JBL Technical Note Volume 1 Number 21 Tn V1n21

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Technical Note, Volume 1, Number 21

JBL’s New Optimized Aperture™

Horns and Low Distortion Drivers

Introduction:

Except in their initial historical development, high

frequency horns and their associated compression

drivers have normally been designed independently of

each other. The reason is of course that drivers of a

given exit diameter are normally expected to work with

the appropriate family of horns, old and new. JBL's

large format family of 100 mm (4 in) diaphragm drivers

is a clear example of this. When the JBL 375

compression driver was designed during the fifties, it

was based on the exit geometry of the original Western

Electric 594 driver. Subsequent variations, such as the

JBL 2441, 2445, and 2450 family all maintained the

original exit geometry and were designed to work with

four decades of horn hardware with 50 mm (2 in) throat

diameter.

As part of JBL's ongoing research in horn/driver

acoustic relationships, it became apparent that

significant improvements could be made in non-linear

performance at high output levels in horn systems if

the exit geometry could be changed. Specifically, the

generation of second and third harmonic distortion at

high levels could be greatly reduced through a more

rapid flare rate from the driver into the horn, and in

order to do this both new drivers and new horns would

have to be developed simultaneously.

JBL has embarked on such a program, and the

driver models 2447 and 2451 have been developed.

These are both 100 mm (4 in) diaphragm drivers with

38 mm (1.5 in) exit diameter. Three medium size

format horn models 2352, 2353, and 2354, offering 90

by 40, 60 by 40, and 40 by 20 degree coverage, have

been designed to work with these drivers. We have

addressed the following points in the new coordinated

designs:

1. Phasing plug coupled directly to the horn throat.

2. Lower distortion than earlier JBL and

competitive combinations.

3. JBL's traditional 100 mm (4 in) titanium

diaphragm integral to the design.

4. Excellent pattern control, extending down to the

crossover range.

5. Consistent on-axis frequency response, model

to model.

While much of this information is apparent from

studying the product specification sheets, we have

developed this Technical Note for the purpose of

showing relevant competitive information on distortion

and pattern control. We will present distortion

measurements on three 90 by 40 degree horn/driver

combinations and show beamwidth data on three

combinations of 90 by 40 and 60 by 40 degree horns.

Background:

As discussed in JBL Technical Note Volume One,

Number Eight, second harmonic distortion in

horn/driver combinations is due to thermodynamic air

overload and, for a given level and driving frequency,

the distortion is inversely proportional to the design

cutoff frequency of the horn. Actually, the exponential

horn flare begins at the diaphragm and is initially

established through the relatively short openings of the

phasing plug. As shown in Figure 1 A, the older driver

technology had a built-in coupler that provided a slow

flare from the 38 mm (1.5 in) diameter of the phasing

plug to the 50 mm (2 in) diameter at the driver's exit.

The original reason for this was to allow internal space

in the driver for relatively deep magnet structures or

field coils.

Figure 1A. Old driver configuration.

Figure 1B. New driver configuration.

Today, with very small, high energy neodymium

magnets and thin profile ferrite magnets, we do not

need that space. The overall depth of the driver can be

significantly reduced, as shown in Figure 1 B, providing

a relatively rapid flare into the throat of the horn.

By our calculations, the initial flare rate in the older

driver design was approximately 160 Hz, reflecting the

need to drive the very large horns that were used in

early motion picture systems. Today, we can double or

quadruple that flare rate, inasmuch as many horns are

now intended for nominal crossover at 800 Hz.

Rapid flare rates offer an opportunity to make

improvements in high frequency pattern control. Since

the exit of the phasing plug is virtually at the apex of

the horn, there is normally an excellent sight line into

the phasing plug, even at the extremes of angular

coverage; this is virtually a guarantee that high

frequency signals will illuminate the entire wave guide

and show little tendency to beam on axis.

Distortion Measurements:

Figure 2 shows the basic setup for measuring

second harmonic distortion. For these measurements,

the output level at one meter from the diffraction slot of

the horn was carefully maintained at 107 dB SPL

through the use of the compressor in the B&K test

gear. This level corresponds to 87 dB SPL at 10

meters (33 ft), and thus represents normal application

of these horns. The action of the compressor

simulates, on swept sine wave input signals, flat

equalization of the system on axis, as would be the

case in real world equalization practice for many

applications. The total range of compression was about

15 dB.

