Swept Measurements of Difference Frequency Intermodulation and Harmonic Distortion of Hearing Aids

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Practical Instrument Set-ups

Swept Measurements of Difference Frequency Intermodulation and Harmonic Distortion of Hearing Aids by Philip S. White, Bruel & Kjaer

ntroduction In general, amplitude non-lineari­

ties of a hearing aid may be mea­sured using three different distor­tion measurements: harmonic, dif­ference-frequency, and intermodula­tion distortion. Up to the present t ime, harmonic distortion measure­ments have been the easiest to make, and hence the most com­monly used. However, with the ad­vent of the new instrumentation dis­cussed in this Application Note, the intermodulation and difference-fre­quency distortion measurements have become significantly easier and thus merit consideration.

Intermodulation distortion is the interaction of two or more frequen­cies in a complex signal that results in the generation of new frequency components not present in the origi­nal signal. These components are "mix ing" products, and hence their frequencies are equal to the sum and difference of the frequencies of the original signals and the integral multiples thereof. A special case of intermodulation distortion is differ-

distortion which ence-frequency only considers which are the

those components difference between

the original components, thus ignor­ing the sum components.

Intermodulation and difference-frequency distortion measurements are of considerable importance be­

cause they represent a more realis­tic simulation of speech or music than a single tone which is used for harmonic distortion measurements. They are also useful in describing in­teraction phenomena between several frequencies, which is inher­ently impossible with a single tone. In addition, since intermodulation components are not musically re­

lated, they are more audible, and in many cases, more annoying.

Another significant advantage of intermodulation distortion measure­ments, is that they permit the meas­urement of non-linearities up to the cut-off frequency of the system, be­cause many of the distortion compo­nents generated are folded back

_n_ Analysis Frequency (2010) Output Frequency (1902)

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U A INTERMODULATION MODE

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+ 2. +3. + 4. + 5. n n n n

f2 - 4f, f2 - 2f, f2 - 3f, f 2 " f 1

f f2 + 2f, f2 + 4f, f2 + f, f2 + 3f,

Frequency

750346/1

F ig .1 . Test signals and analysis frequencies for distortion measurement

1

Br i ie l&Kjasr Briiel & Kjaar Briiel & Kjser D a a n a a n a a a n a n a D a a n a a a n a a a n a o a a D a D D C i Q a o a a D D □ a a D a a at r i J e t & K j S B r Potentiometer Range:———dB Rectifier: - Lower Lim. Freq.:.—_—Hz Wr. Speed: mm/sac . Paper Speed:——mm/sec

Copenhagen 5 0 [ ;

dB

40

Measuring Obj.:

Rec. No^j. Date Sign.:

QP 1124

dB

20

30 15

2010

106

0>-0 10 20 Hz 50 100

Mult ip ly Frequency Scale by

500 1000 2000

Zero Level:

5000 10000 2000

1 6 1 2 / 2 1 1 2

0 40000D A B C

A B C 75108O

Fig.2. Impedance curve of a 2 cc coupler

into the band-pass section of the system. However, with harmonic measurements at higher frequen­cies, the distortion components wi l l fall above the cut-off frequency, and thus wil l be rolled off. Finally, inter-modulation measurements are a more sensitive test of non-lineari­ties since the theoretical amplitude of the intermodulation components is higher than the harmonic compo­nents.

Let's take a look at the advantage of each of the three measuring methods.

Harmonic Distortion Looking at Fig.1a, harmonic dis­

tortion is measured by exciting the hearing aid with a single sinusoidal tone, f2- The sound pressure level of f2 is to remain constant at the microphone of the hearing aid.

One of the merits of measuring harmonic distortion is that it re­quires a fairly simple instrumenta­tion system. When using a spec­trometer for this purpose, the har­monics are simply measured by off­setting the center frequency of the contiguous filters with respect to the sine generator by a constant fac­tor corresponding to the number of the desired harmonic.

