmetic average of the sound-

pressure levels in the 500 Hz, 1,000 Hz, and 2,000 Hz octave bands. In this

example, the arithmetic average of 38 dB, 31 dB, and 24 dB is 31 dB.

3Draw a line (C) with a slope of –5 dB per octave that passes through the

calculated SIL at the 1,000 Hz octave band. This is the reference line for

evaluating the character of the sound spectrum.

4Between 31.5 Hz and 500 Hz, draw a line (D) that is 5 dB above the reference

line (C). Between 1,000 Hz and 4,000 Hz, draw a second line (E) that is 3 dB

above the reference line (C). These two boundary lines (D and E) represent

the maximum permitted deviation to receive a neutral (N) rating.

5Judge the character of the sound quality by observing how the sound

spectrum deviates from the boundary lines drawn in Step Four. Use the

following criteria to choose the appropriate letter descriptor that

characterizes the subjective quality of the noise.

nNeutral (N): The sound level in each of the octave bands between

31.5 Hz and 500 Hz is below line D, and the sound level in each of

the octave bands between 1,000 Hz and 4,000 Hz is below line E.

nRumble (R): The sound level in any octave band between 31.5 Hz

and 500 Hz is above line D.

nHiss (H): The sound level in any octave band between 1,000 Hz and

4,000 Hz is above line E.

sound pressure, dB ref 20

sound pressure, dB ref 20 m

mPa

octave

octave-

-band frequency, Hz

band frequency, Hz

31.5

31.5 63

63 125

125 250

250 500

500 1,000

1,000 2,000

2,000 4,000

4,000

RC-

-25

RC-

-30

RC-

-35

RC-

-40

RC-

-45

RC-

-50

Room Criteria (RC) Curves

Figure 29

TRG-TRC007-EN 25

period two

Sound Perception and

Rating Methods

notes nPerceptible vibration (RV): The sound level in the octave bands

between 16 Hz and 63 Hz falls in the shaded regions (A and B).

These regions indicate sound-pressure levels at which walls and

ceilings can vibrate perceptibly—rattling cabinet doors, pictures,

ceiling fixtures, and other furnishings in contact with them.

Region A: High probability that noise-induced vibration levels in

lightweight wall and ceiling constructions will be felt. Anticipate

audible rattles in light fixtures, doors, windows, and so on.

Region B: Noise-induced vibration levels in lightweight wall and

ceiling constructions may be felt. Slight possibility of rattles in light

fixtures, doors, windows, and so on.

The RC rating for the sound is the numerical SIL value calculated in Step Two

and the letter descriptor determined in Step Five.

26 TRG-TRC007-EN

notes

period two

Sound Perception and

Rating Methods

If we plot the acoustical data for our example office space on the RC chart, we

find that it results in a rating of RC-31(R). The SIL is 31 and the sound-pressure

levels in the 63 Hz and 125 Hz octave bands are above line D, indicating a

rumble characteristic of the sound.

This time, our objective and subjective analyses lead to the same conclusion.

Although the space is quiet enough, the background noise is perceived as

having a rumble. A sound spectrum that falls in the RC neutral category would

be judged as excellent by most observers. It is this conformity of analysis

results that makes the RC rating method a better tool than the other single-

number descriptors for specifying acoustical requirements indoors. Despite

the advantages of the RC rating system, it is less widely used than other single-

number descriptors.

Finally, accurate determination of sound-power levels for the 16 Hz and 31.5 Hz

octave bands requires a very large reverberant room. Most HVAC equipment

manufacturers do not provide sound data in these two octave

bands due the cost of constructing such a large test room and the difficulty in

qualifying it. This makes it difficult to predict sound-pressure levels in these

octave bands.

sound pressure, dB ref 20

sound pressure, dB ref 20 m

mPa

octave

octave-

-band frequency, Hz

band frequency, Hz

31.5

31.5 63

63 125

125 250

250 500

500 1,000

1,000 2,000

2,000 4,000

4,000

RC-

-25

RC-

-30

RC-

-35

RC-

-40

RC-

-45

RC-

-50

Room Criteria (RC) Curves

RC-

-31 (R)

31 (R)

Figure 30

TRG-TRC007-EN 27

period two

Sound Perception and

Rating Methods

notes

The phon is another descriptor used to indicate loudness with a single number.

The loudness of a sound, expressed in phons, is equal to the sound- pressure

level of a standard sound, at 1,000 Hz, that is considered equally loud. For

example, a sound pressure level of 40 dB at 1,000 Hz is considered to have a

loudness of 40 phons. Any sound that falls on this same loudness curve, at any

frequency, would also be described as having a loudness equal to 40 phons.

While the phon scale is logarithmic, the sone is the linear equivalent to the

phon. In principle, the sone scale is linear when compared to the response of

the human ear. For example, two sones is twice as loud as one sone, and half

as loud as four sones.

While the phon and sone scales are not widely used, some HVAC equipment,

primarily non-ducted fans and power ventilators, is still rated in sones. AMCA

Standard 301, Methods for Calculating Fan Sound Ratings from Laboratory Test

Data, provides a method for calculating the sone rating from octave-band data.

Use caution when comparing equipment based on sones. Multiple methods

exist for calculating a sone rating, and they provide different results.

frequency, Hz

50 100

100 200

200 500

500 1,000

1,000 2,000

2,000 5,000

5,000

20 10,000

10,000

120

100

sound pressure, dB ref 20

sound pressure, dB ref 20 m

mPa

Phon and Sone

100

110

120

loudness level (

loudness level (phons

phons)

)

Figure 31

28 TRG-TRC007-EN

notes

period two

Sound Perception and

Rating Methods

Octave-Band Rating Method

A more useful method of rating sound level is to use the octave bands

discussed earlier. While octave-band data is not as simple to interpret as a

single-number rating, it provides much more information about the character of

the sound.

Both sound-power levels and sound-pressure levels can be presented in

octave-band format. When equipment sound data is provided in terms of

sound-power level in each octave band, an “apples to apples” comparison

can be made between various pieces of equipment. In addition, this sound-

power data can be converted to sound-pressure levels when the details of the

environment are known. This type of analysis will be discussed further in

Period Three.

Sound-pressure levels in each octave band, whether predicted from sound-

power data or measured in an existing environment, reveal much more about

the character of sound than any of the single-number rating methods. It is

important to note that any of the single-number ratings described in this section

can be calculated from octave-band sound-pressure data. However, octave-

band data cannot be derived from any of the single-number ratings.

Octave-Band Rating Method

octave

band

octave

band

center

frequency (Hz)

125

250

500

1,000

2,000

4,000

8,000

center

frequency (Hz)

125

250

500

1,000

2,000

4,000

8,000

equipment

sound power

(dB ref 10-12 W)

103

104

100

101

equipment

sound power

(dB ref 10-12 W)

103

104

100

101

sound pressure

in the space

(dB ref 20 m

mPa)

sound pressure

in the space

(dB ref 20 m

mPa)

Figure 32

TRG-TRC007-EN 29

notes

period three

Acoustical Analysis

The primary acoustical design goal for an HVAC system is to achieve a

background noise level that is quiet enough so that it does not interfere with

the activity requirements of the space and is not obtrusive in sound quality.

