MCHAS

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NOISE REDUCTION STRATEGIES
Wave 4 SFR 17/05/04
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NOISE REDUCTION STRATEGIES

• Multi-microphone technology
• Noise Reduction Algorithms
• Expansion/Squelch
• Multi-channel compression i.e. BILL

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Whats The Problem?

• Interference from background noise represents the
  problem
• To improve speech perception in noise, hearing
  aids need to improve the signal-to-noise ratio
• Hearing impaired individuals require a greater
  S/N ratio than normal hearing listeners
• Hearing aids with omnidirectional microphones
  amplify sounds from all directions in the
  listeners environment equally

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Multi-Microphone Technology

• Directional amplification is the most effective
  way to improve signal-to-noise-ratios in hearing
  aids.

Dillon 2001
BKB sentences Moore, Johnson, Clark Pluvinage,
1992
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• Directional microphones enhance the hearing
  instruments sensitivity to sounds from desired
  direction over sound from other directions.
• It is the only technology that can effectively
  distinguish between desired and undesired speech
  signals. It is therefore quite effective in so
  called cocktail party situations.

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Directional Microphone
Important information
Adapted from Dilion 2001
Dillon 2001
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Dual Microphones

• Similar principle to single directional
  microphone but has two microphones with 2
  separate ports.
• Important users have the option to switch between
  omni-directional directional amplification.

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• The 2 microphones can be either
• 2 omni-directional ones whose outputs are coupled
  with electronic time delay and subtraction
  components to make them directional. When omni-
  direction is needed just one of the microphones
  is used.
• 1 directional microphone (which has a front and
  rear port) and the other an omni-directional
  microphone.
• The directional sensitivity of a microphone is
  usually indicated on a polar plot.

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Cardioid Family
Super
90
60
120
150
30
Hyper
180
0
330
210
Figure-8
300
240
270
Dillon 2001
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(No Transcript)
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Microphone Arrays

• FIXED MICROPHONE ARRAYS
• Current hearing aids implement variable polar
  patterns in one of two ways
• The first method introduces circuitry that allows
  either the user or the hearing aid programmer to
  select the desired directional pattern
  (presently, none of the aids on contract have
  this option)
• Such a system means that there will be the same
  polar pattern in all situations

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Microphone Arrays

• ADAPTIVE MICROPHONE ARRAYS
• The second method is designed to adaptively
  switch between polar patterns in response to the
  listening environment.
• In noisy environments adaptive instruments
  automatically shift polar patterns in an attempt
  to maximize S/N ratio in the presence of other
  noise sources.

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Microphone Arrays

• It may appear that adaptive directional
  processing has the potential to improve speech
  recognition across noisy environments
• However, little evidence to suggest more
  beneficial than fixed directional processing
• Ricketts Henry (2002) found advantages only in
  specific environmental listening situations
• Bentler et al (2003) found directional better
  than omnidirectional but adaptive no better than
  fixed.

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Microphone Arrays

• May be of more benefit when
• (Dillon, 2001)
• there is one dominant source of background noise
• When the noise source is very close
• When the aid wearer is in a large open area so
  that echoes and reverberation do not affect the
  operation of the microphone
• However, further research required in this area!

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A Word of Caution

• Increased circuit noise in directional mode
• Less effective as speaker-listener distance
  increases
• Echoes/reverberation
• Microphone drift
• Dirt!

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Patient selection

• Everyone benefits from increased SNR.
• Younger children must be able to hear sounds from
  all directions so must have access to
  omni-directional microphone.
• Directional depends on maturity
• Those with good low frequency hearing will hear
  low frequency noise through vents therefore the
  benefit of the directional microphone will be
  reduced.

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Phonak Aero 211, 311 413 AZ

• Provides omnidirectional and fixed directional
  (supercardiod) microphone performance. Either
  microphone can be implemented in specific
  programs.

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Danalogic 163D

• Provides omnidirectional and fixed directional
  (hypercardiod) microphone performance.
  Accessible in all programs.

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Danalogic 283D

• Provides omnidirectional and fixed directional
  (hypercardiod) microphone performance.
  Accessible in all programs.

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Prisma 2M 2D SP

• Directional microphone accessible in both
  channels.
• Fixed supercardiod
• Self calibrating to avoid microphone drift (uses
  2 omni-directional mics coupled together)

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Oticon Spirit II D

• Directional in program 2 only, and can be
  disabled.
• Fixed supercardiod array.
• In program 2, can access different degrees of LF
  cut.

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Noise Reduction Algorithms

• 2 types of noise reduction systems
• Modulation Detection Synchrony detection
• We will concentrate on modulation detection as it
  is far more common at present.

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Modulation Detection

• Underlying algorithms may vary between
  manufacturers but overall aim is to provide
  improved listener comfort and possibly improved
  speech intelligibility in background noise.
• Common to all algorithms is this need to identify
  which signal is noise and which is speech.

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Modulation Detection

• A speech/non speech detector analyses
  fluctuations in signal amplitude.
• Speech noise envelopes fluctuate in a well
  characterised manner
• Speech modulations tend to be slow and have big
  amplitude fluctuations.
• Noise modulations tend to be more constant with
  rapid and smaller fluctuations.

