mchas modernising children’s hearing aid services noise reduction strategies wave 4 sfr 17/05/04

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

NOISE REDUCTION NOISE REDUCTION STRATEGIESSTRATEGIES

Wave 4 SFR 17/05/04


NOISE REDUCTION STRATEGIESNOISE REDUCTION STRATEGIES

Multi-microphone technologyMulti-microphone technologyNoise Reduction AlgorithmsNoise Reduction Algorithms

Expansion/SquelchExpansion/SquelchMulti-channel compression i.e. BILLMulti-channel compression i.e. BILL


What’s The Problem?What’s The Problem?

Interference from background noise represents Interference from background noise represents the problemthe problem

To improve speech perception in noise, hearing To improve speech perception in noise, hearing aids need to improve the signal-to-noise ratioaids need to improve the signal-to-noise ratio

Hearing impaired individuals require a greater Hearing impaired individuals require a greater S/N ratio than normal hearing listenersS/N ratio than normal hearing listeners

Hearing aids with omnidirectional microphones Hearing aids with omnidirectional microphones amplify sounds from all directions in the listeners amplify sounds from all directions in the listeners environment equallyenvironment equally


Multi-Microphone TechnologyMulti-Microphone Technology

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

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20

40

60

80

100

-4 -3 -2 -1 0 1 2 3 4

Signal to noise ratio (dB)

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Effect of decreasing noise

BKB sentences: Moore, Johnson, Clark & Pluvinage, 1992

Dillon 2001


Directional microphones enhance the Directional microphones enhance the hearing instrument’s sensitivity to hearing instrument’s sensitivity to sounds from desired direction over sounds from desired direction over sound from other directions.sound from other directions.

It is the only technology that can It is the only technology that can effectively distinguish between effectively distinguish between desired and undesired desired and undesired speechspeech signals. It is therefore quite effective signals. It is therefore quite effective in so called ‘cocktail party’ situations.in so called ‘cocktail party’ situations.


Directional MicrophoneDirectional Microphone

Dillon 2001

Adapted from Dilion 2001

Important information

Rear soundsentering back

port are electronically delayed so…….

….. rear sounds entering the front port impinge on the

diaphragm at the same time. Hence the

diaphragm does not move.


Dual MicrophonesDual Microphones

Similar principle to single Similar principle to single directional microphone directional microphone but has two microphones but has two microphones with 2 separate ports.with 2 separate ports.

Important users have the Important users have the option to switch between option to switch between omni-directional & omni-directional & directional amplification.directional amplification.


The 2 microphones can be either:The 2 microphones can be either:– 2 omni-directional ones whose outputs are 2 omni-directional ones whose outputs are

coupled with electronic time delay and coupled with electronic time delay and subtraction components to make them subtraction components to make them directional. When omni- direction is needed directional. When omni- direction is needed just one of the microphones is used.just one of the microphones is used.

– 1 1 directional microphone (which has a front directional microphone (which has a front and rear port) and the other an omni-and rear port) and the other an omni-directional microphone.directional microphone.

The directional sensitivity of a The directional sensitivity of a microphone is usually indicated on a microphone is usually indicated on a polar plot. polar plot.


Microphone ArraysMicrophone Arrays

FIXED MICROPHONE ARRAYSFIXED MICROPHONE ARRAYSCurrent hearing aids implement variable Current hearing aids implement variable

polar patterns in one of two wayspolar patterns in one of two waysThe first method introduces circuitry that The first method introduces circuitry that

allows either the user or the hearing aid allows either the user or the hearing aid programmer to select the desired programmer to select the desired directional pattern directional pattern (presently, none of the (presently, none of the aids on contract have this option)aids on contract have this option)

Such a system means that there will be Such a system means that there will be the same polar pattern in all situationsthe same polar pattern in all situations



ADAPTIVE MICROPHONE ARRAYSADAPTIVE MICROPHONE ARRAYS The second method is designed to The second method is designed to

adaptively switch between polar adaptively switch between polar patterns in response to the listening patterns in response to the listening environment.environment.