What is considered “acceptable” varies dramatically with the intended use

of the space. Obviously, a factory has less stringent acoustical requirements

than a church, while an office has a different set of requirements altogether.

Therefore, the acoustical design goal depends on the required use of the space.

period three

Acoustical Analysis

Fundamentals of HVAC Acoustics

Figure 33

30 TRG-TRC007-EN

notes

period three

Acoustical Analysis

Setting a Design Goal

The first step of an acoustical design is to quantify the goal. Period Two

introduced several single-number descriptors that designers commonly use

to define the acoustical design goal for a space. Each descriptor has its

advantages its and drawbacks.

In general, when defining the acoustical design goal for an interior space, either

an NC value or an RC value is used. To aid HVAC system designers, the

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

(ASHRAE) recommends target RC ratings for various types of spaces, and

encourages the use of the RC rating method whenever the space requires a

neutral, unobtrusive background sound. Figure 34 includes an excerpt from the

ASHRAE Handbook—Applications (Table 43 in Chapter 46 of the 1999 edition).

As mentioned earlier, A-weighting is also used in many hearing-protection

safety standards for industrial environments. These standards generally take

the form of a maximum A-weighted sound-pressure level at a specified

distance from the piece of machinery.

Setting a Design Goal

room type

hotels/motels

guest rooms

banquet rooms

libraries

office buildings

open plan offices

public lobbies

performing arts

theaters

practice rooms

schools

small classrooms

large classrooms

room type

hotels/motels

guest rooms

banquet rooms

libraries

office buildings

open plan offices

public lobbies

performing arts

theaters

practice rooms

schools

small classrooms

large classrooms

RC(N) criteria

25 to 35

30 to 40

40 to 45

25 max

35 max

40 max

35 max

RC(N) criteria

25 to 35

30 to 40

40 to 45

25 max

35 max

40 max

35 max Figure 34

TRG-TRC007-EN 31

period three

Acoustical Analysis

notes

When defining the acoustical design goal for an outdoor environment, to meet

a local noise ordinance for example, the A-weighted scale is typically used. This

generally takes the form of a maximum A-weighted sound-pressure level at the

lot line of the property.

More-sophisticated noise ordinances may specify maximum sound-pressure

levels for each octave band and possibly a restriction on other characteristics

of the sound. For example, a sound ordinance may define that a tone is present

when the sound-pressure level in any one-third octave band exceeds the

arithmetic average of the sound-pressure levels in the two neighboring one-

third octave bands by 5 dB or more.

Setting a Design Goal

Figure 35

32 TRG-TRC007-EN

notes

period three

Acoustical Analysis

Source–Path–Receiver Analysis

Achieving the desired acoustical characteristics in a space, however, requires

more than selecting an appropriate single-number descriptor. Including a

single-number descriptor in a HVAC system specification means that someone

must perform an acoustical analysis to determine if the proposed HVAC system

and equipment will satisfy the space acoustical requirements. To make such a

prediction, the analysis must convert the sound-power level of the source (the

fan in the air handler in this example) to the sound-pressure level in the

occupied space, assessing the effect of installation and environmental factors

along the way.

Sound that reaches the occupied space will be altered by ductwork, wall and

ceiling construction, room furnishings, and many other factors. The validity

of an acoustical analysis, therefore, depends on the analyst’s familiarity with

construction details.

Acoustical Analysis

air handler

(source)

air handler

(source)

ductworkductwork

ductwork diffuserdiffuser

diffuser

occupied space

(receiver)

occupied space

(receiver)

VAV boxVAV box

VAV box

Figure 36

TRG-TRC007-EN 33

period three

Acoustical Analysis

notes

Predicting the sound level in a given space requires making a model of the

system. A source–path–receiver model provides a systematic approach to

predict the acoustical characteristics in a space. As the name suggests, this

modeling method traces sound from the source to the location where we want

to predict the sound (the receiver). How the sound travels between the source

and the receiver, and everything it encounters as it travels along the way,

constitutes the path.

In the example shown in Figure 37, the source is the fan in the mechanical room.

The receiver is the person working in the adjacent office space. The supply duct

provides one of the paths for sound to travel from the source to the receiver.

Using such an analysis, the designer can determine the effect of the paths on

the sound emanating from the source, and can specify the maximum allowable

equipment sound power that will not exceed the sound-pressure target for

the space.

Source–Path–Receiver Model

receiver

path

source

Figure 37

34 TRG-TRC007-EN

notes

period three

Acoustical Analysis

The work, and art, of an acoustical analysis is in identifying and quantifying the

various paths that sound travels from the source to the receiver. There are

primarily three different types of sound paths.

nAirborne: This is a path where sound travels with, or against, the direction

of airflow. In a HVAC system, sound travels along this type of path through

the supply ductwork, return ductwork, or an open plenum.

nBreakout: This type of path is typically associated with sound breaking out

through the duct walls and into the space.

nTransmission: This is a path where sound travels through walls, floors,

and ceilings. In its simplest form, this path involves sound traveling directly

through the air from the source to the receiver.

Typical Sound Paths

IAirborne

KSound that travels through supply ductwork, return

ductwork, or an open plenum

KCan travel with or against the direction of airflow

IBreakout

KSound that breaks out through the walls of the supply

or return ductwork

ITransmission

KSound that travels through walls, floors, or ceilings

Figure 38

TRG-TRC007-EN 35

period three

Acoustical Analysis

notes

Sound can travel between a single source and the receiver along one or

multiple paths. In the case of an air-cooled chiller sitting on the roof of a

building, and a receiver located across a parking lot at the edge of the property,

sound travels along only one path.

Another example is a fan-coil unit installed under a window in an office. Sound

travels primarily along one path, from the fan-coil to the receiver in the same

room.

In other cases there may be several paths for sound to travel from a source

to the receiver. This particular example shows the paths associated with an

air handler that is installed in a mechanical equipment room adjacent to an

occupied space. Only one sound source is included in this analysis, the fan

located in the air handler. The receiver is the person working in the office.

The sound travels from the source to the receiver along four separate paths:

Examples of a Single Sound Path

fan

fan-

-coil

coil

air

air-

-cooled chiller

cooled chiller

Figure 39

Example of Multiple Sound Paths

wall transmission

return

airborne

airborne supply

supply

airborne

supply

breakout

source

Figure 40

36 TRG-TRC007-EN

notes

period three

Acoustical Analysis

1Supply airborne through the supply ductwork and diffusers and into

the space

2Supply breakout as the sound travels through the walls of the supply

ductwork, through the ceiling tile, and into the space

3Return airborne through the air-handler intake, return ductwork and grilles,

and into the space

4Wall transmission as the sound travels through the adjoining wall and

into the space

These paths are typical of most centralized air-handling equipment, including

packaged rooftop and self-contained air conditioners. Most other equipment

types have a subset of these paths.

There are a few important points to remember when identifying sources and

paths for a source–path–receiver acoustical analysis.

nOne piece of equipment may contain several sound sources. For example,

a packaged rooftop air conditioner (shown in Figure 41) contains supply and

exhaust (or return) fans, compressors, and condenser fans.

nSound may travel from a single source to the receiver along multiple paths.

This was demonstrated with the previous example.

nThe total sound heard by the receiver is the sum of all the sounds from

various sources that travel along several paths.

