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Acoustic Feedback Reduction. Introduction. Acoustic Feedback Principle Acoustic Feedback Reduction Strategies Accessing Acoustic Feedback Tests Summary. Acoustic Feedback Principle. Input signal. Output signal. Input signal. Feedback signal. =. ( In phase). Feedback problems.

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Acoustic Feedback Reduction

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acoustic feedback reduction

Acoustic Feedback Reduction


Modernising Children’s Hearing Aid Services

  • Acoustic Feedback Principle
  • Acoustic Feedback Reduction Strategies
  • Accessing Acoustic Feedback Tests
  • Summary
acoustic feedback principle
Acoustic Feedback Principle

Input signal

Output signal

Input signal

Feedback signal


(In phase)

Feedback problems

Feedback signal

GN Resound

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.

feedback management strategies
Feedback Management Strategies




Hearing aid gain (dB)











Frequency (Hz)

Dillon 2001

gain reduction strategies


Volume decreased

Gain Reduction Strategies

Dillon 2001

gain reduction strategies2
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
gain reduction strategies3
Gain Reduction Strategies
  • Active Notch Filters

- Also called Roving Notches

- HA actively monitors it’s 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)

feedback cancellation
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.
feedback cancellation1

External leakage path



Internal feedback path

Feedback Cancellation

Dillon 2001

feedback cancellation2
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)
feedback cancellation3

1. Max gain with DFS

2. Max gain without DFS

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.



Taken from actual Danalogic 163D fitting with 2 mm vent

(GN Resound)

accessing feedback tests
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
accessing feedback tests1
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.


  • Optimize will insert static notch

if necessary

  • Adaptive sets up active notch


  • 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)

thank you




Modernising Children’s Hearing Aid Services