In noisy environments adaptive In noisy environments adaptive instruments automatically shift polar instruments automatically shift polar patterns in an attempt to maximize patterns in an attempt to maximize S/N ratio in the presence of other S/N ratio in the presence of other noise sources.noise sources.



It may appear that adaptive directional It may appear that adaptive directional processing has the potential to improve speech processing has the potential to improve speech recognition across noisy environmentsrecognition across noisy environments

However, little evidence to suggest more However, little evidence to suggest more beneficial than fixed directional processingbeneficial than fixed directional processing

Ricketts & Henry (2002) found advantages only in Ricketts & Henry (2002) found advantages only in specific environmental listening situationsspecific environmental listening situations

Bentler et al (2003) found directional better than Bentler et al (2003) found directional better than omnidirectional but adaptive no better than fixed.omnidirectional but adaptive no better than fixed.


A Word of CautionA Word of Caution

Increased circuit noise in directional Increased circuit noise in directional modemode

Less effective as speaker-listener Less effective as speaker-listener distance increasesdistance increases

Echoes/reverberationEchoes/reverberationMicrophone driftMicrophone driftDirt!Dirt!


Patient selectionPatient selection

Everyone benefits from increased SNR.Everyone benefits from increased SNR.Younger children must be able to hear Younger children must be able to hear

sounds from all directions so must have sounds from all directions so must have access to omni-directional microphone.access to omni-directional microphone.

Directional depends on maturity Directional depends on maturity Those with good low frequency hearing Those with good low frequency hearing

will hear low frequency noise through will hear low frequency noise through vents therefore the benefit of the vents therefore the benefit of the directional microphone will be reduced.directional microphone will be reduced.


Phonak Aero 211, 311 & 413 Phonak Aero 211, 311 & 413 AZAZ

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


Danalogic 283DDanalogic 283D Provides omnidirectional and fixed directional Provides omnidirectional and fixed directional

(hypercardiod) microphone performance. (hypercardiod) microphone performance. Accessible in all programs.Accessible in all programs.


Prisma 2M & 2D SPPrisma 2M & 2D SPDirectional Directional

microphone microphone accessible in both accessible in both channels.channels.

Fixed supercardiod Fixed supercardiod Self calibrating to Self calibrating to

avoid microphone avoid microphone drift (uses 2 omni-drift (uses 2 omni-directional mics directional mics coupled together)coupled together)


Oticon Spirit II DOticon Spirit II D

Directional in Directional in program 2 only, program 2 only, and can be and can be disabled.disabled.

Fixed supercardiod Fixed supercardiod array.array.

In program 2, can In program 2, can access different access different degrees of LF cut.degrees of LF cut.


Noise Reduction AlgorithmsNoise Reduction Algorithms

2 types of noise reduction systems2 types of noise reduction systems

Modulation Detection & Synchrony Modulation Detection & Synchrony detectiondetection

We will concentrate on modulation We will concentrate on modulation detection as it is far more common at detection as it is far more common at present.present.


Modulation DetectionModulation Detection

Underlying algorithms may vary Underlying algorithms may vary between manufacturer’s but overall between manufacturer’s but overall aim is to provide improved listener aim is to provide improved listener comfort and possibly improved comfort and possibly improved speech intelligibility in background speech intelligibility in background noise.noise.

Common to all algorithms is this Common to all algorithms is this need to identify which signal is noise need to identify which signal is noise and which is speech.and which is speech.



A speech/non speech detector analyses A speech/non speech detector analyses fluctuations in signal amplitude. fluctuations in signal amplitude.

Speech & noise envelopes fluctuate in a Speech & noise envelopes fluctuate in a well characterised manner:well characterised manner:– Speech modulations tend to be slow and Speech modulations tend to be slow and

have big amplitude fluctuations. have big amplitude fluctuations. – Noise modulations tend to be more Noise modulations tend to be more

constant with rapid and smaller constant with rapid and smaller fluctuations. fluctuations.


Envelope of speech + noiseEnvelope of speech + noise


The precise way in which the The precise way in which the hearing aid processes the two hearing aid processes the two signals will differ depending on the signals will differ depending on the algorithm.algorithm.

One common approach is to:One common approach is to:1.1. Estimate when the speech signal is Estimate when the speech signal is

present in each channel. present in each channel.