Identifying Sound Sources and Paths

IOne piece of equipment

may contain several

sound sources

ISound may travel from

source to receiver along

multiple paths

ITotal sound heard by the

receiver is the sum of all

sounds from all sources

and all paths

packaged rooftop

air conditioner

Figure 41

TRG-TRC007-EN 37

period three

Acoustical Analysis

notes

Sound-Path Modeling

When all the paths have been identified, they can be individually modeled to

determine the contribution of each to the total sound heard by the receiver.

Sound-path modeling studies how sound from a source changes on its way

to a receiver. The pieces that make up the path from source to receiver can be

called elements of the path.

Returning to the air-handler example, one path that sound travels from the air-

handling unit (source) to the person in the office (receiver) is to follow the

conditioned air supplied to the space. In addition to the source and receiver,

the elements of this path include the components of the air distribution system,

such as straight pieces of duct, possibly duct silencers, elbows, junctions, and

diffusers. The path also includes the acoustical characteristics of the occupied

space, such as its size, floor coverings, furnishings, and wall construction.

receiver

source

Modeling Sound Paths

diffuser

straight duct

elbow

Figure 42

38 TRG-TRC007-EN

notes

period three

Acoustical Analysis

As mentioned previously, the total sound heard by the receiver is the sum of

sounds from multiple sources, following multiple paths. After each path is

modeled to determine its contribution to the sound-pressure level at the

receiver location, the paths must be summed to complete the model. While

separating the individual paths is necessary for modeling, a secondary benefit

is that the magnitude of the various paths can be compared.

In this example, sound travels from a single source to the receiver along four

separate paths: supply airborne, supply breakout, return airborne, and

transmission through the adjacent wall. By modeling these four paths

independently, you can see that the supply airborne path contributes to the

total sound-pressure level in the space much more than the other three paths.

In fact, when the sounds due to all four paths are logarithmically summed, the

total sound heard by the receiver is nearly the same as the sound due to the

supply airborne path alone.

This would indicate that, if the sound-pressure level in the space is too high,

the designer should focus first on reducing the sound due to the supply

airborne path. Reducing the sound due to the return airborne path, without

addressing the supply airborne path, would have no effect on the total sound-

pressure level heard in the space.

sound pressure, dB ref 20

sound pressure, dB ref 20 m

mPa

octave

octave-

-band frequency, Hz

band frequency, Hz

63 125

125 250

250 500

500 1,000

1,000 2,000

2,000 4,000

4,000 8,000

8,000

Example of Multiple Sound Paths

return

airborne

airborne wall

wall

transmission

supply

breakout

total

supply

airborne

Figure 43

TRG-TRC007-EN 39

period three

Acoustical Analysis

notes

Theoretical equations aid the analysis of some path elements, but prediction

equations based on test data and experience prevail. For example, an acoustical

lab may have measured the attenuation and regenerated sound from a number

of different types of duct elbows at various airflow rates. Data recorded from

tests is used to generate an equation that can be used to model the test data.

ASHRAE collected and developed numerous prediction equations for path

components in HVAC systems, and subsequently published them in their

Algorithms for HVAC Acoustics handbook. Similar information can be found in

the National Environmental Balancing Bureau (NEBB) publication titled Sound

and Vibration Design and Analysis.

ASHRAE algorithms are widely used and generally provide good results. When

using the algorithms, it should be remembered that they mainly come from test

data. As a result, if they are used to extrapolate beyond the test conditions, the

accuracy of the algorithms will diminish.

Algorithms for Sound-Path Modeling

Figure 44

40 TRG-TRC007-EN

notes

period three

Acoustical Analysis

Solving these algorithms manually can be tedious and time consuming,

especially when one or more paths need further attenuation and the

calculations have to be repeated. Fortunately, computer software tools are

available to spare analysts from the calculation-intensive equations.

Also, computer programs make it easier to perform tradeoff, or “what if?,”

analyses. Examples may include determining the effects of using a duct

silencer, changing the construction of the equipment-room wall, adding

absorptive materials to a ceiling, or placing a barrier wall between an outdoor

sound source and the property lot line.

Computerized Analysis Tools

Figure 45

TRG-TRC007-EN 41

period three

Acoustical Analysis

notes

Terms Used in Sound-Path Modeling

This section introduces several terms that are fairly specific to the science of

acoustics.

Attenuation refers to the reduction in sound level as sound travels along the

path from a source to a receiver. It is typically used to refer to the reduction

of sound as it travels through a duct system. Straight ducts, elbows, junctions,

and silencers are examples of elements that attenuate sound.

Regenerated sound results from components of the duct system that create

turbulence in the air stream. Turbulence is caused by an abrupt change in

airflow direction or velocity with a corresponding static-pressure loss.

Regenerated sound increases with air velocity or when the air is forced to

make sharp turns. Elbows, junctions, diffusers, silencers, and dampers are all

examples of elements that regenerate sound.

Notice that some elements can both attenuate and regenerate sound. For

example, as air makes a 90-degree turn in a rectangular duct elbow, some

of the sound is reflected back upstream, attenuating the airborne sound

downstream of the elbow. At the same time, however, the turbulence created

by the air turning the sharp corner causes some regenerated sound.

Attenuation and Regeneration

supply

duct

return

duct

diffuser

heat pump

(source)

return

return-

-air

air

grille

Figure 46

42 TRG-TRC007-EN

notes

period three

Acoustical Analysis

The total sound energy that strikes a surface (Wi) is either reflected (Wr),

absorbed by the material (Wa), or transmitted through the material (Wt).

A material provides a barrier to the incident sound energy (Wi) when it reduces

the amount of sound energy that is transmitted through the material (Wt). There

are a number of factors that affect the amount of sound transmitted through the

wall, including the type and thickness of material, frequency of the sound, and

quality of construction.

Materials that are dense (such as masonry block or wallboard) or stiff (such as

glass) are generally better at reducing transmitted sound than materials that

are lightweight or flexible. Increasing the thickness of a material reduces the

amount of sound transmitted through it. Finally, the ability of a material to

reduce transmitted sound depends on frequency. High-frequency sound is

more easily reduced than low-frequency sound.

incident

sound energy

sound energy W

transmitted

sound energy

reflected

sound energy

absorbed

sound energy

Sound Transmission

Figure 47

TRG-TRC007-EN 43

period three

Acoustical Analysis

notes

The ability of a material to reduce transmitted sound is most commonly

referred to in terms of its insertion loss, noise reduction, or transmission loss.

Insertion loss and noise reduction are both based on actual sound-pressure

measurements and are expressed in terms of dB reduction.

Insertion loss (IL) is the difference in sound pressure measured in a single

location with and without a noise-control device located between the source

and receiver. Using the air-handler example (Figure 42), assume there is a door

installed in the wall separating the equipment room from the office space. The

difference in the sound pressure measured in the occupied space with the door

open versus with the door closed is the IL of the door.

Noise reduction (NR) is the difference between sound-pressure

measurements taken on each side of a barrier. For example, the NR for this

same door can be determined by measuring the sound-pressure level inside the

office space, with the door closed, and on the other side of the door inside the

equipment room. The difference in these measurements is the NR of the door.

Transmission loss (TL) is proportional to the ratio of the sound-power level

on the receiver side of a barrier to the sound-power level on the source side.

