MCHAS

1
Acoustic Feedback Reduction
2
Introduction

• Acoustic Feedback Principle
• Acoustic Feedback Reduction Strategies
• Accessing Acoustic Feedback Tests
• Summary

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Acoustic Feedback Principle
Input signal
Output signal
Input signal
Feedback signal

(In phase)

Feedback problems
Feedback signal
GN Resound
4

• Feedback occurs when
• the ear mould is damaged or a poor fit
• if the ear canal is occluded with wax
• when the hearing aid is turned up too much.
• Feedback reduction algorithms try to ensure that
  the hearing aid is never turned up too much by
• the clinician
• the aid wearer
• automatically by the hearing aid itself.

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Feedback Management Strategies
40
Desired
30
Hearing aid gain (dB)
20
10
0
4000
125
8000
2000
1000
500
250
Frequency (Hz)
Dillon 2001
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Gain Reduction Strategies
Dillon 2001
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Gain Reduction Strategies
Dillon 2001
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Gain Reduction Strategies

• The maximum gain that can be achieved in
  different frequency regions is limited.
• Need to know what the maximum amount of gain (in
  each frequency) is, before feedback will occur.
  This safe value can be set by
• the clinician at the time of fitting
• the fitting software at the time of fitting
  (in-situ measurements)
• the hearing aid whenever the hearing aid is worn

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Gain Reduction Strategies

• Static Notch Filters
• Fixed filters which limit the gain only around
  the frequencies where feedback oscillation is
  occurring.
• Filters can only be changed by running a new
  feedback check.
• Limitation Feedback oscillation frequency does
  not remain fixed over time

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Gain Reduction Strategies

• Active Notch Filters
• - Also called Roving Notches
• - HA actively monitors its own output for
  feedback and creates a notch filter at the
  frequency it detects feedback.
• - Can implement two or three spikes
• - More effective than static notches but are
  limited and may be slow to adapt (Hayes, 2003)

11
Feedback Cancellation

• Most sophisticated strategy as there is no gain
  reduction.
• HA detects signal leaking back to the microphone
  and automatically cancels this signal by adding
  some equal but opposite signal.

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Feedback Cancellation
Dillon 2001
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Feedback Cancellation

• Highly adaptive
• Can control several feedback spikes
  simultaneously without sacrificing sound quality
• However, battery life, sampling time and
  processing power are key to its effectiveness
  (Hayes, 2003)

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Feedback Cancellation

• Feedback cancellation is also known as search
  destroy and can allow up to 10 dB of extra gain
  before feedback.
• Limitation Some musical tones (highly periodic)
  and simple test box signals may also be
  suppressed.

2
1
Taken from actual Danalogic 163D fitting with 2
mm vent (GN Resound)
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Accessing Feedback Tests

• Active gain reduction strategies
• Phonak Supero the active notch filter is
  deactivated during the fitting process and in the
  music program.
• In the Siemens Prisma 2M and DSP notch filters
  are removed after 5 mins to see if feedback is
  still present.
• Starkey Strata there is an automatic gain
  reduction in the high channel.
• Feedback Cancellation
• Danalogic 283D use this strategy. The Digital
  Feedback Suppression (DFS) can be deactivated in
  the software.
• Spirit II Direct and Power

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Accessing Feedback Tests.

• All feedback tests can be accessed by clicking on
  a software icon or going into one of the pull
  down menus.
• All tests require noise free environments.
• There may be a number of different options
  available. Instructions normally accompany any
  procedure but if you have difficulties try
  reading the instructions in the help menu.

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SPIRIT II DIRECT POWER
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DANALOGIC 283D
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PRISMA 2M DSP

• Optimize will insert static notch
• if necessary
• Adaptive sets up active notch
• filter(s)
• If feedback occurs aid checks
• for presence after 30 secs
• and 5 minutes. Notch(es)
• will be removed if feedback
• signal no longer present.
• Can implement one notch per
• channel (up to 4)

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STRATA A312
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Phonak Aero Supero
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NHS DSP Hearing Aid FB Strategies
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Thank-you

• Questions?