2.2. If the amplitude envelope at a If the amplitude envelope at a channel’s output is characteristic of channel’s output is characteristic of speech the gain in that channel speech the gain in that channel remains unaltered. remains unaltered.




3.3. If the envelope is relatively constant If the envelope is relatively constant then the signal is assumed to be noise then the signal is assumed to be noise and the gain for that channel is and the gain for that channel is reduced. reduced.

4.4. If more noise is detected than speech If more noise is detected than speech at a certain channel, then the gain will at a certain channel, then the gain will also be reduced.also be reduced.


Important points to note:Important points to note:– If the unwanted signal has similar fluctuations to If the unwanted signal has similar fluctuations to

speech, then it is unlikely to be attenuated.speech, then it is unlikely to be attenuated.

– Best results occur with steady state noise that Best results occur with steady state noise that has a narrow frequency band, or is of low has a narrow frequency band, or is of low frequency (to reduce the risk of upward spread frequency (to reduce the risk of upward spread of masking)of masking)

– If any other ‘constant’ amplitude signals (e.g. If any other ‘constant’ amplitude signals (e.g. pure tones) are identified then they will also be pure tones) are identified then they will also be treated as a ‘noise’ signal and be attenuated. treated as a ‘noise’ signal and be attenuated. This means that pure tones should not be used This means that pure tones should not be used to either set or test the hearing aid, unless the to either set or test the hearing aid, unless the noise reduction feature has been turned off.noise reduction feature has been turned off.


Modulation Detection:Modulation Detection:Advantages and LimitationsAdvantages and Limitations

ADVANTAGE: Channel specific gain ADVANTAGE: Channel specific gain reductions to ensure comfort whenever reductions to ensure comfort whenever noise is presentnoise is present

LIMITATION: Cannot distinguish LIMITATION: Cannot distinguish between noise only versus speech plus between noise only versus speech plus noisenoise

Tryanski & PogashTryanski & Pogash

(AAA 2204)(AAA 2204)


Synchrony DetectionSynchrony Detection

Assumption: you can find speech versus Assumption: you can find speech versus noise but you cannot separate themnoise but you cannot separate them

Designed to look for the unique structure Designed to look for the unique structure of speech (energy)of speech (energy)

Maintains full response whenever speech Maintains full response whenever speech is present….only goes into comfort mode is present….only goes into comfort mode when speech is no longer presentwhen speech is no longer present

Tyranski & PogashTyranski & Pogash(AAA 2004)(AAA 2004)


Synchrony Detection:Synchrony Detection:Advantages & LimitationsAdvantages & Limitations

ADVANTAGE: Can protect audibility of ADVANTAGE: Can protect audibility of speech whenever speech is presentspeech whenever speech is present

LIMITATION: Anytime speech is LIMITATION: Anytime speech is present there will be no comfort-based present there will be no comfort-based changes, even if noise present and changes, even if noise present and high levelhigh level

Tyranski & PogashTyranski & Pogash

(AAA 2004)(AAA 2004)


The Future of Noise The Future of Noise Reduction?Reduction?

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


ConclusionsConclusions

Very little research on NR systems and none at all with the Very little research on NR systems and none at all with the paediatric populationpaediatric population

Alcantara et al (2003) found there to be no improvement in Alcantara et al (2003) found there to be no improvement in sound quality or listening comfort with noise reduction turned sound quality or listening comfort with noise reduction turned onon

Boymans & Dreschler (2000) found no extra benefit with NR Boymans & Dreschler (2000) found no extra benefit with NR & Directional when compared to directional alone& Directional when compared to directional alone

Therefore, research published to date would suggest that Therefore, research published to date would suggest that directional microphone systems lead to a better improvement directional microphone systems lead to a better improvement in the S/N ratioin the S/N ratio


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

Light indicates up to 3 dB gain Light indicates up to 3 dB gain reductionreduction

Moderate indicates up to 6 dB Moderate indicates up to 6 dB gain reductiongain reduction

Strong indicates up to 12 dB gain Strong indicates up to 12 dB gain reductionreduction

OFF deactivates the dNC in any OFF deactivates the dNC in any programprogram


Danalogic speech/ noise Danalogic speech/ noise analysis analysis

in 14 frequency bandsin 14 frequency bands

Noise SpeechNoise NoiseSpeech Speech Speech

Noise NoiseSpeech Speech Speech Speech Speech

reducedgain

reducedgain

normal gainnormal gain


Danalogic 283D Noise Danalogic 283D Noise ReductionReduction

Uses modulation or level fluctuations to identify ‘speech’ from Uses modulation or level fluctuations to identify ‘speech’ from noise.noise.