Using the same door example, the transmission loss of the door is determined

by the manufacturer by taking measurements in a special laboratory and

expressing the results as sound power. It is also expressed in terms of dB

reduction.

Sound Transmission

IInsertion loss (IL)

INoise reduction (NR)

ITransmission loss (TL)

Figure 48

44 TRG-TRC007-EN

notes

period three

Acoustical Analysis

Absorptive materials work by converting acoustical energy into heat energy.

The absorbed energy (Wa) is the portion of the incident sound energy (Wi)

that is neither transmitted through the material nor reflected off the material.

The absorptivity of a material depends on several factors, including thickness,

frequency of the sound, and whether there is a reflective surface located behind

the absorptive material. Materials that are porous (such as open cell foam) or

fibrous (such as fiberglass insulation) are more absorptive than materials that

are smooth and dense (such as sheet metal or gypsum board). Increasing the

thickness of a material, and installing a reflective surface behind the material,

both increase its absorptivity. It is also important to note that absorption is

dependent on frequency. High-frequency sound is more easily absorbed than

low-frequency sound because it has a shorter wavelength and more cycles

occur within the thickness of the absorptive material.

The absorptivity of a material is typically described in terms of an absorption

coefficient. The absorption coefficient is the ratio of sound energy absorbed

by the material to the sound energy incident upon the surface of the material.

Preferably, absorption coefficients are reported for each octave band, but may

also be expressed in terms of a single Noise Reduction Coefficient (NRC).

The NRC is simply the arithmetic average of the absorption coefficients for the

250, 500, 1,000 and 2,000 Hz octave bands.

Absorption

incident

sound energy

transmitted

sound energy

reflected

sound energy

absorbed

sound energy

Figure 49

TRG-TRC007-EN 45

period three

Acoustical Analysis

notes

Finally, some of the incident sound energy (Wi) bounces off of (or is reflected

from) the material. Reflected sound becomes especially important when the

sound source and the receiver are located in the same room.

Consider a mechanical equipment room that contains a water chiller, pumps,

and other sound sources. Often the walls of the equipment room are

constructed of masonry—either cement block or poured concrete. Neither of

these materials absorb or transmit very much of the incident sound energy,

so most of it is reflected back into the room. The reflected sound adds to the

sound coming from the source, greatly increasing the sound level in the room.

The best way to reduce reflected sound is to add an absorptive material to as

much of the walls, floor, and ceiling as possible.

On occasion, reducing reflected sound may also lower the sound levels in

adjacent spaces. Using the equipment room example, reducing the reflected

sound energy lowers the sound level in the equipment room. Given a fixed

transmission loss for the walls, this will result in a decrease in sound that

travels to the adjacent space. Said another way, if it is quieter in the equipment

room, it will be quieter in the adjacent spaces.

Reflected Sound

Figure 50

46 TRG-TRC007-EN

notes

period three

Acoustical Analysis

Receiver sound correction, also called room effect, is the relationship

between the sound energy (sound power) entering the room and the sound

pressure at a given point in the room where the receiver hears the sound.

This reduction in sound is due to a combination of effects, including distance

and the absorptive and reflective properties of the surrounding surfaces.

In an outdoor environment, such as a field or parking lot, the absorption of

sound is nearly perfect. Sound leaves the source in all directions and

diminishes as it travels away from the source. Only the portion of the sound

that travels in a direct line from the source ever reaches the receiver. In this

environment, the receiver sound correction is mainly a function of distance

between the source and receiver.

In contrast, sound entering a room bounces off walls and other surfaces.

Therefore, the receiver will hear sound reflecting off the surfaces, as well as the

sound coming directly from the source. The amount of sound that reaches

the receiver is dependent on the size of the room and the absorptivity and

reflectivity of the surfaces in the room. For example, in a completely “hard”

room (with concrete walls and floors and no furnishings) the room effect is very

small. Conversely, in a “soft” room (with carpeted floors and wall coverings)

the room effect can be quite substantial. Receiver sound correction will nearly

always result in a reduction in sound level in each octave band.

Sound spreading refers to the reduction of sound energy as a listener moves

away from the sound source. It is a factor in room acoustics and, typically, is the

primary factor in outdoor sound calculations.

Receiver Sound Correction

Figure 51

TRG-TRC007-EN 47

notes

period four

Equipment Sound Rating

As explained in Period One, sound pressure can be directly measured,

however, sound power cannot. Because sound pressure is influenced by the

surroundings, the most accurate sound data that can be provided for a piece of

equipment is sound power.

Sound-power levels are determined by measuring sound-pressure levels in an

environment with known acoustical characteristics, and adding back any effects

attributed to the surroundings.

period four

Equipment Sound Rating

Fundamentals of HVAC Acoustics

Figure 52

48 TRG-TRC007-EN

notes

period four

Equipment Sound Rating

Fields of Measurement

To measure sound pressure correctly, it is important to understand the behavior

of sound in various environments, or fields.

In theory, a free field is a homogeneous, isotropic medium that is free from

boundaries. In practice, an example of a free field over a reflecting plane would

be a large open area void of obstructions, like a parking lot or meadow.

An ideal sound source, that is, one that radiates sound equally in all directions,

placed in a free field generates sound-pressure waves in a spherical pattern.

At equal distances from the source, the sound pressure is same in all directions.

As the sound waves travel farther away from the source, the area of the sphere

increases. Doubling of the distance from the source spreads the sound over

four times as much surface area.

Free Field

source

Figure 53

TRG-TRC007-EN 49

period four

Equipment Sound Rating

notes

This type of relationship between distance and surface area provides the

following simple mathematical model for estimating how sound will change

as the distance from the source increases.

Lp2 = Lp1 - 20 log10 [r2 / r1]

where,

Lp2 = sound-pressure level at distance r2

Lp1 = sound-pressure level at distance r1

r1 = distance from the source where Lp1 was measured

r2 = distance from the source to where the sound pressure (Lp2) is desired

Using this expression, it can be shown that doubling the distance from the

sound source results in a 6 dB reduction in the sound-pressure level. This is a

handy fact to know when making estimates of outdoor sound levels.

Lp2 = Lp1 - 20 log10 [2 /1] = Lp1 - 6

Distance Correction in a Free Field

Lp2 = Lp1 – 20 log10

Figure 54

50 TRG-TRC007-EN

notes

period four

Equipment Sound Rating

In practice, this equation is commonly used to determine how loud a piece of

equipment will be at a given distance. For example, the manufacturer of an air-

cooled chiller lists the sound-pressure level of the chiller as 95 dB at a distance

of 30 ft (9.1 m) from the chiller. This equation can be used to estimate the

sound-pressure level at a lot line, which is 120 ft (36.6 m) from the chiller. In this

example, the sound-pressure level at the lot line is 83 dB.

Lp2 = 95 dB – 20 log10 [120 ft / 30 ft] = 83 dB

(Lp2 = 95 dB – 20 log10 [36.6 m / 9.1 m] = 83 dB)

This equation is only valid for sound pressure. It cannot be used to convert

sound power (Lw) to sound pressure (Lp). Also, it is only valid in a free-field

environment.

