Hearing aid components

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Hearing aid components

• Block diagrams
• Microphones
• Amplifiers
• Digital circuits
• Tone controls and filters
• receivers

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Block diagrams 1
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Block diagrams 2
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Microphones principle of operation
Electret microphone
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Microphone frequency responses
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Microphone imperfections

• Exposed to chemical agents such as perspiration
• Random electronic noise
• Vibration and internal feedbacks
• Wind noise

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Directional microphone
External and internal time delay
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Polar diagram and directivity index (DI)
Dual microphone Adaptive directional microphone
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Amplifiers linear amplifiers class A
Constant current flows Inefficient Fast battery
drain Asymmetric clipping Inadequate headroom
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Headroom
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Amplifier linear class B (push-pull)
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Amplifier linear class D
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Amplifier compression
An amplifier that turns down its gain as the
input increases
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Digital circuit

• Greater precision
• Less internal noise
• Consume little power
• Ability to do complex operations

• ADC and DAC
• Sampling rate
• Anti-aliasing filter
• bits

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Sampling rates
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Bits 2n Dynamic range Relationship with speech
perception
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Filters and tone controls
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Multi-band or multi-channel?

• Interchangeable
• Band frequency range
• Channel physical chain of device
• Channel ????Band ????

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Receivers principle of operation
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Receiver frequency responses