Noise Reduction either Noise Reduction either ON or OFFON or OFF


Prisma 2M & 2D SPPrisma 2M & 2D SP Can be deactivated.Can be deactivated. Reduction of gain can Reduction of gain can

be altered to ‘min=12 be altered to ‘min=12 dB’, ‘med=18 dB’ and dB’, ‘med=18 dB’ and ‘max=24 dB’. ‘max=24 dB’. Defaults to min.Defaults to min.

Gain reduction is Gain reduction is prioritised in channel prioritised in channel 1 & 4.1 & 4.

C1 C2 C3 C4Off - - - -Min min - - minMed med min min medMax max med med max


Starkey Strata 312Starkey Strata 312

Off = 0dB Off = 0dB reductionreduction

Min = Up to a 10dB Min = Up to a 10dB channel gain channel gain reduction reduction (default)(default)

Max = Up to a Max = Up to a 20dB channel gain 20dB channel gain reductionreduction


Oticon Spirit IIOticon Spirit II

No control over settings.No control over settings.Identifies speech & noise signals Identifies speech & noise signals

based on modulation index.based on modulation index.If signal characteristic of noise the If signal characteristic of noise the

release time will be delayed by up to release time will be delayed by up to 4 times. If characteristic of speech 4 times. If characteristic of speech there will be no change in release there will be no change in release time.time.


Expansion/SquelchExpansion/Squelch

Expansion or squelch occurs when the Expansion or squelch occurs when the compression ratio is less than 1:1, compression ratio is less than 1:1, therefore gain decreases as the input therefore gain decreases as the input level decreases.level decreases.

It is useful for decreasing the It is useful for decreasing the audibility of very low level sounds. audibility of very low level sounds. Good as long as Good as long as ALLALL inaudible sounds inaudible sounds are unwanted e.g. microphone are unwanted e.g. microphone internal noise, computer hum.internal noise, computer hum.


Expansion & SquelchExpansion & Squelch

By removing low level electronic noises it By removing low level electronic noises it can be thought of as a low level noise can be thought of as a low level noise reduction strategy.reduction strategy.

Important to distinguish between ‘true Important to distinguish between ‘true noise reduction algorithms’ and squelch.noise reduction algorithms’ and squelch.

NHS DSP Aids with expansion/squelch:NHS DSP Aids with expansion/squelch:– Phonak Phonak AeroAero, Oticon Spirit II, Oticon Spirit II, , Starkey Strata.Starkey Strata.


Multi-channel compression- Multi-channel compression- BILLBILL

BILL compression circuits were designed to BILL compression circuits were designed to improve speech intelligibility in background improve speech intelligibility in background noise by providing more gain for soft low noise by providing more gain for soft low frequency sounds than for loud low frequency frequency sounds than for loud low frequency sounds.sounds.

Based on the premise that background noise Based on the premise that background noise is predominantly low frequency, quite loud is predominantly low frequency, quite loud and hence would be attenuated while high and hence would be attenuated while high frequency sounds would remain unaffected.frequency sounds would remain unaffected.


Multi-channel compression- Multi-channel compression- BILLBILL

Any aid that has……Any aid that has……– Multi-channel compressionMulti-channel compression– Adjustable kneepointsAdjustable kneepoints– Adjustable compression ratiosAdjustable compression ratios

………………. could be configured to a BILL . could be configured to a BILL response.response.


Questions?Questions?

mchas modernising children’s hearing aid services noise reduction strategies wave 4 sfr 17/05/04

Documents

hearing aid programmer

omni directional microphone

single directional microphone

directional sensitivity

bill slide

desired directional

omnidirectional ones

noise ratio