Distance Correction in a Free Field

2= 120 ft

= 120 ft

(36.6 m)

1= 30 ft

= 30 ft

(9.1 m)

Lp1

p1 = 95 dB

= 95 dB L

Lp2

p2 = 83 dB

= 83 dB

air

air-

-cooled chiller

cooled chiller

lot

line

Figure 55

TRG-TRC007-EN 51

period four

Equipment Sound Rating

notes

The near field is an area adjacent to the source where sound does not behave

as it would in a free field. Most sound sources, including all HVAC equipment,

do not radiate sound in perfectly spherical waves. This is due to the irregular

shape of the equipment and different magnitudes of sounds radiating from the

various surfaces of the equipment. These irregularities cause pressure-wave

interactions that make the behavior of the sound waves unpredictable.

Sound-pressure measurements should not, therefore, be made in the near field.

The size of the near field is dependent on the type of source and dimensions

of the equipment.

Near Field

near

field

near

field

source

Figure 56

52 TRG-TRC007-EN

notes

period four

Equipment Sound Rating

A reverberant field is nearly the opposite of a free field. Reverberant fields

exist in rooms with reflective walls, floors, and ceilings. When a sound source is

placed in an enclosed room, the sound waves from the source bounce back and

forth between the reflective walls many times. This can create a uniform, or

diffuse, sound field. In a perfectly reverberant room, the sound-pressure level is

equal at all points within the room.

Special reverberant rooms are designed, built, and qualified for the purpose of

measuring the sound emitted by a piece of equipment. This type of facility will

be discussed later in this period.

Reverberant Field

source

reflective

walls

reflective

walls

Figure 57

TRG-TRC007-EN 53

period four

Equipment Sound Rating

notes

Most rooms in buildings are somewhere between a free field and a reverberant

field environment. Called a semireverberant field, these spaces have some

characteristics of both free field and reverberant field environments. The walls,

floor, and ceiling prevent the sound from behaving as it would in a free field.

These surfaces are not, however, perfectly reflective. Some of the sound is

reflected by these surfaces, but a portion of the sound is absorbed or

transmitted.

An understanding of how sound behaves in a semireverberant field is

important when taking sound measurements. The characteristics of the sound

field change with distance when a small sound source is placed in the center

of a room. Close to the source, in the near field, sound measurement is

unpredictable. Near the wall, in the reverberant field, the reflected sound begins

to add to the sound coming directly from the source. The reduction in sound

level due to the distance from the source tends to be cancelled out by the

addition of the sound reflecting off the wall. This results in a near-constant

sound-pressure level near the wall.

In the semireverberant field, sound behaves similarly to how it would in a free

field. The sound level will decrease as the distance from the source increases,

but not as much as it would in an ideal free field.

The construction of the room plays a significant role in determining what

portion of the room behaves as a reverberant field, and what portion behaves

as a semireverberant field. Small rooms with hard, reflective surfaces behave

similarly to reverberant rooms. This description often fits a mechanical

equipment room that is constructed of concrete and is small with respect to

the size of the sound source.

Semireverberant Field

semireverberant

field

semireverberant

field

wallwall

wall

source

near

field

near

field

Figure 58

54 TRG-TRC007-EN

notes

period four

Equipment Sound Rating

HVAC Equipment Sound Rating

As mentioned earlier, because sound pressure is influenced by the

surroundings, often the best way for an equipment manufacturer to provide

sound data is to provide sound-power levels.

Sound-power levels for many types of HVAC equipment are determined in an

acoustics laboratory, usually by the manufacturer. Sound-power levels are

determined by measuring sound-pressure levels in a test facility with known

acoustical characteristics, and adding back any environmental effects attributed

to the surroundings. Formal written standards qualify such test facilities and

methods, in order to promote uniformity of data between different

manufacturers across the industry. This allows for objective comparisons of

similar equipment.

The two most common methods of determining the sound power of HVAC

equipment are the reverberant-room and free-field methods.

Rating HVAC Equipment

Figure 59

TRG-TRC007-EN 55

period four

Equipment Sound Rating

notes

The most common test method for HVAC equipment is the reverberant-room

method. The objective of a reverberant room is to create a uniform, or diffuse,

sound field by reflecting and mixing the sound waves. The walls, floor, and

ceiling of the reverberant room are hard, in order to cause multiple reflections

of sound waves. In this environment, the sound pressure is essentially the same

at all locations within the room.

Sound-pressure levels are measured in the reverberant room and used to

calculate sound-power levels for the piece of equipment. The reverberant-room

method is commonly used to determine the sound power of fans, air handlers,

compressors, in-room air conditioners, terminal equipment (such as fan-coils

and VAV boxes), and diffusers.

Reverberant-Room Method

Figure 60

56 TRG-TRC007-EN

notes

period four

Equipment Sound Rating

The free-field method is commonly used for HVAC equipment that is too

large to be tested in a reverberant room. This includes water chillers, cooling

towers, and the outdoor sound from packaged rooftop air conditioners and

air-cooled condensers.

This type of equipment is placed on a hard surface in an anechoic (or sound-

absorbing) room or on a large parking lot outdoors. This approximates the

characteristics of sound in a free field above a reflecting plane. That is, the

sound-pressure waves travel evenly in a hemispherical pattern, away from the

equipment. Sound-power levels are determined by measuring sound-pressure

levels on an imaginary hemispherical surface surrounding the equipment.

Free-Field Method

Figure 61

TRG-TRC007-EN 57

period four

Equipment Sound Rating

notes

Formal standards are written by industry organizations to promote uniformity

of data between different manufacturers. Air-Conditioning & Refrigeration

Institute (ARI), the Air Movement and Control Association International (AMCA),

and the American Society of Heating, Refrigerating, and Air-Conditioning

Engineers (ASHRAE) are three such organizations.

However, methods for predicting sound data may still vary from manufacturer

to manufacturer, hampering comparisons of similar equipment. Done properly,

collecting accurate sound data for an entire line of products is an expensive and

time-consuming endeavor. A single product line may consist of many models,

and each model may come in a range of sizes with various options that alter the

sound generated by the equipment.

(The ARI, AMCA, and ASHRAE logos are registered trademarks of their

respective organizations.)

Industry Standards

Figure 62

58 TRG-TRC007-EN

notes

period four

Equipment Sound Rating

One of the best examples to demonstrate the complexity of gathering complete

and accurate sound data is air-handling equipment. This involves any type of

HVAC equipment that contains a fan and is used to condition and move air

through a duct system.

Consider that each fan in an air-handling product line may run at multiple

speeds and within a range of flow and static-pressure conditions. The fact

that each type of fan (forward-curved, backward-inclined, and so forth) has a

different operating characteristic further complicates testing.

A fan performs differently inside an air handler than it does in a stand-alone

application. The air-handler casing generally changes the airflow patterns at

the fan inlet and discharge openings, which can change the sound power for

a given flow and static-pressure condition.

Additionally, an air handler may have only one source of sound or it may have

several. For example, a ducted, packaged rooftop air conditioner has multiple

sources. It contains a supply fan, refrigeration compressors, air-cooled

condenser fans, and possibly an exhaust or return fan.

Finally, sound may leave the air handler in multiple ways. In the case of the

indoor air handler, sound travels along with the conditioned air into the supply

duct system. It also travels back out the return-air inlet, against the direction of

airflow. Finally, sound is also radiated by the casing of the air handler into the

equipment room. In order to properly design the HVAC system, the designer

needs to know the sound power from all of these paths.

