Compression system in hearing aids

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Compression system in hearing aids

• Major role of compression
• Basic characteristics of compression
• dynamic and static characteristics
• The rational for the use of compression
• Combinations of compressors in HA
• Benefits and disadvantages of different
  compression systems

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Compression reduces the signal dynamic range
Intense sound have to be amplified less than weak
sound
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Dynamic characteristics of compression attack
and release time
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Output envelope
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Attack and release time

• Attack time usually around 5 msec
• Release time rarely less than 20 msec
• Dropout, pumping and breathing
• Adaptive release time

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Compression static characteristics
Compression threshold Compression
ratio Compression range Expansion Curvilinear
compressor
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Input and output automatic gain control
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Multichannel compression

• Reasons for multiband compression
• Drawbacks of multiband compression
• TILL and BILL

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Rational for use of compression 1. Avoiding
discomfort, distortion and damage
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2. Reducing inter-syllabic and inter-phonemic
intensity difference syllabic or phonemic
compression
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3. Reducing difference in long term level
Automatic volume control Input varies suddenly
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4. Increase sound comfort
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5. Loudness normalization
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6. Reducing noise Reduce low frequency gain
especially at high input levels
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• 7. Maximizing intelligibility
• 8. Empirical approaches
• eg. Fast acting compression for low frequency
  and slow acting compression for high frequency
• Combination of compressor in hearing aids

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Benefit and disadvantages of compression system

• Relative to linear amplification
• Limiting
• Typical input levels
• Low level inputs
• High level inputs
• Multichannel relative to single channels