The document describes the design and construction of a digital hearing aid with recording capabilities. It aims to minimize background noise and allow playback of conversations. Key features include: - A directional microphone and telecoil input to the amplifier circuit. - An audio recording system using an IC memory to store up to 60 seconds of playback. - A testing process that showed improved hearing for users with disabilities, except those who are completely deaf. The hearing aid was designed and constructed to address issues like noise filtering, sound localization, and social stigma. Test results demonstrated benefits for various levels of impaired hearing. Recommendations include reducing size, improving noise cancellation and feedback management, and increasing storage capacity.

1. DESIGN AND CONSTRUCTION OF A
TELECOIL-ENABLED DIGITAL HEARING
AID WITH A RECORDING SYSTEM
Presented by:
ATAKORA-AMANIAMPONG LARRY
CHINAKA CHINOMSO CHUKWUEBUKA
1

2. INTRODUCTION
• Hearing is the ability to perceive pressure waves generated by vibrating air
molecules 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 better
hearing ear in adults and a hearing loss greater than 30dB in the better
hearing ear in children.
• Hearing aid is a small electronic device that is worn in or behind the ear to
amplifier sounds so that a person with hearing loss can listen, communicate
and participate fully in daily activities.
2

3. 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 become
distorted.
• Also, there are difficulties in determining the origin(direction) of sounds from
unseen source (localization) due to processing time and transmission of sound
to the ear.
• Moreover, hearing aid users face stigmatization due to the negative social
attitudes toward the hard-of-hearing in the community.
3

4. JUSTIFICATION
• This project seeks to design and construct a hearing aid device with enhanced
signal receivers to minimize the signal-to-noise ratio of the device.
• The device also incorporates an audio recording system which the user can
play-back previous conversations when needed.
4

5. AIM
To design and construct a telecoil enabled digital hearing aid incorporated
with an audio recording system for disabling hearing loss.
5

6. OBJECTIVES
• To construct a hearing aid using digital signal processing.
• To use a directional microphone as the main input of acoustic sound to the
amplifier.
• To incorporate a telecoil as a peripheral input of sound to the amplifier.
• To integrate a recording system for play-back of conversation using IC
memory.
6

7. 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 effective
digital 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 the
speaker which creates loop leading to a painful, irritating high pitched noise.
Dillon H. (2001) and Chung K. (2004)
7

8. MATERIALS
COMPONENT 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

9. BLOCK DIAGRAM
Input
Transducers
Audio
Amplifier
Recording
System
Audio
Output
9

10. DESIGN ANALYSIS
• The hearing aid device is designed to consume very small or optimal
amount of power within the range of 1 − 10𝑚𝑊.
• The voltage requirement of every major component in the circuit is within
the range of 1.8 − 15𝑉 and therefore, a 5𝑉 DC power supply unit is used to
drive the circuit.
• Frequency component used in the analysis was the minimum audible
frequency (20Hz) unless specified.
• Operating and stand by currents are specified on the data sheets.
10

11. BIASING OF MICROPHONE
• The directional microphone is fed with an optimal steady current 𝐼 𝑑𝑐
of 1𝑚𝐴.
𝐶1 =
1
2𝜋𝑓𝑋𝑐
𝐶1 =
1
2𝜋 × 20 × 80 × 103
∴ 𝐶1 = 100𝑛𝐹
11

12. BIASING OF MICROPHONE(cont.)
12

13. PRE-AMPLIFIER DESIGN
• The target gain for the design was approximately 500. The coupling
capacitor C2 is same as that of C1.
R3
680Ω
R4
33Ω
Q1
BC547BP
R2
330kΩ
C2
100nF
13

14. PRE-AMPLIFIER DESIGN(cont.)
According to the data sheet of the manufacturer of the transistor, 𝑉 𝐵𝐵 = 12𝑉 and the current
amplification factor 𝛽 = 100. The base resistor, 𝑅2 is set as 330𝑘Ω.
𝐼 𝐵 =
𝑉 𝐵𝐵
𝑅2
𝐼 𝐵 =
12
330 × 10−3
= 36 × 10−6
∴ 𝐼 𝐵 = 36𝜇𝐴
The relationship between the collector current, 𝐼 𝐶 and the base current, 𝐼 𝐵 is given as:
𝐼 𝐶 = 𝛽𝐼 𝐵
𝐼 𝐶 = 100 × 36 × 10−6
∴ 𝐼 𝐶 = 3.6𝑚𝐴
14

