Hearing and Hearing Aids

1. Hearing and Hearing Aids 蘇璟瑋, 林宜宏, 李彥鋒 Department of Electrical Engneering National Taiwan University Monday, April 9, 2007

2. Outline • The Structure and Function of Ear • Hearing Loss • Hearing Aids • Summary • Reference Outline

3. The Structure and Function of Ear • The ear of human can be classified into three parts : external ear, middle ear and inner ear. Outer ear Middle ear Inner ear The Structure of Ear

4. The Structure and Function of Ear (Cont’d) • External Ear : Preamplifier • Pinna Collects sound waves and amplifies sound • Ear Canal : Amplifies frequencies in the range 3 kHz to 12 kHz External Ear

5. Total gain Gain from Pinna Gain from Ear Canal Frequency (kHz) The Structure and Function of Ear (Cont’d) • External Ear : Preamplifier • Pinna Collects sound waves and amplifies sound • Ear Canal : Amplifies frequencies in the range 3 kHz to 12 kHz The frequency response of external ear External Ear

6. The Structure and Function of Ear (Cont’d) • Middle Ear • Conduction Conduct sound from the outer ear to the inner ear • Protection Middle ear muscles provide protection from loud sounds Barrier between external ear and inner ear Middle Ear

7. The frequency response of Middle Ear The Structure and Function of Ear (Cont’d) • Middle Ear • Transducer Converts acoustic energy to mechanical energy Converts mechanical energy to hydraulic energy • Amplifier Middle Ear

8. The Structure and Function of Ear (Cont’d) • Inner Ear • Hearing • Frequency Selectivity • Balance Inner Ear

9. Hearing Loss • The type of hearing loss • Conductive hearing loss Sound cannot be conducted efficiently through the outer ear canal to the eardrum and the tiny bones, or ossicles, of the middle ear.  • Sensorineural hearing loss There is damage to the inner ear (cochlea) or to the nerve pathways from the inner ear (retrocochlear) to the brain. Sensorineural hearing loss cannot be medically or surgically corrected. • Mixed hearing loss Hearing Loss

10. Hearing Loss (Cont’d) (1) Decreased Audibility • Conductive hearing loss • Sensorineural hearing loss Decreased Audility

11. Hearing Loss (Cont’d) (2) Decreased Dynamic Range • Normally, the dynamic range of hearing equals 120dBs roughly. Decreased Dynamic Range

12. Hearing Loss (Cont’d) (3) Decreased Frequency Resolution • SNR Loss • Sensitivity Loss • Clarity Loss (3) Decreased Temporal resolution Decreased Frequency Resolution

13. Hearing Aids • Basic Structure and Design of Modern Hearing Aids

14. Hearing Aids (Cont’d) • The Function of Hearing Aids • Compression

15. Hearing Aids (Cont’d) • The Function of Hearing Aids (Cont’d) • Directional Microphone and Microphone array This method enables the design of highly-directive-hearing instruments which are comfortable, inconspicuous, and convenient to use. The array provides the user with a dramatic improvement in speech perception over existing hearing aid designs, particularly in the presence of background noise, reverberation, and feedback.

16. Hearing Aids (Cont’d) • The Function of Hearing Aids (Cont’d) • Directional Microphone and Microphone Array (Cont’d)

17. Hearing Aids (Cont’d) • The Function of Hearing Aids (Cont’d) • Noise Reduction • Method 1 Using high pass filter, but the eliminates both speech and noise in the frequency region between 0.4kHz and 1.0 kHz.

18. Hearing Aids (Cont’d) • The Function of Hearing Aids (Cont’d) • Noise Reduction (Cont’d) • Method 2 Adaptive noise cancellation, need at leat two microphones

19. Hearing Aids (Cont’d) • The Function of Hearing Aids (Cont’d) • Speech Enhancement

20. Hearing Aids (Cont’d) • The Function of Hearing Aids (Cont’d) • feedback cancellation Acoustic feedback in hearing aids, which, if severeenough, causes the aid to emit a high-intensity oscillation (known as ‘whistling,’ ‘screeching,’ or ‘howling’) is a persistent and annoying problem. Block diagram of amplification and feedback in a hearing aid. Continuous adaptation feedback cancellation scheme

21. Hearing Aids (Cont’d) • The Function of Hearing Aids (Cont’d) • feedback cancellation (Cont’d) An adaptive algrithom attempts to identify the hearing-aid feedback path. Once the feedback path has been identified, an estimate of its output is subtracted from the microphone signal. Thus the hearing-aid user hear the amplified input signal with a reduction of the distortion caused by the feedback path. Continuous adaptation feedback cancellation scheme

22. Hearing Aids (Cont’d) • The types of Hearing Aid • 線性電路助聽器 • 電腦程控式助聽器 • 壓縮類比式電路助聽器 • Advantage • 價格低 • 調整快速 • Disadvantage • 調整彈性低

23. Hearing Aids (Cont’d) • The types of Hearing Aid (Cont’d) • 數位助聽器 • Advantage • 價格較高 • Performance(清晰度、音質、SNR等)高 • Disadvantage • 調整彈性不易

24. Summary Although many techniques are provided to solve the problem of hearing loss, but those methods are still poor for practical application. • Hearing aid makes sound too loud • Poor performance in background noise • Hear things, but not clear • Acoustic feedback • Cost

25. Summary (Cont’d) Hearing-aid design involves in many region • Electronics (circuit, system, analog/digital signal processing etc.) • Medicine • Even though Art designer

26. Reference • Sound, http://www.britannica.com/ebc/article-64016 • Reliability of computer-automated hearing thresholds in cochlear-impaired listeners using ER-4B Canal Phone Earphones, James A. Henry, PhD et al., http://www.rehab.research.va.gov/jour/03/40/3/henry.htm • 富聆聽力中心, http://www.new-care.com.tw • Suband Adaptive Filtering Applied to Acoustic Feedback Reduction in Hearing Aids, M.G. Siqueira et al., http://citeseer.ist.psu.edu/siqueira96subband.html • B. Widrow, “A microphone array for hearing aids,” IEEE Circuits Syst. Mag., vol. 1, pp. 26-32, 2001. • 人耳聽覺與聽力損失, http://www.iosh.gov.tw/netbook/noear2.htm

27. Thank you for your attention.