Ducted Air-Handling Equipment

coilcoil

coil

supply fansupply fan

supply fan outdoor-air

intake

outdoor

outdoor-

-air

air

intake

return-air

inlet

return

return-

-air

air

inlet

supply-air outletsupply

supply-

-air outlet

air outlet

Figure 63

TRG-TRC007-EN 59

period four

Equipment Sound Rating

notes

In order to isolate these different paths, the air handler must be tested using

a number of different configurations. For example, to determine the portion

of the sound that is discharged with the air into the supply ductwork, the air

handler is installed outside of the reverberant room with the supply air ducted

into the room.

In order to determine the sound that travels back down the return duct in a

ducted inlet application, the air handler is again installed outside of the

reverberant room, but the return air duct is connected to the reverberant room.

In order to determine the portion of the sound that is radiated by the casing,

the air handler is installed inside the reverberant room with both the return and

supply air ducted outside of the room.

Finally, in cases where the return air travels back to the air handler through

an open plenum and into the open equipment room, the combined “free”

(or open) inlet plus casing radiated sound level must be known. Therefore,

the air handler is installed inside the reverberant room with a free inlet and

the supply air is ducted to outside of the room.

Air Handler Test Configurations

ducted

discharge

discharge ducted

ducted

inlet

unducted

unducted inlet

inlet

plus casing

radiated

casing

radiated

Figure 64

60 TRG-TRC007-EN

notes

period four

Equipment Sound Rating

Historically, there have been several methods used to generate sound data for

air handling equipment.

Though increasingly less common, there are still cases where fan-sound levels

are based on prediction equations such as the generic fan algorithm published

long ago by ASHRAE. Using this algorithm is much less costly than using an

acoustic test facility, but results in much less accurate data. In 1995, after tests

proved that sound levels predicted by the algorithm could vary from actual

measured readings by as much as 10 dB in a given octave band, ASHRAE

removed the fan prediction algorithm from the handbook, stating that:

The sound power generated by a fan performing at a given duty is best

obtained from manufacturers’ test data taken under approved test conditions.

[1999 ASHRAE Handbook–Applications, Chapter 46, page 4]

Another sometimes-used rating method measures actual sound data for the fan

by itself, then uses acoustical equations to predict the effects of the cabinet,

coils, filters, and other components that make up the air handler. These

prediction algorithms vary from manufacturer to manufacturer, and since they

are usually proprietary, it is difficult to judge their accuracy. This prevents

designers from effectively comparing data between manufacturers or applying

that data in an analysis.

AMCA Standard 300–1996, Reverberant Room Method for Sound Testing of

Fans, defines the test methodology for collecting fan-only sound-power data.

However, it has also been used to test entire air handlers. Just because the

data was “taken in accordance with AMCA 300” does not indicate whether

the sound data is for the fan only or for the entire air handler. This leaves the

designer to determine whether the data reflects the entire air handler or, as

intended, just the fan by itself.

Former Methods of Sound Testing

IASHRAE generic fan

algorithm

IMeasured fan-sound

data plus prediction

equations

IAMCA Standard 300

Figure 65

TRG-TRC007-EN 61

period four

Equipment Sound Rating

notes

The objective of ARI Standard 260–2001, Sound Rating of Ducted Air Moving

and Conditioning Equipment, is to deliver sound data that accurately represents

the acoustical impact of the air-handling equipment after it is installed.

This standard uses the reverberant-room method to measure the sound

generated by the entire air handler, not just the fan. As mentioned, a fan

performs differently inside an air handler than it does in a stand-alone

application. The air-handler casing generally changes the airflow patterns at

the fan inlet and discharge openings. This effect is the major reason for the

difference between fan-only sound data and the actual sound produced by

the air handler after it is installed.

To eliminate such inaccuracies, ARI 260 requires that the entire air handler be

tested in all of the configurations in which the equipment is commonly applied

in the field. This involves the four test configurations discussed in Figure 64,

as well as various combinations of options such as inlet and discharge

plenums, different types of filters, dampers, coils, and so forth.

Unlike the other methods, ARI 260 requires that secondary sound sources,

such as return or exhaust fans and compressors, be tested to determine their

acoustical impact on the air handler. Any secondary source that alters the

sound spectrum of the supply fan must be included in cataloged ratings.

In summary, ARI 260 addresses a wide range of air-handling equipment with a

consistent test method. It ensures accurate, verifiable sound data, and focuses

on the entire air handler in all of its common installation arrangements. ARI 260

enables designers to make equitable product comparisons, and to more

accurately predict sound-pressure levels for occupied spaces.

ARI Standard 260

IUses reverberant-

room method

ITests entire air

handler, not just fan

IAll common

configurations and

components included

IIncludes effects of

secondary sound

sources

Figure 66

62 TRG-TRC007-EN

notes

period four

Equipment Sound Rating

Depending on the type of equipment, sound ratings for HVAC equipment are

typically given as sound-power levels by octave band or as a single dBA rating.

Outdoor equipment, such as an air-cooled chiller or condensing unit, may be

rated in terms of A-weighted sound-pressure level (dBA) at a specific distance

from the equipment. This generally assumes a free-field environment and may

be useful for comparing equipment from various manufacturers. Nevertheless,

the sound-pressure level by octave band should still be available from the

manufacturer for use in an acoustical analysis.

As mentioned, air-handling equipment is typically rated in terms of sound-

power level per octave band. Typically, this data is given for the octave bands

from 63 Hz through 8,000 Hz. The availability of accurate, tested sound data for

HVAC equipment is vitally important to any acoustical analysis.

Sound Power by Octave Band

octave

band

octave

band

center

frequency (Hz)

125

250

500

1,000

2,000

4,000

8,000

center

frequency (Hz)

125

250

500

1,000

2,000

4,000

8,000

equipment

sound power

(dB ref 10-12 W)

103

104

100

101

equipment

sound power

(dB ref 10-12 W)

103

104

100

101

85 Figure 67

TRG-TRC007-EN 63

notes

We will now review the main concepts that were covered in this clinic on the

fundamentals of HVAC acoustics.

Period One explained some of the basic concepts of sound. Sound is the

audible emissions resulting from the vibration of molecules within an elastic

medium. It is generated at many different frequencies at the same time. Noise

is defined as unwanted, or obtrusive, sound.

Sound power and sound pressure are both terms that are used when describing

sound. Sound power is the acoustical energy emitted by the sound source and

is not affected by the environment. Sound pressure is a disturbance in the

atmosphere and can be measured directly. Its intensity is influenced not only by

the strength of the source, but also by the surroundings and the distance from

the source to the listener. Sound pressure is what our ears hear and what sound

meters measure.

period five

Review

Fundamentals of HVAC Acoustics

Figure 68

Review—Period One

ISound power

KAcoustical energy emitted by source

KUnaffected by the environment

KCorrelates to bulb wattage

ISound pressure

KPressure disturbance in atmosphere

KAffected by strength of source,

surroundings, and distance from source

KCorrelates to brightness in a particular

location

Figure 69

period five

Review

64 TRG-TRC007-EN

notes

period five

Review

Period Two discussed how the human ear perceives sound. As a selective

sensory organ, the human ear is more sensitive to high frequencies than to

low frequencies. The sensitivity of the human ear at a particular frequency also

changes with loudness.

