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Design your own hearing aid!

Ray Meddis, Nick Clark, Wendy Lecluyse

Essex University, UK.(Aalto 2015)

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Abstract• • A hearing aid makes sounds louder so that people with a hearing impairment can hear them better.

Right? No, wrong! The saying ‘I may be deaf but there is no need to shout’ is very instructive. Equally instructive is the fact that hearing aids are very expensive but owners frequently refuse to wear them, despite pressure from friends and family. We do not know for sure why this is. One possibility is that there are many different types of hearing impairment that are not differentiated by standard hearing tests resulting in a ‘one size fits all’ failure to solve the problem. Also, we often think of human hearing in terms of linear systems even though the response of the ear to sound is strongly nonlinear. A hearing aid needs therefore to be designed so that it recreates natural hearing by feeding nonlinearly processed sound into a receiver (the impaired listener) that is itself nonlinear but damaged in some unspecified way. This is a signal processing challenge worthy of the 21st century! Until recently, it was difficult to build a hearing aid for research purposes. However, the arrival of the smartphone now means that new hearing aid algorithms can be implemented and tested quickly not only in the laboratory but disseminated as an app on a worldwide basis to obtain user feedback. The talk will report our experiences with ‘BioAid’, an (open source) app that can be downloaded free of charge from the iTunes store to an iPhone. Its algorithm also attempts to address the problem of how to fit the aid without professional support in a novel way. The audience is invited to download the app and bring it with their questions to the talk!

• •

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Mobile phone = hearing aid?

= + +

Microphone computer speaker

=

Implemented as iPhone/iPod app‘BioAid’

4http://bioaid.org.uk/

Nick Clarkhttp://mimi.io

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Testimonials“I was amazed, took a chance and could not be more pleased .. Don’t have hearing aids yet and this app has made such a difference. Very pleased .. Would definitely pay for this .. thanks for the free app Great” – Me2533, USA

“I love this app for watching TV or around the house” – Anon, UK

“Having tested a few other apps, this one surpasses the rest. It provides a simple user friendly way of selecting the frequencies you need amplifying, other hearing aids just make everything louder which makes them uncomfortable most of the time.– FraggerBlagger, UK

“I am amazed with this app. I have two CIC hearing aid worth more than 2500 pounds and this one beats the pants off. 5 stars without a doubt. Its a great substitute or even better than the hearing aids sold in market for thousands of pounds!!!!!! Keep up the good work.” – MG, UK

http://bioaid.org.uk/testims.html

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BioAid a hearing aid on an iPhone

Uses

iPhone microphone+

iPhone processor+

iPhone ear phones

7CPU 20% Sound latency = 7 msec.

All calculations in the time domain

Choose a setting 1/6

Noise gate

VolumeControl

(on side of phone)

User selects own ‘best-fit’

BioAid

Settinginformation

Fine-tuneSetting x4

On/Off

Performance

Nick Clark

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AUD1 - son of BioAidexperimental stereo system

(small charge for this; requires stereo microphone)

Nick Clark

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BioAid

• is free on iTunes– http://bioaid.org.uk

• Its code is open source – http://bioaid.org.uk/source.html

• AUD1– http://www.aud1.com

Make your own

amplifier

Works well with conductive hearing loss butmost impaired listeners will experience discomfort at high signal levels

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Dynamic range problem

O

50 1

00

Absolute threshold (normal)

Sound level (dB SPL)

Absolute threshold (impaired)

Discomfort threshold (normal)

Discomfort threshold (impaired)

Dyn

amic

rang

e

too loud!

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Add Automatic gain control(AGC)

amplifier

Works well with flat hearing loss butmost impaired listeners will experience uneven frequency distribution

(AGC)

O

5

0

100

Absolute threshold (normal)

Sound level

(dB SPL)

Absolute threshold (impaired)

Discomfort threshold (normal)

Discomfort threshold (impaired)

Dyn

amic

rang

e

too loud!

Dynamic range problem

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Different patterns of lossHigh-frequency loss

(typical of ageing)Low-frequency loss

(e.g. Menieres disease)

Solution: graphic equalizer!

Hearing loss (dB SL)

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Add multiple frequency bands

OK, but why stop there?

AGC

ampfilter

Σampfilterampfilterampfilter

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Add separate AGCs!