One of the disadvantages of mea­suring harmonic distortion is related to the hardware; the type of coupler used in conjunction wi th the ear­phone of the hearing aid normally has one or more resonances at high frequencies (Fig.2). With a reson­ance peak at 12 kHz it means that the amount of second order har­monic distortion at f = 6000 Hz wi l l be more than 20dB too high. Same for third harmonic at f = 4000 Hz.

Another disadvantage of har­monic distortion measurements is due to the frequency response of the hearing aid.

Looki ng at Fig. 3, showi ng fre­quency response and harmonic dis­tortion of a behind-the-ear hearing aid, we see that the sharp roll-off of the frequency response above 5 kHz causes the harmonics to roll-off at correspondingly lower frequencies. This, however, does not imply that

the hearing aid does not exhibit am­plitude non-linearity at higher fre­quencies within the pass-band of the hearing aid. This quantity can be measured as:

Difference Frequency (DF) Distor­tion

DF distortion {Fig. 1) is measured by exciting the hearing aid wi th a twin-tone test signal with frequen­cies f i and f2- The sound pressure levels of f i and f2 are equal. The two tones are swept through the fre­quency range of interest while keep­ing a small fixed interval in hertz be­tween the two tones.

DF distortion considers only the components which are the differ­ence between the original fre­quency components ( f i & f2> and their harmonics (Fig.4).

For hearing aids, normally second order (f2 — f i ) and third order (2 f i — f2) would be measured (Fig.5).

f2 — f1 w a s chosen to be 160 Hz, which means that when sweeping to measure the second or­der DF distortion, the filter of the an­alyzer is steadily tuned to 160 Hz. Some standards call for f2 — f i equal to 80 Hz. This may in some cases be inconvenient as some hear­ing aids have a very low output cap­ability at 80 Hz. It is therefore sug­gested that f2 — f i be chosen such that the hearing aid has a reason­able output at f2 — f T . When sweeping to measure the third or­der minus DF distortion at (2 f i — f 2 ) Hz the analysis frequency of the filter is (f2 — f 1) Hz lower than the lower tone (f<|) of the test signal, meaning that the third order DF dis­tortion is measurable throughout the pass-band of the hearing aid. Similarly when measuring third or­der plus (2 f2 — f i ) , the filter of the analyzer is tuned just (f2 — f i ) Hz higher than the upper tone (f2) of the test signal.

Briiel & Kjaer Briiel & Kjaer Briiel & Kjaer D n a a a n n a D D a n o a D a a n D D a a a n n D n a a n n n a n D n n a a D n n a a n D a D a r l i e l & K j a s r Potentiometer Range:—™—dB Rectifier: Lower Lim. Freq.: Hz Wr. S p e e d : — — m m / s e c . Paper Speed: m m / s e c

10 i75 Copenhagen 5 0 r 2 5 F

dB

40 20

Measuring Obj.:

Freq. re-sponse and „ _ . _ harmonic 3 0 P 5

distort, of a behind the-ear hearing a i d _ 2 0 1 0

Acoustical gain 40 d B _ L

1 0 *

Rec. No^i Date Sign.: ok»

0 0 0 0 2 0 0 0 0 40000D A B C liiv

Q P 1 1 2 4 Mult ip ly Frequency Scale by Zero l e v e l : 1 6 1 2 / 2 1 1 2 A 6 c 751079

Fig.3. Frequency response and harmonic distortion of a hearing aid

Since the analysis frequency is very close to the test signal(s) when measuring third order DF distortion, a mandatory requirement is that the filter of the analyzer be very steep and have a narrow bandwidth. In other words, the filter must let through only the signal at the analy­sis frequency and cut-out the test signal(s). The filter should also have low distortion and a wide dynamic range.

Thus, for example, if f2 — f i = 80 Hz and we want to measure third order distortion (2 f i — f 2 ) down to 0 , 1 % (— 60 dB) we see from Fig.6 that the filter should have a bandwidth B ^ 8 0 / 3 , 5 - 23 Hz — a 10 Hz bandwidth would be appropriate.

