1. Design your own hearing aid! Ray Meddis, Nick Clark, Wendy Lecluyse Essex University, UK. (Aalto 2015) 1

2. 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 21 st 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! 2

3. Mobile phone = hearing aid? = + + Microphonecomputerspeaker = 3

4. Implemented as iPhone/iPod app ‘BioAid’ 4 http://bioaid.org.uk/ Nick Clark http://mimi.io

5. 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 5

6. BioAid a hearing aid on an iPhone Uses iPhone microphone + iPhone processor + iPhone ear phones 6

7. CPU 20%Sound latency = 7 msec. All calculations in the time domain Choose a setting 1/6 Noise gate Volume Control (on side of phone) User selects own ‘best-fit’ BioAid 7 Setting information Fine-tune Setting x4 On/Off Performance Nick Clark

8. AUD1 - son of BioAid experimental stereo system (small charge for this; requires stereo microphone) 8 Nick Clark

9. BioAid is free on iTunes – http://bioaid.org.uk http://bioaid.org.uk Its code is open source – http://bioaid.org.uk/source.html http://bioaid.org.uk/source.html AUD1 – http://www.aud1.com http://www.aud1.com 9

10. Make your own amplifier Works well with conductive hearing loss but most impaired listeners will experience discomfort at high signal levels 10

11. Dynamic range problem O 50 100 Absolute threshold (normal) Sound level (dB SPL) Absolute threshold (impaired) Discomfort threshold (normal) Discomfort threshold (impaired) Dynamic range 11 too loud!

12. Add Automatic gain control (AGC) amplifier Works well with flat hearing loss but most impaired listeners will experience uneven frequency distribution (AGC) 12 O 50 100 Absolute threshold (normal) Sound level (dB SPL) Absolute threshold (impaired) Discomfort threshold (normal) Discomfort threshold (impaired) Dynamic range too loud! Dynamic range problem

13. Different patterns of loss High-frequency loss (typical of ageing) Low-frequency loss (e.g. Menieres disease) Solution: graphic equalizer! 13 Hearing loss (dB SL)

14. Add multiple frequency bands OK, but why stop there? AGC ampfilter Σ ampfilter ampfilter ampfilter 14

15. Add separate AGCs! Lots of things to think about! 1.Channel bandwidths 2.Channel gains 3.Channel time constants Σ ampfilterAGC ampfilterAGC ampfilterAGC ampfilterAGC 15

16. Feedback The art of hearing aids But also: 1.Noise cancellation 2.Sound focusing (Beam forming) 3.Feedback cancellation Fitting procedure? 1. Test and fit (audiogram) 16 Lots of things to think about! 1.Channel bandwidths 2.Channel gains 3.Channel time constants

17. BioAid is biologically inspired! Hearing aid algorithm is based on a computer model of the response of the human inner ear to sound 17

18. BioAid amp filter AGC? multi-channel (variable no) Why so complicated? user global volume control 18

19. 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 19

20. Innovation no. 1 Instantaneous compression 20

21. Human hearing has nonlinear I/O (linear then nonlinear!) linear Sound input dB SPL output (dB) compressed O 50 100 Nominal output threshold compression threshold Hearing loss (linear) 21 Hearing loss (nonlinear)

22. Bioaid reproduces the human nonlinear function linear Sound input dB SPL output (dB) compressed 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 22

23. Natural compression is instantaneous Impaired AGC Ideal Assisted tones (rising amplitude) ✔ ✖ 23

24. 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 24

25. Input filters 1 & 2 broken-stickfilters 3 & 4 Pure 4-kHz tone 3.7 & 4-kHz tones Power spectra Compression sandwich removes most distortion 25

26. Comparison with animal data (basilar membrane measurements) BioAid animal 7.6 kHz + 8.4 kHz tones @ 60 dB SPL Data from robles et al (1997, J. Neurophys) FFT 26

27. 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 27

28. Innovation no.2 Slow, delayed, negative feedback loop 28

29. Efferent attenuation 29 middle ear Basilar membrane Auditory nerve brainstem Inner hair cell Acoustic reflex Outer hair cell MOC reflex

30. Slow, delayed, negative feedback (single channel) These circuits are replicated in BioAid 30 middle ear Basilar membrane Auditory nerve brainstem Inner hair cell Acoustic reflex Outer hair cell MOC reflex

31. MOC –what is it for? Do we need it? (It extends the linear window) linear compressed Compression threshold MOC depresses response Liner range: before after Complete linear range 31

32. Automatic speech recognition in noise (with and without MOC efferent) human Within-channel feedback no attenuation background babble dB SNR % correct 32 Brown, G. J., Ferry, R.T., and Meddis, R. (2010). JASA.

33. 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) Gain ‘compress’ is instantaneous compression, inspired by cochlear measurements. DFAC is negative feedback, inspired by MOC efferent 33

34. Innovation no.3 User selects his/her own settings. 34

35. Innovation no.4 (AUD1) Stereo Common platform guarantees left/right timing & level information Excellent 3D sound with low latency Nick Clark 35

36. 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. 36

37. Thank you 37