DESIGN AND CONSTRUCTION OF A TELECOIL-ENABLED DIGITAL HEARING AID WITH A RECORDING SYSTEM

Presented by:

ATAKORA-AMANIAMPONG LARRY

CHINAKA CHINOMSO CHUKWUEBUKA

1

INTRODUCTION

• Hearing is the ability to perceive pressure waves generated by vibrating airmolecules known as sound.

• The frequency range of hearing for a normal human is 20Hz to 20kHz.

• Disabling hearing loss refers to hearing loss greater than 40dB in the betterhearing ear in adults and a hearing loss greater than 30dB in the betterhearing ear in children.

• Hearing aid is a small electronic device that is worn in or behind the ear toamplifier sounds so that a person with hearing loss can listen, communicateand participate fully in daily activities.

2

PROBLEM STATEMENT

• Most hearing aids are unable to filter background noise from desired sounds.As a result, some portions of speech in a noisy environment becomedistorted.

• Also, there are difficulties in determining the origin(direction) of sounds fromunseen source (localization) due to processing time and transmission of soundto the ear.

• Moreover, hearing aid users face stigmatization due to the negative socialattitudes toward the hard-of-hearing in the community.

3

JUSTIFICATION

• This project seeks to design and construct a hearing aid device with enhancedsignal receivers to minimize the signal-to-noise ratio of the device.

• The device also incorporates an audio recording system which the user canplay-back previous conversations when needed.

4

AIM

To design and construct a telecoil enabled digital hearing aid incorporatedwith an audio recording system for disabling hearing loss.

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OBJECTIVES

• To construct a hearing aid using digital signal processing.

• To use a directional microphone as the main input of acoustic sound to theamplifier.

• To incorporate a telecoil as a peripheral input of sound to the amplifier.

• To integrate a recording system for play-back of conversation using ICmemory.

6

LITERATURE REVIEW• The hearing aid amplifiers are designed to increase the loudness depending

on the specific gain of the device.

Barry SJ (1998), Usman F (2012) and Yusuf MA et al. (2013)

• Digital Signal Processing technology has allowed the creation of effectivedigital feedback control and management systems within hearing aids.

Stetzler T et al. (2000) and Shaji NM et al. (2002)

• Feedback occurs when the microphone picks up sound emitting from thespeaker which creates loop leading to a painful, irritating high pitched noise.

Dillon H. (2001) and Chung K. (2004)

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MATERIALSCOMPONENT TYPE/SPECIFICATION QUANTITY

Directional microphone 600Ω 1

Telecoil TA20AD01 1

Amplifier LM386 1

IC Recorder APR9600 1

Transistor BC547 1

Resistor 10-330k Ω 18

Capacitor 0.1-470𝜇𝐹 19

Headphone/Speaker 16 Ω with internal class D

amplifier

1

Battery 9V 1

PCB 6cm X 5cm 1

Switch SPST, PB_NO, DSWPK_3 1,2,1

LED Red 2

Connecting wires 8

BLOCK DIAGRAM

Input Transducers

Audio Amplifier

Recording System

Audio Output

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DESIGN ANALYSIS

• The hearing aid device is designed to consume very small or optimalamount of power within the range of 1 − 10𝑚𝑊.

• The voltage requirement of every major component in the circuit is withinthe range of 1.8 − 15𝑉 and therefore, a 5𝑉 DC power supply unit is used todrive the circuit.

• Frequency component used in the analysis was the minimum audiblefrequency (20Hz) unless specified.

• Operating and stand by currents are specified on the data sheets.

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BIASING OF MICROPHONE

• The directional microphone is fed with an optimal steady current 𝐼𝑑𝑐of 1𝑚𝐴.

𝐶1 =1

2𝜋𝑓𝑋𝑐

𝐶1 =1

2𝜋 × 20 × 80 × 103

∴ 𝐶1 = 100𝑛𝐹

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BIASING OF MICROPHONE(cont.)