Interpretation of DF Distortion Curves

Again, looking at Fig.5 we see that the third order distortion is qu­ite significant and bears some semblance to the frequency sponse curve. Specifically we that the peaks in the frequency re­sponse curve also occur in the third order distortion curve offset by 320 Hz. This is due to the fact that f2 — f1 = 160 Hz and the analysts frequency fa = 2 f1 — f2 . Solving for fa we see that fa = f2 — 320, hence the frequency offset of the two curves. The peaks in the distor­tion curve may be attributed to the fact that at the peaks (high gain) in the frequency response, the "loop gain" wil l be lower because of satu­ration phenomena or too heavy drain on the power supply (battery). Symmetrical clipping of the signal wil l result in "odd order" distortion. The second order distortion ("even order") may e.g. be attributed to a rectification phenomenon. This could be the case if the hearing aid employs a compressor circuit or if an amplifier stage causes assymetri-cal clipping of the signal.

re-re-

Intermodulation (IM) Distortion IM distortion {Fig. 1) is measured

by exciting the test specimen with a twin-tone test signal ^ and f2 . Keeping the amplitude of both sig­nals constant, the amplitude of f i is 12dB higher than the amplitude of f2 . ^ is kept at a fixed low fre­quency, while f2 is swept through the frequency range of interest. In a non-linear system the IM distortion

3rd 3rd +

2nd f f.

4th CM

CM

CM CM

CM

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CM

0 100 200 300 400 500 600 700 800 900 1k 1,1 k Hz

750845

Fig.4. DF distor t ion of 8 0 0 and 9 0 0 Hz signals

Bruel & Kjaer Bruel & Kjaer Bruel & Kjaer o n n n a a a a a a a D a n n n a n n n a n a a a a a o a o a n n a n n a n a n D D n n n Q Q D n t

B r U 6 l & KJ9BT Potentiometer Range: ^0 ^B Rectifier: Lower Lim Freq.: Hz Wr. Speed: .mm/sec. Paper Speed: mm/sec Copenhagen 50 r25

40

Measuring Obj.:

£req> re­sponse and diff. freq. distort of a behind-the-

45

aid

Rec. No.: Date Sign.:

QP 1124

ear hearing

AcQjjstjca gain 40 dB_ ^

10 20 Hz 60 100

Mult ip ly Frequency Scale by Zero Level: 1 6 1 2 / 2 1 1 2 A B C

7-QJ0

751078

Fig.5. Frequency response and DF distor t ion of a hearing aid

0

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10

CD 20

30

40

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Bandwidths from centre frequency 171104

Fig. 6. Typical filter characteristics of a Type 2 0 1 0 Heterodyne Analyzer

3

m Brual & Kjaer Briiel & Kjaer Bruel & Kjaer n a a o a a n n n a a D a a a a a a n a a a D a D D D D a a a a D n n a a D a D a n a n n n n a □ [

B r u e t & K J 3 8 r Potentiometer Range:———dB Rectifier: Lower Lim. Freq.: - H z Wr. S p e e d : - — - m m / s e c . Paper Speed: .mm/sec Copenhagen 50r2

Measuring Obj.:|_

Freq. re-nd

3 OH 5 sponse and inter mod. distort, of a behind-the-ear hearing .

^ -2010

Acoustical gain 40 dB

Rec. N q ^ Date Sign.:

|10i75

QP1124

10 20 Hz 50 100 Multiply Frequency Scale by Zero Level:

20000 40000D A B C

1612 /2112 A B C 751077

Fig.7. Frequency response and IM distortion of a hearing aid

shows up as a number of side bands located on both sides of f2 — the distance between the side bands being equal to f 1.

Similarly to DF measurements, the filter of the analyzer should be very steep and have a narrow band­width, especially if f j is chosen such that the IM sidebands are very close to the upper test tone f 2.

An example of an IM distortion measurement is shown in Fig.7.

System Description With the advent of the

1 9 0 2 / 2 0 1 0 combination, a distor­tion measurement control unit and heterodyne analyzer, the user now has a unique tool for making swept harmonic, difference frequency (DF) and intermodulation (IM) distortion measurements accurately and con­veniently on electroacoustic de­vices. The 1 9 0 2 / 2 0 1 0 combination automatically generates the neces­sary test signal(s) as well as tunes its filter to the desired distortion component, one through fifth order.

