Hearing and Hearing Aids

蘇璟瑋 , 林宜宏 , 李彥鋒

Department of Electrical Engneering

National Taiwan University

Monday, April 9, 2007

Dept. of Electrical Engeering, NTU

2

Su, Lin and Lee

Outline

• The Structure and Function of Ear

• Hearing Loss

• Hearing Aids

• Summary

• Reference

Outline

Dept. of Electrical Engeering, NTU

3

Su, Lin and Lee

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

Dept. of Electrical Engeering, NTU

4

Su, Lin and Lee

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

Dept. of Electrical Engeering, NTU

5

Su, Lin and Lee

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

Frequency (kHz)

The frequency response of external ear

Total gainGain from PinnaGain from Ear Canal

External Ear

Dept. of Electrical Engeering, NTU

6

Su, Lin and Lee

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

Dept. of Electrical Engeering, NTU

7

Su, Lin and Lee

The Structure and Function of Ear (Cont’d)

• Middle Ear Transducer

Converts acoustic energy to mechanical energy

Converts mechanical energy to hydraulic energy

Amplifier

The frequency response of Middle Ear

Middle Ear

Dept. of Electrical Engeering, NTU

8

Su, Lin and Lee

The Structure and Function of Ear (Cont’d)

• Inner Ear Hearing

Frequency Selectivity

Balance

Inner Ear

Dept. of Electrical Engeering, NTU

9

Su, Lin and Lee

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

Dept. of Electrical Engeering, NTU

10

Su, Lin and Lee

Hearing Loss (Cont’d)

(1) Decreased Audibility

Decreased Audility

Conductive hearing loss Sensorineural hearing loss

Dept. of Electrical Engeering, NTU

11

Su, Lin and Lee

Hearing Loss (Cont’d)

(2) Decreased Dynamic Range Normally, the dynamic range of hearing equals

120dBs roughly.

Decreased Dynamic Range

Dept. of Electrical Engeering, NTU

12

Su, Lin and Lee

Hearing Loss (Cont’d)

(3) Decreased Frequency Resolution SNR Loss Sensitivity Loss Clarity Loss

(3) Decreased Temporal resolution

Decreased Frequency Resolution

Dept. of Electrical Engeering, NTU

13

Su, Lin and Lee

Hearing Aids

• Basic Structure and Design of Modern Hearing

Aids

MicrophoneAudio

AmplifierMicroprocessor earphonePre-amplifierVoice Voice

Dept. of Electrical Engeering, NTU

14

Su, Lin and Lee

Hearing Aids (Cont’d)

• The Function of Hearing Aids Compression

Dept. of Electrical Engeering, NTU

15

Su, Lin and Lee

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 instru

ments 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.

Dept. of Electrical Engeering, NTU

16

Su, Lin and Lee

Hearing Aids (Cont’d)

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

Dept. of Electrical Engeering, NTU

17

Su, Lin and Lee

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.

Dept. of Electrical Engeering, NTU

18

Su, Lin and Lee

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

Dept. of Electrical Engeering, NTU

19

Su, Lin and Lee

Hearing Aids (Cont’d)

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

Dept. of Electrical Engeering, NTU

20

Su, Lin and Lee

Hearing Aids (Cont’d)

• The Function of Hearing Aids (Cont’d) feedback cancellation Acoustic feedback in hearing aids, which, if severe enough, caus

es 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

Dept. of Electrical Engeering, NTU

21

Su, Lin and Lee

Hearing Aids (Cont’d)

• The Function of Hearing Aids (Cont’d) feedback cancellation (Cont’d) An adaptive algrithom attempts to identify the hearing-aid feedb

ack 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

Dept. of Electrical Engeering, NTU

22

Su, Lin and Lee

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

Dept. of Electrical Engeering, NTU

23

Su, Lin and Lee

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

Dept. of Electrical Engeering, NTU

24

Su, Lin and Lee

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

Dept. of Electrical Engeering, NTU

25

Su, Lin and Lee

Summary (Cont’d)

Hearing-aid design involves in many region

Electronics (circuit, system, analog/digital signal processing etc.) Medicine Even though Art designer

Dept. of Electrical Engeering, NTU

26

Su, Lin and Lee

Reference

• Sound, http://www.britannica.com/ebc/article-64016• Reliability of computer-automated hearing thresholds in cochlear-im

paired 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 i

n 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

Dept. of Electrical Engeering, NTU

27

Su, Lin and Lee

Thank you for your attention.Thank you for your attention.