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PRE-AMPLIFIER DESIGN

• The target gain for the design was approximately 500. The couplingcapacitor C2 is same as that of C1.

R3680Ω

R433Ω

Q1

BC547BP

R2330kΩ

C2

100nF

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PRE-AMPLIFIER DESIGN(cont.)

According to the data sheet of the manufacturer of the transistor, 𝑉𝐵𝐵 = 12𝑉 and the current

amplification factor 𝛽 = 100. The base resistor, 𝑅2 is set as 330𝑘Ω.

𝐼𝐵 =𝑉𝐵𝐵𝑅2

𝐼𝐵 =12

330 × 10−3= 36 × 10−6

∴ 𝐼𝐵 = 36𝜇𝐴

The relationship between the collector current, 𝐼𝐶 and the base current, 𝐼𝐵 is given as:

𝐼𝐶 = 𝛽𝐼𝐵

𝐼𝐶 = 100 × 36 × 10−6

∴ 𝐼𝐶 = 3.6𝑚𝐴

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PRE-AMPLIFIER DESIGN(cont.)

For maximum undistorted output swing, the quiescent collector voltage is set to half of the supply

voltage. Thus, 𝑉𝐶𝐸 = 0.5𝑉𝐶𝐶

𝑉𝐶𝐸 = 𝑉𝐶𝐶 − 𝐼𝐶𝑅3

𝑅3 =0.5𝑉𝐶𝐶𝐼𝐶

𝑅3 =0.5 × 5

3.6 × 10−3

∴ 𝑅3 = 694Ω

The closest standard resistor to this value is 680Ω, hence 𝑅3 = 680Ω.

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PRE-AMPLIFIER DESIGN(cont.)

However, the emitter to ground voltage, 𝑉𝐸 is typically one-tenth of supply voltage and the emitter

current is set at 𝐼𝐸 = 15𝑚𝐴. Thus;

𝑉𝐸 = 0.1𝑉𝐶𝐶

𝑉𝐸 = 0.1 × 5 = 0.5𝑉

The emitter to ground resistor can then be calculated from the formula below:

𝑅4 =𝑉𝐸𝐼𝐸

𝑅4 =0.5

15 × 10−3

∴ 𝑅4 = 33Ω

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CONNECTING THE TELECOIL

• In order to obtain the best EMI frequencies suppression, the wires from thetelecoil to the hearing instrument amplifier would have to be twisted, asshort as possible and shielded.

• The required capacitor C11 value equals 10nF to obtain a high pass filterwith a cut-off frequency of f1 = 80Hz.

• For extra suppression for frequencies above 10kHz can be obtained byusing C12 = 3.9nF, where f2 is the required cut-off frequency.

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CONNECTING THE TELECOIL(cont.)

𝐶11 =1

2𝜋𝑓1(𝑅𝑖𝑛 + 𝑅𝑜𝑢𝑡 )

𝐶11 =1

2𝜋 × 80(200 × 103 + 3 × 103)

∴ 𝐶11 = 9.8𝑛𝐹

𝐶12 =𝑅𝑖𝑛 + 𝑅𝑜𝑢𝑡

2𝜋𝑓2𝑅𝑖𝑛𝑅𝑜𝑢𝑡

𝐶12 =200 × 103 + 3 × 103

2𝜋 × 10 × 103(200 × 103 × 3 × 103)

∴ 𝐶12 = 5.38𝑛𝐹

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MEDIUM POWER AMPLIFIER STAGE

• The medium power amplifier increases the output of the pre-amplifier to anaudible level.

• Resistor R6 and capacitor C3 form the RC decoupling circuit. They areconnected across the power supply to smooth out noise.

• The time constant (Τ) produced by this RC circuit is given by: T = RC

• A shorter time constant is better suited to deal with higher frequencyvariations in voltage.

• An electrolytic capacitor of value 47𝜇𝐹 is used to achieve a 7𝑚𝑠 timeconstant.

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MEDIUM POWER AMPLIFIER STAGE(cont.)

• Hence, the closest standard resistor, R6=150Ω is used to produce a time constant approximate to 7ms.

