Electroacoustic Testing of DSP Hearing Aids

Christine Cameron & Mary Hostler

MCHAS Team

University of Manchester

Introduction• Testing DSP hearing aids – guidelines

prepared for the MCHAS project

• People it will involve: Teachers of the Deaf and Educational Audiologists

working “in the field”

Overview • Change is necessary.

• What are the options available?

• What are the best solutions?

Settings?

Test signals

ICRA?

• Regular electroacoustic testing is still important

• Test boxes currently in use are less than ideal for testing complex hearing aids with multichannels and compression.

• ToDs and Ed Auds cannot change or “read” hearing aid parameters without NOAH, manufacturer’s software, programming cables and hi-pro box.

• Full specification checks are still desirable at clinic based hearing aid reviews, or if a hearing aid is found on

routine testing to deviate from usual performance. (+/- 3dB from Baseline FRC, or THD unacceptable)

• Do this in “Test mode” in h.a. software

Guidelines for Testing DSP Hearing Aids

Note how the hearing aids have been set:

• Feedback management• Memory 2• Directional / Omnidirectional mic• Noise reduction

2. Carry out a subjective listening check

Test box

65dB SPL 50dB SPL

80dB SPL

Input signal:

• Pure Tones

• Speech shaped noise

• Pulsed or modulated speech shaped signal

3. Generate baseline FRCs / Run FRCs at three input levels

FRCs for Digifocus II

4. Compare curves obtained with baseline FRCs, they should be within + / - 3dB

5. Check THD using pure tone sweep. This should be <10% for the 65dB SPL input curve

6. Troubleshoot. Arrange full specification check at clinic if necessary

Issues raised by Step 1

1st Major issue is that

INFORMATION SHARING

between Audiology and Education

IS CRUCIAL

Issues raised by Step 1 • Any changes to the programming require

new baseline FRCs

• Frequency of generating new baseline FRCs may increase:– “fine tuning” taking place– When new earmoulds fitted – new RECDs should

be measured and adjustments made to the hearing aid to ensure match to target is still as close as possible

– Gradual introduction of features

Issues raised by Step 1 Feedback management: if this has been activated at fitting, or at

subsequent clinic appointments, it will affect – the baseline FRCs – e.g. presence of a notch or high

frequency gain reduction. (For most of the current NHS DSP aids)

– The testing - if pure tones are being used and the aid has active feedback management or suppression. (For eg Danalogic 283D, Supero 413, Prisma 2DSP)

– *demo 1*– Feedback management notch

Issues raised by Step 1 What is contained in Memory 2 is

important to note. If it contains a different program, two sets of baseline FRCs will be needed – they may be very different.

* demo 2*

Normal use program Vs “noisy environment”

COMPARE LIKE WITH LIKE!!

Issues raised by Step 1

If the directional microphone facility has been activated (usually in memory 2 – but could be on function switch) this may have an effect on test results.

* demo 3 *

Directional mic Vs Omnidirectional mic

Issues raised by Step 1 If a Noise Reduction feature has been

activated, then the INPUT SIGNAL in use can have a particularly dramatic effect on test results.

* demo 4 *DSP hearing aid with Noise Reduction feature on - FFT (speech noise) stimulus

FFT = Fast Fourier Transform

Issues raised by Step 2

May need attenuators for your stetoclip (and parents’ stetoclips) as the DSP aid may have no Volume Control… or not a great deal of adjustment on the VC

Take care when listening!!

Care when listeningUse attenuators or “VC” in the stetoclip –Use attenuators or “VC” in the stetoclip –especially if there is no volume control on the especially if there is no volume control on the hearing aidhearing aid

Issues raised by Step 3

2nd Major issue is the

INPUT SIGNAL USED TO TEST DSP HEARING AIDS

Issues raised by Step 3

Input Signal has been shown to have a big effect on test results when DSP aids have Noise Reduction features on. (eg Danalogic 283 D)

It is also an issue for ALL complex hearing aids with multichannels and compression…

*demo 5* “blooming effect”

Issues raised by Step 3 • Ideally, speech or a speech shaped signal

which is pulsed or modulated should be used – but most Services for H.I. don’t have this signal available…yet…test boxes for Education are available via the project.

• Next best is speech shaped noise (such as the “composite” signal on FONIX FP 40 test boxes)

• Worst is pure tones – various possible funny effects!!!… but STILL MUCH BETTER THAN NOTHING IF YOU ARE AWARE OF THESE EFFECTS….

Issues raised by Step 3 Possible effects with pure tones…to recap…are:• Filters followed by compressors in

multichannel hearing aids may result in the compressor undoing the effect of the filter – so the FRC looks different from one measured with a broadband speech noise or “digital test signal” i.e. “Blooming effect” – low frequency increase, and high frequency decrease

• Spikey FRC if there is active DFS• Drop in gain / output if Noise Reduction

feature is on.

Issues raised by Step 3 When using broadband, noise like signals, where

all the frequencies are present at once,• GAIN rather than OUTPUT must be measured

* demo 6*– Output Vs gain with FFT speech noise

• THD cannot be tested (only possible to do this with pure tones)

• Pure tones must be used for estimating OSPL 90

Step 3 – demonstration*demo 7*

• Three baseline FRCs – obtained with speech weighted noise (using GAIN /Frequency response curves) These curves show the nonlinear functioning of the hearing aid, with different gain applied for different input levels.

• Input/output graphs will also show the nonlinear function and are worth running at 500Hz and 2kHz.

So far…..These guidelines have been in use by First

Wave & Second wave sites…• Every site in 1st wave studies had some

hearing aids fitted as WDRC but which were in fact functioning linearly – it is important to know how the aids are functioning for counselling the user and for setting up FM systems appropriately. Our definition of WDRC: that the kneepoint is low, < 60 dB SPL at either 500Hz or 2kHz. CR typically is low, <5:1.

• 50 dB input FRC difficult to obtain if noisy test conditions in some schools.

Conclusion

• Blooming effects!!!

• Sounds more problematic than it is!!

• Worth extra hassle if children benefit

• CONTINUE TO TEST DSP AIDS !!

Thank You!!