Advanced concepts in hearing aids

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Advanced concepts in hearing aids
Harvey Dillon National Acoustic Laboratories,
Australian Hearing, CRC for Cochlear Implant and
Hearing Aid Innovations
ASA, Melbourne, April 2001
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Aim of this talk

• To describe current signal processing features of
  hearing aids
• their benefits,
• their limitations,
• their prescription and fitting

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This talk can be down-loaded from the NAL Web-site

• www.nal.gov.au
• Research_at_NAL.GOV.AU

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Programmability
50 of market in USA
12 years old
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Programmability

• No direct benefit, but .
• Adjustments more likely to be done

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Programmability

• No direct benefit, but .
• Adjustments more likely to be done

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Programmability

• No direct benefit, but .
• Adjustments more likely to be done

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Programmability

• Patient selection
• Everyone

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Digital signal processing
Microphone
Earphone
Processor
1 0 0 1 0 0 1 1 .
Analog to digital converter
Digital to analog converter
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Digital signal processing
Benefits

• No direct benefit, but ..
• Some performance features only feasible with
  digital signal processing

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Digital signal processing

• Patient selection
• Anyone who needs a feature that cannot be
  provided by analog hearing aids.
• (But eventually, everyone)

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Compression limiting versus peak clipping
To prevent excessive loudness

• Peak clipping chops the tops off sounds
• Compression limiting rapidly turns the volume
  down

Benefits Comfort, Sound quality
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Peak clipping versus compression limiting
Compressed
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Compression limiting

• Patient selection
• Everyone, except those who need higher output
  levels than can be achieved with compression
  limiting. (Peak clippers can provide 6-10 dB
  higher output levels when measured with speech
  signals.)

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Wide dynamic range compression
Intense
Automatic
Non-linear
Moderate
Weak
Normal
Impaired
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Wide dynamic range compression

• Patient selection
• Most patients
• Unclear how to choose not to use WDRC
• Profound loss will limit the amount of
  compression that can be achieved

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Multi-channel compression
Hearing loss varies across frequencies so should
compression
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Multi-channel hearing aids
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Multi-channel hearing aids

• Benefits (re single channel compression)
• Small ! 5 to 10 in intelligibility at
  most.
• But .. benefit at low and high levels, re
  single channel compression has not been evaluated.

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Multi-channel compression

• Patient selection
• Probably beneficial for people with moderately or
  steeply sloping hearing loss.
• Not too harmful for anyone

More evidence urgently needed
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Squelch or expansion
Gain (dB)
Input level
90
80
Output (dB SPL)
70
60
30
40
50
60
70
80
90
Input level
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Expansion or squelch

• Patient selection
• Patients who complain about internal noise
• Perhaps patients who complain about low-level
  external sounds

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The biggest problem noise
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Suppressing noise adaptively
Hearing aid automatically alters its response
depending on the environment
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Adaptive noise suppression
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Noise Reduction
Signal
Input Spectral Level
Noise
Frequency
Gain
Signal
Output Spectral Level
Noise
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Steady HF-noise
Impulsive HF-noise
Babble-noise
Keidser Grant
Traffic-noise
Speech in quiet
Less LF/more HF gain in dB
More LF/less HF gain in dB
NAL-response
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Noise Reduction
Gain
Signal
Output Spectral Level
Noise
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Noise Reduction
Input Spectral Level
Noise
Frequency
Gain
Signal
Output Spectral Level
Noise
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Adaptive noise suppression

• Benefits
• A definite comfort advantage.
• Occasionally there may be an intelligibility
  advantage.
• Disadvantage
• May sometimes misinterpret what is the signal and
  what is the noise.

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Adaptive noise suppression

• Patient selection
• Everyone, but benefit will be greatest for people
  who need gain at all frequencies, which implies
  low frequency hearing loss should be greater than
  about 30 dB HL.