𝑇 = 𝑅6𝐶3

7 × 10−3 = 𝑅5(47 × 10−6)

∴ 𝑅6 = 149Ω R5

10kΩ

Key=A

50%

LM386

3

2

4

7

6

51

8

C4

470µF

R6

150Ω

R710Ω

C7

100µF

C810µF

C51µF

C9100µF

R810Ω

C60.01µF

C347µF

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RECORDING SYSTEM

• An IC memory is used to achieve the recording and reproducing aspect ofthe hearing aid.

• Recording IC APR9600 can be used to record sound for a maximum of 60seconds.

• Playback and record operations are managed by an on-chip circuitry.

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TAPE MODE OF APR9600

22Photos courtesy of Aplus Integrated Circuit Inc.

CIRCUIT DIAGRAM

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RESULTS

Input frequency (Hz) Log of input

frequency (Hz)

Input voltage, 𝑉𝑖𝑛

(mV)

Output voltage, 𝑉𝑜𝑢𝑡

(V)

Gain (𝑉𝑖𝑛/𝑉𝑜𝑢𝑡) Gain (dB)

10 1.0 20 9.7 485 53.7

20 1.3 20 9.7 485 53.7

50 1.7 20 9.7 485 53.7

100 2.0 20 9.7 485 53.7

500 2.7 20 9.7 485 53.7

1000 3.0 20 9.7 485 53.7

5000 3.7 20 9.7 485 53.7

10000 4.0 20 9.7 485 53.7

20000 4.3 20 9.2 460 53.3

50000 4.7 20 3.0 150 43.5

100000 5.0 20 0.3 15 23.5

1000000 6.0 20 0.1 5 14.0

GAIN MEASUREMENT OF AMPLIFIER

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RESULTS(cont.)

0

10

20

30

40

50

60

1.7 2 2.7 3 3.7 4 4.3 4.7 5 6

GA

IN IN

DEC

IBEL

LOG OF FREQUENCIES (HZ)

A plot of frequency response of the amplifier

Graph25

RESULTS(cont.)

RESPONSE OF THE HEARING AID

Degree of hearing loss Response to the use of the device

Disabling hearing loss Improvement in hearing ability

Total deafness No improvement in hearing ability

Normal Improvement in hearing ability

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RESULTS(cont.)

SAMPLING RATE AND DURATION OF RECORDING

Resistance (𝑘Ω) Sampling

frequency (𝑘𝐻𝑧)

Input bandwidth

(𝑘𝐻𝑧)

Duration (sec)

84 4.2 2.1 60

38 6.4 3.2 40

24 8.0 4.0 32

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PICTURE OF COMPLETE ASSEMBLY

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DISCUSSION

• The frequency response curve shown above indicated that the amplifiedsignals were within the audio frequency domain.

• This means that the amplifier satisfies the purpose of which it was designed.

• On the test of the hearing aid device on individuals with disabling hearing lossshowed appreciable improvement in their hearing ability for all cases.

• The volume control was also found very useful as the users get to select orcontrol the level of intensity of sound to listen.

• Signals that are two loud may course further damage to the ears.

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CONCLUSION

• The aim of the research was to design and construct a hearing aid devicewith a recorder incorporated to it.

• The device was tested on several individuals with various levels of hearingimpairment.

• The result showed significant improvement in the hearing abilities for allpatients tested except for patients with complete deafness.

• The recorder also recorded and played back messages in an auto rewindoption of the normal mode.

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RECOMMENDATIONS

• We recommend that VLSI technology should be employed to reduce the sizeand weight of the device.

• Manufacturers should research in the area of water resistance to resolve theissues of water damage.

• Manufacturers should also focus on technological improvement inbackground noise and feedback management.

• For social and general acceptance for hearing aids, we suggest the design ofhearing aids created and modelled in a similar selection criteria aseyeglasses.

• Signal storage capacity should be increased to accommodate largermessages since the APR9600 records for 60 seconds only.

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THANK YOU

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