Many single-number rating systems have been developed over the years and

each has its advantages and drawbacks. This clinic focused on some of the

more commonly used rating systems, including A-weighting, Noise Criteria

(NC), and Room Criteria (RC). A-weighting is typically used to describe the

sound in outdoor environments, and for determining whether or not hearing

protection is required in certain industrial environments. Noise Criteria and

Room Criteria are used to describe the sound in indoor environments. The RC

method has the added feature of describing the character, or quality, of the

sound, as well as its loudness. However, because most HVAC equipment

manufacturers do not have sound-power data for the 16 Hz and 31.5 Hz octave

bands, it is difficult to predict the sound-pressure levels in these octave bands.

Finally, both sound-power levels and sound-pressure levels can be described

using either full or one-third octave bands. Octave-band sound-power data is

commonly used for describing the sound generated by HVAC equipment. It can

also be used to describe the sound in either indoor or outdoor environments.

Review—Period Two

IA-weighting

KOutdoor environments

and hearing protection

INoise criteria (NC)

KIndoor environments

IRoom criteria (RC)

KIndoor environments

KAlso describes sound

character

IOctave-band data

sound pressure, dB ref 20

sound pressure, dB ref 20 m

mPa

octave

octave-

-band frequency, Hz

band frequency, Hz

31.5

31.5 63

63 125

125 250

250 500

500 1,000

1,000 2,000

2,000 4,000

4,000

chart

Figure 70

TRG-TRC007-EN 65

period five

Review

notes

Period Three walked through the steps of an acoustical analysis, including

setting the design target for the indoor or outdoor environment, and

performing a source–path–receiver analysis. This method of analysis traces

sound from the source to the location where we want to predict the sound

(the receiver). How the sound travels between the source and the receiver,

and everything it encounters as it travels along the way, constitutes the path.

Remember:

nOne piece of equipment may contain several sound sources.

nSound may travel from a single source to the receiver along multiple paths.

nThe total sound heard by the receiver is the sum of all the sounds from

various sources that travel along several paths.

Computer software analysis tools are available to aid in performing this type of

calculation-intensive analysis.

Review—Period Three

wall transmission

return

airborne

airborne supply

supply

airborne

supply

breakout

source

Figure 71

66 TRG-TRC007-EN

notes

period five

Review

Period Four introduced two common methods used by HVAC equipment

manufacturers to provide accurate sound data. Because sound pressure is

influenced by the surroundings, the most useful sound data that can be

provided for most pieces of equipment is sound power.

Sound-power levels are determined by measuring sound-pressure levels in an

environment with known acoustical characteristics, and adding back any effects

attributed to the surroundings. The most common method uses a special

acoustical testing facility called a reverberant room.

The availability of accurate, tested sound data for HVAC equipment is vitally

important to any acoustical analysis. ARI Standard 260–2001 is one example

of an industry standard for rating the sound level of equipment. This standard

addresses a wide range of air-handling equipment with a consistent test

method. It assures accurate, verifiable sound data, and focuses on the entire

air handler in all of its common installation arrangements. As with other

existing sound standards, ARI 260 enables designers to make equitable product

comparisons and to more accurately predict sound levels for occupied spaces.

Review—Period Four

Figure 72

TRG-TRC007-EN 67

period five

Review

notes

For more information, refer to the following references:

nAcoustics in Air Conditioning Applications Engineering Manual (Trane

literature order number FND-AM-5)

nTrane Acoustics Program (TAP™)

nASHRAE Handbook – Fundamentals (chapter 7, 2001)

nASHRAE Handbook – Applications (chapter 46, 1999)

nA Practical Guide to Noise and Vibration Control, ASHRAE, 1991

nApplication of Manufacturer’s Sound Data, ASHRAE, 1998

nAlgorithms for HVAC Acoustics, ASHRAE, 1991

nSound and Vibration Design and Analysis, National Environmental

Balancing Bureau (NEBB), 1994

Visit the ASHRAE Bookstore at www.ashrae.org and the NEBB Bookstore at

www.nebb.org.

For information on additional educational materials available from Trane,

contact your local Trane sales office (request a copy of the Educational Material

price list—Trane order number EM-ADV1) or visit our online bookstore at

www.trane.com/bookstore/.

Figure 73

68 TRG-TRC007-EN

Questions for Period 1

1What unit of measure is used to describe frequency?

2Define a tone.

3Sound _______ (power or pressure) is what our ears hear and is influenced

by the surroundings.

Questions for Period 2

4Which of these two single-number descriptors, A-weighted sound pressure

level or Noise Criteria (NC) rating, is better for describing outdoor sound?

5Which of the above mentioned descriptors is better for describing sound in

an office space or classroom?

6What is the NC rating of a space with the following sound-pressure levels?

octave

band

center

frequency

(Hz)

measured

sound pressure

(dB ref 20 µPa)

163 48

2125 47

3250 45

4500 39

51,000 35

62,000 27

74,000 19

88,000 14

Quiz

TRG-TRC007-EN 69

Quiz

notes

Questions for Period 3

7True or False: Sound can only travel from a source to the receiver along one

path.

8True or False: One piece of HVAC equipment may contain several sound

sources.

9What term is used to describe the reduction in sound that enters a room as

it travels to the receiver? It is influenced by distance and the absorptive and

reflective characteristics of the surfaces and furnishings in the room.

Questions for Period 4

10 A _______ (free or reverberant) sound field is characterized by a uniform, or

diffuse, sound field where the sound pressure is equal at all points in the

field.

11 What recent ARI standard, which rates the sound due to ducted air moving

and conditioning equipment, requires testing of the entire air handler, not

just the fan?

NC-

-15

NC-

-20

NC-

-25

NC-

-30

NC-

-35

NC-

-40

NC-

-45

NC-

-50

NC-

-55

NC-

-60

NC-

-65

sound pressure, dB ref 20

sound pressure, dB ref 20 m

mPa

octave

octave-

-band frequency, Hz

band frequency, Hz

63 125

125 250

250 500

500 1,000

1,000 2,000

2,000 4,000

4,000 8,000

8,000

Figure 74

70 TRG-TRC007-EN

1Hertz (cycles per second)

2A sound at a single frequency. A sound at a narrow band of frequencies

that is significantly greater than the sound at adjacent frequencies would

be similar to a tone.

3Sound pressure

4A-weighted sound-pressure level is better for describing outdoor sound

5Noise Criteria rating is better for describing sound in an office or classroom

6NC-35 (see Figure 75)

7False

8True

9Receiver sound correction (or room effect)

10 Reverberant

11 ARI Standard 260–2001, Sound Rating of Ducted Air Moving and

Conditioning Equipment

NC-

-15

NC-

-20

NC-

-25

NC-

-30

NC-

-35

NC-

-40

NC-

-45

NC-

-50

NC-

-55

NC-

-60

NC-

-65

sound pressure, dB ref 20

sound pressure, dB ref 20 m

mPa

octave

octave-

-band frequency, Hz

band frequency, Hz

63 125

125 250

250 500

500 1,000

1,000 2,000

2,000 4,000

4,000 8,000

8,000

NC-

-35

Figure 75

Answers

TRG-TRC007-EN 71

absorbed sound Sound energy that strikes a material and is converted from

sound energy to heat energy within the material.

absorption coefficient The ratio of the sound energy absorbed by the material

to the total sound energy incident upon the surface of that material.