15. 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 − 𝐼𝐶 𝑅3
𝑅3 =
0.5𝑉𝐶𝐶
𝐼𝐶
𝑅3 =
0.5 × 5
3.6 × 10−3
∴ 𝑅3 = 694Ω
The closest standard resistor to this value is 680Ω, hence 𝑅3 = 680Ω.
15

16. 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 𝐼 𝐸 = 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 =
𝑉𝐸
𝐼 𝐸
𝑅4 =
0.5
15 × 10−3
∴ 𝑅4 = 33Ω
16

17. CONNECTING THE TELECOIL
• In order to obtain the best EMI frequencies suppression, the wires from the
telecoil to the hearing instrument amplifier would have to be twisted, as
short as possible and shielded.
• The required capacitor C11 value equals 10nF to obtain a high pass filter
with a cut-off frequency of f1 = 80Hz.
• For extra suppression for frequencies above 10kHz can be obtained by
using C12 = 3.9nF, where f2 is the required cut-off frequency.
17

18. CONNECTING THE TELECOIL(cont.)
𝐶11 =
1
2𝜋𝑓1(𝑅𝑖𝑛 + 𝑅𝑜𝑢𝑡 )
𝐶11 =
1
2𝜋 × 80(200 × 103 + 3 × 103)
∴ 𝐶11 = 9.8𝑛𝐹
𝐶12 =
𝑅𝑖𝑛 + 𝑅𝑜𝑢𝑡
2𝜋𝑓2 𝑅𝑖𝑛 𝑅𝑜𝑢𝑡
𝐶12 =
200 × 103
+ 3 × 103
2𝜋 × 10 ×103(200 × 103 × 3 × 103)
∴ 𝐶12 = 5.38𝑛𝐹
18

19. MEDIUM POWER AMPLIFIER STAGE
• The medium power amplifier increases the output of the pre-amplifier to an
audible level.
• Resistor R6 and capacitor C3 form the RC decoupling circuit. They are
connected 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 frequency
variations in voltage.
• An electrolytic capacitor of value 47𝜇𝐹 is used to achieve a 7𝑚𝑠 time
constant.
19

20. MEDIUM POWER AMPLIFIER STAGE(cont.)
• Hence, the closest standard resistor, R6=150Ω is used to produce a
time constant approximate to 7ms.
𝑇 = 𝑅6 𝐶3
7×10−3
= 𝑅5(47×10−6
)
∴ 𝑅6 = 149ΩR5
10kΩ
Key=A
50%
LM386
3
2
4
7
6
51
8
C4
470µF
R6
150Ω
R7
10Ω
C7
100µF
C8
10µF
C5
1µF
C9
100µF
R8
10Ω
C6
0.01µF
C3
47µF
20

21. RECORDING SYSTEM
• An IC memory is used to achieve the recording and reproducing aspect of
the hearing aid.
• Recording IC APR9600 can be used to record sound for a maximum of 60
seconds.
• Playback and record operations are managed by an on-chip circuitry.
21

22. TAPE MODE OF APR9600
22Photos courtesy of Aplus Integrated Circuit Inc.

23. CIRCUIT DIAGRAM
23

24. 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
24

25. RESULTS(cont.)
0
10
20
30
40
50
60
1.7 2 2.7 3 3.7 4 4.3 4.7 5 6
GAININDECIBEL
LOG OF FREQUENCIES (HZ)
A plot of frequency response of the amplifier
Graph
25

26. 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
26

27. 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
27

28. PICTURE OF COMPLETE ASSEMBLY
28

29. DISCUSSION
• The frequency response curve shown above indicated that the amplified
signals 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 loss
showed appreciable improvement in their hearing ability for all cases.
• The volume control was also found very useful as the users get to select or
control the level of intensity of sound to listen.
• Signals that are two loud may course further damage to the ears.
29

30. CONCLUSION
• The aim of the research was to design and construct a hearing aid device
with a recorder incorporated to it.
• The device was tested on several individuals with various levels of hearing
impairment.
• The result showed significant improvement in the hearing abilities for all
patients tested except for patients with complete deafness.
• The recorder also recorded and played back messages in an auto rewind
option of the normal mode.
30

31. RECOMMENDATIONS
• We recommend that VLSI technology should be employed to reduce the size
and weight of the device.
• Manufacturers should research in the area of water resistance to resolve the
issues of water damage.
• Manufacturers should also focus on technological improvement in
background noise and feedback management.
• For social and general acceptance for hearing aids, we suggest the design of
hearing aids created and modelled in a similar selection criteria as
eyeglasses.
• Signal storage capacity should be increased to accommodate larger
messages since the APR9600 records for 60 seconds only. 31

32. THANK YOU
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