The 1 9 0 2 / 2 0 1 0 combination readily lends itself to harmonic dis­tortion measurements of hearing aids, but things get slightly more complicated when it is desired to make difference frequency and inter­modulation distortion measure­ments.

The problem is this: when making frequency response and distortion measurements the sound pressure level at the hearing aid should be kept constant throughout the fre­quency range of interest. IEC Rec­ommendation No. 118 specifies that the sound source be kept at a

constant level (± 2%) between 200 Hz and 5 kHz. For this purpose a compressor loop is Often em­ployed. In the case of harmonic dis­tortion measurements this is straight forward and simple, but in the case of a twin-tone test signal such as used in IM and DF testing, special consideration has to be given to the compressor loop(s).

In the case of DF measurements, the two test signals f i and f2 are normally close to each other in fre­quency (typically f2 — f i = 80 Hz). When this twin-tone signal is fed into the sound source, e. g. the speaker of Hearing Aid Test Box, Type 4212 , there wil l be some dif­ference in amplitude between the two tones because the frequency re­sponse of the speaker is not com­pletely flat.

Fig.8 shows the relative deviation between the two test tones when using one common compressor. It is seen that if f2 — f i = 80 Hz, the dif­ference in level is approximately 1 dB (300 — 10000Hz) (Fig.8a). An instrument set-up utilizing one common compressor is described la­

ter. If the two test frequencies f2 and f i are further apart, larger am­plitude deviations wil l occur (Fig.8b-c). The roll-off at lower frequencies of i-\ can be attributed to the roll-off of the speaker.

If better amplitude accuracy is de­sired and the test signals f2 and ^ lie far apart, we have to use two se­parate, filtered compressor loops. This could be accomplished by us­ing two separate tracking/slave f i l ­ters. However, by using the B & K Type 2020 Heterodyne Slave Filter, only one filter is necessary. The con­cept is this:

The 2020 Slave Filter has two outputs: one is the regular band­pass output with the center fre­quency equal to the BFO frequency and the other a rejection output passing all frequencies except the BFO frequency. f2 is the BFO fre­quency and we can thus separate the two signals by taking f2 out of the bandpass output and f i out of the rejection output and feeding them into two separate compres­sors.

System for Measurement of Har­monic, Difference Frequency, In-termodulation Distortion with dual Compressor Loop

The system described here is par­ticularly suited for twin-tone meas­urements where good amplitude sta­bility of both test signals is required and where the frequency difference between f 1 and f 2 is large. The sys­tem is shown in Fig.9.

The two test signals ^ and f2 , al­though both normally available at the output of 1902, are in this case derived separately from the outputs of 1902 ( f i ) and 2010 (f2).

The compressor microphone hooked up to a measuring amplifier (2608) receives the twin-tone test signal and feeds it into a hetero­dyne slave filter (2020). The slave filter is tuned to the BFO frequency (f2) of the 2010 Heterodyne Anal­yzer and at the socket "Output" on 2020 we have f2 available while "Rejection Output" gives us f ^ The twin-tone signal is thus split up for two separate compressor loops. f 2

is via a Measuring Amplifier (2608) fed into the "Compressor Input" of the Heterodyne Analyzer (2010). The 2608 serves as a signal condi-

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dB dB

40h20

Measuring Obj.:

3015

2010

105

Rec. No^ P-Sli

2010

Rec. Nqj_ Date

106

Sign.:

All SPL's are 67 dB nominally

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f2 - f1 = 80 Hz

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fo - f, = 160 Hz

f2 — fT = 320 Hz

ok)

Freq. scale ref. is f2 (f 1 is offset by f2 — f 1 ) mm

ll 0i75

QP 1124

10 20 Hz SO 100

Multiply Frequency Scale by

500 1000 2000

Zero Level:

5000 10000 20000 40000D A B C linr

1612/2112 A B C 751081

Fig.8. Amplitude deviation between f-| and f2 when using common compressor in conjunction with Hearing Aid Test Box, Type 4 2 1 2

tioner and readout for the level of fa-

f-|, originating from the generator section of 1902, utilizes the com­pressor circuitry of a 1405 Random Noise Generator, f 1 is from the "Re­jection Output" of 2020 fed into a Measuring Amplifier 2608 serving as a signal conditioner and read­

out of the level of f . From 2608 the signal is fed into the "Ext. Gen. Input" of 1405 which is operated in the compressor mode.*

* For 1405 Noise Generators w i th serial nos. pre 571 512 , a minor modification reducing compressor distortion is recommended; con­sult factory.

f.