AMCA Air Movement and Control Association International (www.amca.org)

ARI Air-Conditioning & Refrigeration Institute (www.ari.org)

ASHRAE American Society of Heating, Refrigerating and Air Conditioning

Engineers (www.ashrae.org)

attenuation The reduction in the sound level as it travels along the path from

a source to the receiver.

A-weighting A single number used to describe sound. It uses weighting

factors, by octave band, to approximate human response to sound in the range

where no hearing protection is needed. It is most appropriately used for low-

volume (or quiet) sound levels and is expressed as dBA.

broadband sound Sound energy that occurs at many frequencies, usually

covering the entire audible range.

center frequency Single frequency used to identify an octave band. It is

calculated by taking the square root of the product of the lowest and highest

frequencies in the octave band.

decibel (dB) A dimensionless ratio of two quantities that is used to describe

both sound power and sound pressure. It is defined as ten times the logarithm

to the base ten (log10) of the measured quantity divided by the reference

quantity.

dynamic insertion loss The sound insertion loss of a duct silencer with air

flowing through it.

free field A homogeneous, isotropic medium, free from boundaries. An

example of a free field over a reflecting plane would be a large open area void

of obstructions, like a parking lot or meadow.

free-field method A common method for testing HVAC equipment that is too

large to be tested in a reverberant room, such as water chillers and cooling

towers. The equipment is placed on a hard surface on a large parking lot

outdoors to approximate the sound conditions in a free field above a reflecting

plane. The sound pressure waves travel evenly in a hemispherical pattern away

from the equipment. Sound-power levels are determined by measuring sound-

pressure levels on an imaginary hemispherical surface surrounding the

equipment.

frequency The number of cycles, or oscillations, per second of a wave in

periodic motion. Expressed in hertz.

Glossary

72 TRG-TRC007-EN

Glossary

hertz (Hz) The unit of measure for frequency. One hertz is equal to one cycle

per second.

insertion loss (IL) The difference in sound pressure measured in a single

location, with and without a noise control device (installed between the source

and receiver) in place.

near field The area adjacent to the source where sound does not behave as

it would in a free field, due to the fact that the source does not radiate sound

equally in all directions.

NEBB National Environmental Balancing Bureau (www.nebb.org)

noise Unwanted or obtrusive sound. Generally, people object to sound when

it interferes with speech, concentration, or sleep.

noise criteria (NC) A single number used to describe sound in a room. It uses

a series of curves for plotting sound pressure by octave band and determining

the NC value.

noise reduction (NR) A term used to measure the effect of a barrier on

reducing the amount of transmitted sound. It is the difference between sound-

pressure measurements taken on each side of a barrier.

noise reduction coefficient (NRC) A single number used to describe the

sound-absorbing characteristics of a material. It is the arithmetic average of

the absorption coefficients for the 250, 500, 1,000 and 2,000 Hz octave bands.

octave band A range of frequencies that is defined such that the highest

frequency in the band is two times the lowest frequency. The octave band is

identified by its center frequency.

phon A unit of measure, using a logarithmic scale, used to describe the

loudness of a sound.

pitch A subjective quantity used to describe a sound. It is primarily based on

frequency, but is also dependent on sound-pressure level and composition.

Pitch is not measured, but is described with terms like bass, tenor, and soprano.

receiver sound correction The relationship between the sound energy (sound

power) entering the room and the sound pressure at a given point in the room

where the receiver hears the sound. This reduction is due to a combination of

effects, including distance and the absorptive and reflective properties of the

surrounding surfaces. Also called room effect.

reflected sound The sound that bounces off, or is reflected by, a barrier back

toward the source.

regenerated sound The noise caused by turbulent flow in air and water

systems.

TRG-TRC007-EN 73

Glossary

reverberant field A uniform, or diffuse, sound field that is the opposite of a

free field. In a perfectly reverberant field, the sound-pressure level is equal at all

points.

reverberant room A specially constructed room with reflective walls, floors,

and ceilings. When a sound source is placed in this room, the sound waves

bounce back and forth between the reflective walls many times. In a perfectly

reverberant room, the sound-pressure level is equal at all points in the room.

reverberant-room method A common method for testing HVAC equipment.

It uses a specially-constructed room to create a uniform, or diffuse, sound field

by reflecting and mixing the sound waves. The walls, floor, and ceiling of the

reverberant room are hard in order to cause multiple reflections of sound

waves. In this environment, the sound pressure is essentially the same at all

locations in the room. Sound-pressure levels are measured in the reverberant

room and used to calculate sound-power levels for the piece of equipment.

room criteria (RC) A single number used to describe sound in a room. It uses a

series of curves and reference lines for plotting sound pressure by octave band

and determining the RC value and a descriptor of the sound quality (i.e., hiss,

rumble).

room effect See receiver room correction.

semireverberant field A sound field that is somewhere between a free field

and a reverberant field. The walls and ceiling of a room prevent the sound from

behaving in a free field manner, however, these surfaces are not perfectly

reflective. Some of the sound is reflected by these surfaces, but a portion is

absorbed.

sone A unit of measure, using a linear scale, used to describe the loudness of

a sound. A sone is the linear equivalent to a phon.

sound Audible emissions resulting from the vibration of molecules within an

elastic medium. It is generated by either a vibrating surface or the movement of

a fluid. In the context of building HVAC systems, this elastic medium can be

either air or the building structure. For structurally-borne sound to become

audible, however, it must first become airborne.

sound power The acoustical energy emitted by the sound source. It is not

affected by the environment.

sound pressure An audible pressure disturbance in the atmosphere that can

be measured directly. Its magnitude is influenced not only by the strength of the

source, but also by the surroundings and the distance from the source to the

listener. Sound pressure is what our ears hear and what sound meters

measure.

source–path–receiver model A systematic approach to analyzing the sound in

a space. It traces sound from the source to the location where we want to

predict the sound (the receiver). How the sound travels between the source and

74 TRG-TRC007-EN

notes

Glossary

the receiver, and everything it encounters as it travels along the way,

constitutes the path.

tone A sound in a single frequency. A sound in a narrow band of frequencies

that is significantly greater than the sound at adjacent frequencies would be

similar to a tone.

transmitted sound The sound that travels though a barrier.

transmission loss (TL) A term used to measure the effect of a barrier on

reducing the amount of transmitted sound. It is the ratio of sound power on the

receiver side of a barrier to the sound power on the source side.

Tr a n e

An American Standard Company

www.trane.com

For more information contact

your local district office or

e-mail us at comfort@trane.com

Literature Order Number TRG-TRC007-EN

File Number E/AV-FND-TRG-TRC007-1101-EN

Supersedes New

Stocking Location La Crosse

Since Trane has a policy of continuous product and product data improvement,

it reserves the right to change design and specifications without notice.

Trane Trg Trc007 En Users Manual

TRG-TRC007-EN to the manual 61fece07-31c3-43b9-9bb0-681fc3820853

Navigation menu

Versions of this User Manual:

Views

Navigation