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Multi-memory
Different programs for different situations
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Multi-memory hearing aids

• Patient selection
• Active lifestyle, with hearing aid use in a
  variety of situations
• High frequency loss gt 55 dB HL
• Low frequency gain gt 0 dB
• Able to understand and use the controls, like to
  be control

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Improving signal to noise ratio
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Effect of decreasing noise
BKB sentences Moore, Johnson, Clark Pluvinage,
1992
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Directional microphones
Benefit 4 dB improvement in SNR
40 improvement in speech understanding
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Directivity Index
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Dual microphones

• Benefits
• directional when needed
• omni-directional when needed

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The cardioid family
90
Super
60
120
150
30
-10
-20
-30
180
0
Hyper
330
210
300
240
270
Figure-8
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Adaptive directional microphone
Front
?
-

?
T
Output
Figure 7.4 A simple adaptive directional
microphone with steerable nulls.
Source Dillon (2001) Hearing Aids
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45 mm
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Directional microphones

• Patient selection
• Any one can benefit from an improvement in SNR,
  from normal hearing to profound hearing loss.
• Hearing aid is only directional when it has gain.

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Feedback management

• Whistling occurs when amplification gt leakage
• Standard solutions
• plug ear tightly
• decrease high frequency amplification
• turn volume down

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Feedback management
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Unsophisticated aid
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Feedback management
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Feedback cancelling
-
Internal feedback path
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Feedback cancelling

• Benefits
• Approximately 10 dB more gain before feedback
• More open ear -gt better own voice quality
• More gain -gt more intelligible speech
• Less whistling -gt less embarrassment

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Feedback management and supression

• Patient selection
• Anyone who has trouble with feeback, particularly
• patients with profound loss,
• patients with near normal LF loss, and severe HF
  loss.

Frequency (Hz)
Hearing threshold (dB HL)
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Transposition
Figure 7.14 Input and output spectra for a
frequency transposition scheme in which the
output frequency equals half the input frequency.
The amplifier also provides some high frequency
pre-emphasis. The arrows show the reduction in
frequency of each formant.
Intensity
1000
250
4000
Frequency
Source Dillon (2001) Hearing Aids
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Spectral Enhancement
Intensity
1000
250
4000
Frequency
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Spectral enhancement
Figure 7.15 Spectrograms of the syllable /ata/
(a) unprocessed and (b) spectrally enhanced,
showing more pronounced formants (Fisher, Dillon
Storey, in preparation).
Time (seconds)
Source Dillon (2001) Hearing Aids
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Environmentally sensitive hearing aids
?
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Two linear benefits
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Wireless / Direct audio input
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Wireless systems

• Patient selection
• Anyone will benefit from the SNR increase.
• Use if the logistics of the situation allow it.
• (Bluetooth is coming)

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Bilateral or binaural amplification
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Bilateral hearing aids

• Patient selection
• Anyone with a hearing loss in both ears, unless
  the loss in either ear is too profound to be
  aidable, or too mild to be aidable.
• Benefit is greatest for those with bilateral
  moderate losses or greater.

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Implantable hearing aids
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Implantable hearing aids
Output vibrator
Microphone
Magnet coil
Magnet
Coil
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Bone-anchored hearing aid
Figure 16.6 Bone-anchored hearing aid, showing
its attachment through the skin to the bone.
Amended by permission from Entific Medical
Systems.
Source Dillon (2001) Hearing Aids
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Summary of features

• Programmability - enabling feature
• Digital signal processing - enabling feature
• Tone controls .......sound quality,
  ?
• Compression limiting .....sound
  quality
• Wide dynamic range compression .. 20 to
  50
• Multi-channel compression .... 5
• Directional microphones .... 50
• Wireless ... 100
• Bilateral hearing aids . 50
• Feedback management/cancelling quality,
  convenience, ?
• Small size in-ear location ...cosmetic,
  wind
• Adaptive noise suppression .. comfort
• Multi-memory ... comfort
• Volume control.comfort,
  intelligibility

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Thats all Folks