Distortion Measurement Control Unit 1902

Noise Generator 1405

■li!BW

• • • *

Compressor Microphone f1 and f

z

Measuring Amplifier 2608

Measuring Microphone

f-l and f2

Heterodyne Analyzer 2010

I

5 k£2

Power 2 5 * V 2 W Amplifier

2706

External Filter

Heterodyne Slave Filter 2020

Hearing Aid Test Box 4212

I

L synchronisation I

I

Level Recorder 2307

2020 fixed freq., variable freq.

f,

f.

•

® %

Measuring Amplifier 2608

(2609) (2425)

751054

Fig.9. Set-up for diff. freq., harm, and IM distortion measurements with dual compressor loop

5

We have now formed two separ­ate compressor loops. The two sig­nals from the generator outputs of 1405 and 2010, ^ and f2 , are "summed" in two 5 kO resistors at the input of the Power Amplifier (2706), which drives the speaker of the Hearing Aid Test Box 421 2.*

The hearing aid parameters to be established are measured by means of the Microphone/Artificial Ear of 4212, the measuring amplifier and heterodyne filter of 2010 in con­junction with Distortion Measure­ment Control Unit 1902. A 2305 or 2307 Graphic Level Recorder serves as a read-out of test results. The artificial ear filter switch on the 4212 Hearing Aid Test Box should be in the "Off" position. This is im­portant when measuring second or­der DF distortion, because the high pass filter cuts-off below 150 Hz. A summary of typical knob settings for a difference frequency test (f2 — f i = 80 Hz) is given in Fig. 10.

System for Measurement of Har­monic, Difference Frequency, In-termodulation Distortion with single Compressor Loop

If it is desired to make DF distor­tion measurements with a small in­terval between f2 and f ^, say around 100 Hz or less, and small deviations between the sound pres­sure levels of ij and f2 can be ac­cepted, the system shown in Fig. 1 1 will suffice. The twin-tone signal from the Type 1902 Distortion Measurement Control Unit is fed into the compressor of the Type 1405 Noise Generator. It is recom­mended to keep the generator sig­nal at 1 V or below. From the regu­lator microphone of the Hearing Aid Test Box, the feedback signal is fed into the compressor via a 2608 Measuring Amplifier serving as a signal conditioner and read-out of the sound pressure level of f2 .

With the system in the same con­figuration, it may also be used for harmonic distortion testing. By ad­ding a high-pass or notch filter to

2608 0) Filters: Ext. Meter Function; Fast Ail other knobs: As required

2020 Input fi lter: In Output: 0° BFO Mode: Sine Bandwidth: 31.6 Gain: 0 dB Bandwidth Compensation: Off

2603(11) Filters: Linear, 2 - 200000 Meter Function: Fast All other knobs: As required

2010

1902 (Mode Selector): Difference — frequency Distortion Order: As required Difference — frequency: 80 Hz or as required Generator Stop: f^On) Output Voltage: Fully clockwise Attenuator: 1V

1405 Compr. Voltage: As required Compr. Speed: 30 (Mode Selector): Compr, only Output: n/a

2706 Current Limit: 1.8 A RMS Attenuator: 10 Gain Control: Fully clockwise

Read Out Selector:DC Lin Effective Averaging Time T: 0.1 sec. Selectivity Control: 10 Hz B&T program: Manual BFO Attenuator: IV BFO Output Voltage: 7 Frequency Scale: x 1 Log Compressor Speed: 100 All other knobs: As required

2307 Potentiometer Range: 50 Rectifier Response: DC Lower Limiting Frequency: 200 Writing Speed: 250 mm/sec Paper Speed: 1 mm/sec AM other knobs: As required