The three horn/driver combinations used in these

measurements were:

1. JBL 2380/2450

2. JBL 2352/2451

3. EV HP940/DH1A.

Figure 2. Block diagram of test setup for

measuring distortion.

These systems are commercially equivalent in

terms of nominal pattern control, and the horns are

medium format in size. The use of the B&K

compression circuitry enables a direct comparison of

distortion, as a function of output level, to be made by

inspection.

The JBL 2380/2450 combination is shown in

Figure 3A. The compressed fundamental is shown as

the solid line. Second harmonic distortion (dashed

curve) has been raised 20 dB, and distortion

percentage is indicated along the right axis of the

graph. For this combination, second harmonic

distortion lies between 1 % and 3% from 1 kHz to

about 5.5 kHz. Between 5 kHz and 10 kHz it rises to a

little over 10%.

Figure 3. Hom/Driver Distortion Measurements.

A. JBL 2380/24SO combination.

B. JBL 2352/2451 combination.

C. EV HP940/DH1A combination.

The JBL 2352/2451 combination is shown at 3B.

Again, second harmonic distortion has been raised 20

dB. Here, midband distortion lies below 1 % up to

about 2.5 kHz, reaching a value of 3% at about 9 kHz.

The EV HP940/DH1A combination is shown at 3C.

Note the rise in distortion between 1 and 2 kHz; this

appears to be due to mechanical suspension

nonlinearity in the driver rather than to thermodynamic

effects. In general, the distortion is significantly higher

than that of either the JBL 2380/2450 or JBL

2352/2451 combination.

Beamwidth Measurements:

As a rule, JBL publishes beamwidth data plotted

on a logarithmic vertical scale, since that scale relates

inversely to directivity index. However, in Figures 4 and

5, a linear vertical scale is used for beamwidth

measurements since it provides better resolution in the

80 to 120 degree range for ease in making competitive

comparisons.

Figure 4 shows beamwidth performance versus

frequency for the JBL 2352, as compared with the

JBL2380 and the EV HP940. At 4A the new JBL 2352

is compared with the older JBL2380. While both horns

have much in common, note that the horizontal

beamwidth is virtually flat out to 16 kHz. Also, the

pattern control in the 400 Hz to 1 kHz range is much

smoother in the new horn.

Figure 4B compares the JBL2352 with the EV

HP940. The 2352 exhibits better control in the

horizontal plane, while the EV HP940 horn exhibits

smoother control in the vertical plane. Overall, the 2352

exhibits smoother pattern control in the 400 Hz to 1

kHz range.

Figure 4. -6 dB Beamwidth Measurements.

A. JBL 2352 and 2380 comparison.

B. JBL 2352 and EV HP940 comparison.

Figure 5 shows beamwidth performance versus

frequency for the JBL 2353, as compared with the JBL

2385 and the EV HP640. At 5A the new JBL 2353 is

compared with the older JBL2385. Note that the 2353's

horizontal beamwidth follows its target 60degree

response from 630 Hz to 12 kHz. Vertical pattern

control is maintained from 2.5 kHz to 16 kHz within +10

degrees.

The JBL2353 and EV HP640 are compared in

Figure 5B. Note that the 2353 exhibits horizontal

pattern control that is within 10 degrees of the target 60

degrees from 630 Hz to 12 kHz. Both horns have

roughly equivalent vertical pattern control. As with the

90 by 40 degree horn, note that the 2353 maintains

better pattern control in the 500 Hz to 1 kHz range than

either the older JBL2385 or the EV HP640.

Figure 5. -6 dB Beamwidth Measurements.

A. JBL 2353 and 2385 comparison.

B. JBL 2353 and EV HP640 comparison.

Conclusions:

JBL's new Optimized Aperture horns and their

associated drivers offer a significant reduction in

distortion when compared to all earlier JBL horns and

all competitive models. These improvements in

distortion have been attained with no loss of pattern

control, as we can see when they are directly

compared with the earlier JBL 2380 series Flat-Front

Bi-Radial designs. In some cases the new horns offer

better pattern control than the older Flat-Fronts. The

combination of large diaphragm diameter and rapid

flare exit geometry points the way for future

development for all classes of horns.

JBL Professional

8500 Balboa Boulevard, PO. Box 2200

Northridge, California 91329 U.S A.

A Harman International Company

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