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Envelope of speech noise
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Modulation Detection

• The precise way in which the hearing aid
  processes the two signals will differ depending
  on the algorithm.
• One common approach is to
• Estimate when the speech signal is present in
  each channel.
• If the amplitude envelope at a channels output
  is characteristic of speech the gain in that
  channel remains unaltered.

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Modulation Detection

• If the envelope is relatively constant then the
  signal is assumed to be noise and the gain for
  that channel is reduced.
• If more noise is detected than speech at a
  certain channel, then the gain will also be
  reduced.

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• Important points to note
• If the unwanted signal has similar fluctuations
  to speech, then it is unlikely to be attenuated.
• Best results occur with steady state noise that
  has a narrow frequency band, or is of low
  frequency (to reduce the risk of upward spread of
  masking)
• If any other constant amplitude signals (e.g.
  pure tones) are identified then they will also be
  treated as a noise signal and be attenuated.
  This means that pure tones should not be used to
  either set or test the hearing aid, unless the
  noise reduction feature has been turned off.

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Summary

• Hearing aids with omni-directional microphones
  fail to provide the required S/N improvement
• Perhaps not so blatantly obvious
• This is true for ALL hearing aids with
  omni-directional microphones...
• including digital hearing aids with adaptive
  noise reduction or speech recognition
  algorithms.

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Whats Wrong with this Picture? (Starkey
laboratories, 2003)
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Modulation DetectionAdvantages and Limitations

• ADVANTAGE Channel specific gain reductions to
  ensure comfort whenever noise is present
• LIMITATION Cannot distinguish between noise only
  versus speech plus noise
• Tryanski Pogash
• (AAA 2204)

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Synchrony Detection

• Assumption you can find speech versus noise but
  you cannot separate them
• Designed to look for the unique structure of
  speech (energy)
• Maintains full response whenever speech is
  present.only goes into comfort mode when speech
  is no longer present
• Tyranski Pogash
• (AAA 2004)

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Synchrony DetectionAdvantages Limitations

• ADVANTAGE Can protect audibility of speech
  whenever speech is present
• LIMITATION Anytime speech is present there will
  be no comfort-based changes, even if noise
  present and high level
• Tyranski Pogash
• (AAA 2004)

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The Future of Noise Reduction?

• One company have recently launched a system that
  combines the two??

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Conclusions

• Very little research on NR systems and none at
  all with the paediatric population
• Alcantara et al (2003) found there to be no
  improvement in sound quality or listening comfort
  with noise reduction turned on
• Boymans Dreschler (2000) found no extra benefit
  with NR Directional when compared to
  directional alone
• Therefore, research published to date would
  suggest that directional microphone systems lead
  to a better improvement in the S/N ratio

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Phonak Aero 211AZ, 311AZ Supero 413 digital
Noise Canceler (dNC)

• Light indicates up to 3 dB gain reduction
• Moderate indicates up to 6 dB gain reduction
• Strong indicates up to 12 dB gain reduction
• OFF deactivates the dNC in any program

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Danalogic speech/ noise analysis in 14 frequency
bands
normal gain
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Danalogic 283D Noise Reduction

• Uses modulation or level fluctuations to identify
  speech from noise.

Noise Reduction either ON or OFF
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Prisma 2M 2D SP

• Can be deactivated.
• Reduction of gain can be altered to min12 dB,
  med18 dB and max24 dB. Defaults to min.
• Gain reduction is prioritised in channel 1 4.

C1 C2 C3 C4 Off - - - - Min min - - min Med med m
in min med Max max med med max
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Starkey Strata 312

• Off 0dB reduction
• Min Up to a 10dB channel gain reduction
  (default)
• Max Up to a 20dB channel gain reduction

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Oticon Spirit II

• No control over settings.
• Identifies speech noise signals based on
  modulation index.
• If signal characteristic of noise the release
  time will be delayed by up to 4 times. If
  characteristic of speech there will be no change
  in release time.

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Expansion/Squelch

• Expansion or squelch occurs when the compression
  ratio is less than 11, therefore gain decreases
  as the input level decreases.
• It is useful for decreasing the audibility of
  very low level sounds. Good as long as ALL
  inaudible sounds are unwanted e.g. microphone
  internal noise, computer hum.

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Expansion Squelch

• By removing low level electronic noises it can be
  thought of as a low level noise reduction
  strategy.
• Important to distinguish between true noise
  reduction algorithms and squelch.
• NHS DSP Aids with expansion/squelch
• Phonak Aero, Oticon Spirit II, Starkey Strata.

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Multi-channel compression- BILL

• BILL compression circuits were designed to
  improve speech intelligibility in background
  noise by providing more gain for soft low
  frequency sounds than for loud low frequency
  sounds.
• Based on the premise that background noise is
  predominantly low frequency, quite loud and hence
  would be attenuated while high frequency sounds
  would remain unaffected.

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Multi-channel compression- BILL

• Any aid that has
• Multi-channel compression
• Adjustable kneepoints
• Adjustable compression ratios
• . could be configured to a BILL response.

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Questions?