Lots of things to think about!1. Channel bandwidths2. Channel gains3. Channel time constants

Σ

ampfilter AGC

ampfilter AGCampfilter AGC

ampfilter AGC

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Feedback

The art of hearing aidsBut also:1. Noise cancellation2. Sound focusing (Beam forming)3. Feedback cancellation

Fitting procedure?1. Test and fit (audiogram)

Lots of things to think about!1. Channel bandwidths2. Channel gains3. Channel time constants

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BioAidis biologically inspired!

• Hearing aid algorithm is based on a computer model of the response of the human inner ear to sound

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BioAid

amp

filter

AGC?

multi-channel(variable no)

Why so complicated?user global

volume control

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Why so complicated?

• Many users are dissatisfied with existing aids• Standard aids are engineering solutions

– They amplify and limit– but pay limited attention to natural human

hearing

• BioAid aims to restore natural hearing– by replicating processes that take place in the

human cochlea

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Innovation no. 1

Instantaneous compression

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Human hearing has nonlinear I/O(linear then nonlinear!)

linear

Sound input dB SPL

outp

ut (d

B)

compressed

O 50 100

O

50

100

Nominal output threshold

compression threshold

Hearing l

oss (lin

ear)

Hearing loss (nonlinear)

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Bioaid reproduces the human nonlinear function

linear

Sound input dB SPL

outp

ut (d

B)

compressed

O 50 100

O

50

100

absolute threshold

compression threshold

The output never exceeds the discomfort threshold

log(y)= a + log(x)

log(y)= b + c.log(x)c=0.2

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Natural compression is instantaneous

Impaired

AGC

Ideal Assisted

tones (rising amplitude)

✔

✖

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Instantaneous?!

• BioAid works in the time domain– Compression is applied using a lookup table

• (instantaneously)

• What about distortion?– Natural hearing contains some distortion

(combination tones)– Natural hearing contains only a limited range of

distortion products– This is simulated in BioAid by post-filtering output

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Input filters 1 & 2 broken-stickfilters 3 & 4

Pure 4-kHz tone

3.7 & 4-kHz tonesPower spectra

Compression sandwich removes most distortion

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Comparison with animal data(basilar membrane measurements)

BioAid

animal

7.6 kHz + 8.4 kHztones @ 60 dB SPL

Data from robles et al (1997, J. Neurophys)

FFT

FFT

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Bioaid so far

• Instantaneous compression– Restores natural I/O function– Restores natural combination tones– Improves on AGC attack and recovery

characteristics• Anything else?

– Yes, biology shows slow continuous negative feedback adjustment of gain depending on current sound levels

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Innovation no.2

Slow, delayed, negative feedback loop

Efferent attenuation

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middle ear

Basilar membrane

Auditory nerve

brainstem

Inner hair cell

Acoustic reflexOuter hair cell

MOC reflex

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Slow, delayed, negative feedback(single channel)

These circuits are replicated in BioAid

middle ear

Basilar membrane

Auditory nerve

brainstem

Inner hair cell

Acoustic reflexOuter hair cell

MOC reflex

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MOC –what is it for? Do we need it?

(It extends the linear window)

linea

rcompressed

Compression threshold

MOCdepressesresponse

Liner range: before

after

Complete linear range

Automatic speech recognition in noise(with and without MOC efferent)

human

Within-channel feedback

no attenuation

background babble dB SNR

%correct

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Brown, G. J., Ferry, R.T., and Meddis, R. (2010). JASA.

A biologically-inspired hearing aid

output

∑

input

Band pass

inst.compress

Band pass

DFAC(tau)

Band pass

inst.compress

Band pass

DFAC(tau)

Band pass

inst.compress

Band pass

DFAC(tau)

Band pass

inst.compress

Band pass

DFAC(tau)

GainGainGainGain‘compress’ is instantaneous compression, inspired by cochlear measurements.

DFAC is negative feedback, inspired by MOC efferent

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Innovation no.3

User selects his/her own settings.

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Innovation no.4 (AUD1)

• Stereo• Common platform

guarantees left/right timing & level information

• Excellent 3D sound with low latency

Nick Clark

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The future

• Make BioAid available as a BTE device– Parameters controlled from a smartphone

• Better, make a platform that researchers can use to innovate and produce better hearing aids.

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Thank you