4212 Art. Ear Filter: Off Attenuator for speaker: H

Fig.10. Typical knob settings for a difference frequency distortion measurement

760124

the 2608 Measuring Amplifier and rerouting the cables as shown with the dotted lines, the system is ready for IM testing. As the tone f has a fixed frequency and amplitude which is 12dB higher than f2 , it is necessary to filter out f -\ to be able to compress f 2- A high-pass or notch filter such as found in Fre­quency Analyzers 2 1 2 0 / 2 1 2 1 wil l suffice, but any other good 4-pole high-pass or notch filter, passive or active, which rejects f 1 by mini­mum 22dB, will do.

System for Measurement of Har­monic and Intermodulation Distor­tion

By eliminating the compressor (Type 1405) from the scheme de­scribed in Fig. 11 the system wil l perform harmonic and intermodula­tion distortion measurements. This system is shown in Fig. 12. The high-pass or notch filter used for IM testing is similar to the one shown in Fig.11. Note that the Power Am­plifier Type 2706 has been omitted:

The generator outputs of 1902 and 2010 have some power capabil­ity — enough to drive 4212's speaker at low-to-moderate sound pressure levels (50 — 70dB SPL). Using the speaker (no current limit­ing resistor to be used) as a "float­ing" load, it is simply connected di­rectly between the two center posts (hot leads) of the generator outputs. The two "grounds" of the generator outputs should be connected to­gether. The two generators have a current drive capability of 70 mA, so voltage levels above approx. 0 ,4V into the speaker ( 6 0 ) should be avoided to keep the distortion of the test signal low (the outputs are protected against excessive cur­rents). If higher sound pressure le­vels, up to approx. 90 dB are re­quired, the power amplifier scheme outlined in Fig.9 and Fig. 11 should be used.

A word of caution here: since the speaker in 4 2 1 2 is rated at max. 4V-RMS and 2 7 0 6 is capable of delivering 15V-RMS in 3 0 , it is strongly recommended to insert a minimum 25 OV12 W resistor in series wi th the speaker leads to prevent speaker burn-out. When doing a sweep it is also rec­ommended to turn down the "Gain Con­t ro l " of 2 7 0 6 whi le the BFO is at low fre­quencies, say below 100 Hz, to avoid exces­sive excursions of the speaker diaphragm.

6

Measuring Microphone

Distortion Measurement Control Unit 1902

I Heterodyne ]_Analyzer2010

Level Recorder 2307

f l & f 2

Noise Generator 1405

Power Amplifier 2706

f,

-vwv-■-WW-

5 k Measuring Amplifier

Filter for IM testing

25 ohm vwv-12W

Compressor Microphone

High-pass, 4 pole, f0 = 2 f , or Notch, f = f , Hearing Aid

Test Box 4212 760126

Fig. 11 - Set-up for diff- freq-, harm, and intermod. distortion measurements wi th single compressor loop

Measuring Microphone f! & f2 Compressor Microphone

Distortion Measurement Control Unit 1902

■®»• * • * -

i '

"

Heterodyne Analyzer 2010

f 1

Measuring Amplifier 2608

s*

%

4-pole High-pass or Notch Filter

Hearing Aid Test Box 4212

*) connect this lead to "common" for harmonic distortion measurements.

Level Recorder 2307 760127

Fig. 1 2 . Set-up for I M and harm measurements

Distortion Measurement Control Unit 1902

H.A. Electronics Earphone

Test Object

mechanical or electrical synchronization

Heterodyne Analyzer 2010

Level Recorder 2305 or 2307

760125

Fig. 1 3 . General set-up for measurement of individual parts of hearing aids

Distortion Measurements on Indi­vidual Parts of Hearing Aids

While the previously discussed set-ups were mainly for establish­ing the overall performance of the hearing aid under assumed, well de­fined working conditions, it is a sim­ple affair also to use the 1 9 0 2 / 2 0 1 0 combination for mea­suring frequency response and dis­tortion levels for individual parts of the hearing aid.

