mchas modernising children’s hearing aid services acoustic feedback reduction

MCHASMCHASModernising Children’s Hearing Aid ServicesModernising Children’s Hearing Aid Services

Acoustic Feedback Acoustic Feedback ReductionReduction


IntroductionIntroduction

Acoustic Feedback PrincipleAcoustic Feedback PrincipleAcoustic Feedback Reduction Acoustic Feedback Reduction

StrategiesStrategiesAccessing Acoustic Feedback TestsAccessing Acoustic Feedback TestsSummarySummary


Acoustic Feedback Principle Acoustic Feedback Principle

Input signal

Feedback signal

Output signal

(In phase)

Input signal

Feedback signalFeedback problems

=

GN Resound


Feedback occurs when: Feedback occurs when: – the ear mould is damaged or a poor fitthe ear mould is damaged or a poor fit– if the ear canal is occluded with waxif the ear canal is occluded with wax– when the hearing aid is turned up too when the hearing aid is turned up too

much.much.

Feedback reduction algorithms try to Feedback reduction algorithms try to ensure that the hearing aid is never ensure that the hearing aid is never turned up too much by:turned up too much by:– the clinicianthe clinician– the aid wearerthe aid wearer– automatically by the hearing aid itself.automatically by the hearing aid itself.


Feedback Management Feedback Management StrategiesStrategies

Desired

Dillon 2001

0

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40

Frequency (Hz)

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(d

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0

10

20

30

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Frequency (Hz)

He

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aid

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in (

dB

)

8000400020001000500250125

Desired

Gain Reduction StrategiesGain Reduction Strategies

Volume decreased

Dillon 2001


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10

20

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Frequency (Hz)

He

ari

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aid

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in (

dB

)

8000400020001000500250125


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Static Notch FiltersStatic Notch Filters– Fixed filters which limit the gain only Fixed filters which limit the gain only

around the frequencies where feedback around the frequencies where feedback oscillation is occurring.oscillation is occurring.

– Filters can only be changed by running a Filters can only be changed by running a new feedback check.new feedback check.

– Limitation: Feedback oscillation Limitation: Feedback oscillation frequency does not remain fixed over frequency does not remain fixed over timetime




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


Feedback CancellationFeedback Cancellation

Most sophisticated strategy as there Most sophisticated strategy as there is no gain reduction. is no gain reduction.

HA detects signal leaking back to the HA detects signal leaking back to the microphone and automatically microphone and automatically cancels this signal by adding some cancels this signal by adding some equal but opposite signal.equal but opposite signal.


External leakage path

+

- Internal feedback

path


Dillon 2001



Highly adaptiveHighly adaptiveCan control several feedback spikes Can control several feedback spikes

simultaneously without sacrificing simultaneously without sacrificing sound qualitysound quality

However, battery life, sampling time However, battery life, sampling time and processing power are key to its and processing power are key to its effectiveness effectiveness (Hayes, 2003)(Hayes, 2003)



Feedback Feedback cancellation is also cancellation is also known as ‘search & known as ‘search & destroy’ and can destroy’ and can allow up to 10 dB of allow up to 10 dB of extra gain before extra gain before feedback.feedback.

Limitation: Some Limitation: Some musical tones (highly musical tones (highly periodic) and simple periodic) and simple test box signals may test box signals may also be suppressed.also be suppressed.

2

1

1. Max gain with DFS

2. Max gain without DFS

Taken from actual Danalogic 163D fitting with 2 mm vent

(GN Resound)


Accessing Feedback TestsAccessing Feedback Tests

Active gain reduction strategiesActive gain reduction strategies– Phonak Supero the active notch filter is Phonak Supero the active notch filter is

deactivated during the fitting process and in deactivated during the fitting process and in the music program.the music program.

– In the Siemens Prisma 2M and DSP notch filters In the Siemens Prisma 2M and DSP notch filters are removed after 5 mins to see if feedback is are removed after 5 mins to see if feedback is still present.still present.

– Starkey Strata there is an automatic gain Starkey Strata there is an automatic gain reduction in the high channel.reduction in the high channel.

Feedback CancellationFeedback Cancellation– Danalogic 283D use this strategy. The ‘Digital Danalogic 283D use this strategy. The ‘Digital

Feedback Suppression’ (DFS) can be Feedback Suppression’ (DFS) can be deactivated in the software.deactivated in the software.

– Spirit II Direct and PowerSpirit II Direct and Power


Accessing Feedback Tests.Accessing Feedback Tests.

All feedback tests can be accessed by All feedback tests can be accessed by clicking on a software icon or going into clicking on a software icon or going into one of the ‘pull down’ menus.one of the ‘pull down’ menus.

All tests require noise free All tests require noise free environments.environments.

There may be a number of different There may be a number of different options available. Instructions normally options available. Instructions normally accompany any procedure but if you accompany any procedure but if you have difficulties try reading the have difficulties try reading the instructions in the help menu.instructions in the help menu.


SPIRIT II DIRECT & POWER


DANALOGIC 283D


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)


STRATA A312


Phonak Aero & SuperoPhonak Aero & Supero


NHS DSP Hearing Aid FB NHS DSP Hearing Aid FB StrategiesStrategies

Hearing AidHearing Aid Limits gain Limits gain in channels in channels

Static Static Notch Notch FilterFilter

Adaptive Adaptive Notch Notch FilterFilter

Phase Phase CancellationCancellation

Phonak Aero 211 Phonak Aero 211 & 311 AZ& 311 AZ

Phonak SuperoPhonak Supero Oticon Spirit II Oticon Spirit II Direct & PowerDirect & Power

Siemens Prisma Siemens Prisma 2M & DSP2M & DSP

Starkey Strata Starkey Strata A312A312

** Active high Active high channel gain channel gain

reductionreduction

Danalogic 283DDanalogic 283D


Thank-youThank-youQuestions?Questions?

mchas modernising children’s hearing aid services acoustic feedback reduction

Documents

feedback spikes

feedback reduction algorithms

aid wearer

feedback oscillation

new feedback check

maximum gain

signal leaking

opposite signal