A general set-up is shown in Fig.13.

This type of measurement is valu­able when it is desired, for design purposes, to relate distortion and frequency response to individual parts of the hearing aid. Amplifier and compressor circuits are typical examples of this. Also, utilizing an artificial ear, measurements on the ear-phone alone can be performed.

7

Performance of the Measurement System

To establish the overall perfor­mance of the system, we let the sys­tem analyze itself. The two main goals were:

1 Establishing accuracy of compres­sor regulated sound pressure le­vel when using a) a single-tone test signal (harm, distortion) and b) a twin-tone signal (IM and DF distortion) with single or dua compressor loop(s).

2. Establishing the level of har­monic, difference frequency and intermodulation distortion of the system itself.

Compressor Loop(s) the outlined Using the system outnnea in

Fig.9, and looking at Fig. 14a to c, we see that in all cases the sound pressure level is kept constant to within a fraction of a dB throughout the frequency range of interest (200 Hz to 5 kHz). in graph 14b the combined sound level of f-| and f2 was recorded. Graph 14c shows the sound level of f2 only — f i re­mains at a fixed amplitude and fre­quency anyway.

The bandwidth of the 2020 Slave Filter was set at B = 31,6 Hz. This was found to yield the best com­promise between compressor loop stability and compressor loop fre­quency selectivity.

When using the single compres­sor loop system for DF measure­ments as outlined in Fig.11, refer to Fig.8 and section System Descrip­tion, where the merits of this method are discussed.

Residual Distortion of the Meas­urement System

While most of the distortion in the system no doubt can be attrib­uted to the speaker in 4212 and the compressor (1405), it was found that the system's distortion was generally well below 0,3% at any distortion component in the fre­quency range of interest (200 Hz to 5 kHz). Generally, hearing aids ex­hibit distortion figures considerably higher than this. Figures 14a and 14c show second and third order

Bruel & K j» r Bruel & Kjaer Briiel & Kjaer D n o n a n a a Q a a a Q a a a a a D a a o a a o o n a Q n a D D n n n n o a a Q D D D D D D D a t

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Measuring Obj.: _

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SPL,

Residual difference frequency distortion of system

Briiel & Kjaer Bruel & Kja»r Bruel & Kjaer a n a D a D a n a a n a n a a n n a n a n a a a D a a D a Q a a a a a n a a a a n D a D D D a D n c

B r i i e l & K jser Potentiometer Range:—§9—dB Rectif ier^ Lower Lim. Freq.: Hz Wr. Speed: mm/sec . Paper S p e e d : J _ m m / s e c Copenhagen 5 0 [ " 2 5 [

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OP 1124

10 20 H z 4 0 d B 100

Mult iply Frequency Scale by

200 500 1000 2000

Zero Level:

000 20000 40000D A B C l i n ^ °

1 6 1 2 / 2 1 1 2 A B C 751Q56

Fig. 14. Performance curves of hearing aid test system for harm., diff. freq. and I M distortion

harmonic and IM distortion curves. Fig. 14b shows only third order dif­ference frequency distortion — the second order component was too low to be measured, because it ap­proached the level of the ambient noise.

8

Conclusion The 1 9 0 2 / 2 0 1 0 combination,

Distortion Measurement Control Unit and Heterodyne Analyzer al­ready covering a wide range of appli­cations in the electro-acoustic field by providing convenient, accurate swept measurements of harmonic, difference frequency and intermodu-lation distortion components can also be used to measure these par­

ameters on hearing aids. The use of frequency selective compressor loops ensure a uniform sound pres­sure level throughout the frequency range of interest when making twin-tone tests. When making DF meas­urements with* single compressor loop and f2 — f i equal to approxi­mately 100 Hz or less, the relatively flat response of the speaker of the

4212 Hearing Aid Test Box ensures that only a small level difference be­tween f i and f 2 wi l l occur.

The system itself has low residual distortion and is able to detect dis­tortion components, second through fifth order, down to a fraction of one percent.

■

Bruel & Kjaer Instruments, 185 Forest Street

Marlborough, Massachusetts 01752 (617)481-7000