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NOISE REDUCTION STRATEGIES

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NOISE REDUCTION STRATEGIES

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    1. MCHAS Modernising Children’s Hearing Aid Services NOISE REDUCTION STRATEGIES All hearing aids allow for the frequency response of the hearing aid to be manipulated. How each aid does this differs, however.All hearing aids allow for the frequency response of the hearing aid to be manipulated. How each aid does this differs, however.

    2. NOISE REDUCTION STRATEGIES Multi-microphone technology Noise Reduction Algorithms Expansion/Squelch Multi-channel compression i.e. BILL

    3. What’s 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

    4. Multi-Microphone Technology Directional amplification is the most effective way to improve signal-to-noise-ratios in hearing aids. Evidence based ……Only proven form of signal processing that can improve SNR in a way that leads to improved intelligibility. Benefit decreases as environment becomes more reverberant. Not a fix for poor acoustics, better to try and fix acoustics if possible. Up to 7 dB improvements in SNR have been shown with directional microphones (Valente et al 1995), BTE on average give 4 dB Sentence test: Steepest part of slope improvement in 18% per 1 dB SNR improvement, however for isolated speech sounds can be as little as 3%. Take 10% to be conservative, this means a directional mic that can provide say 3-4 dB improvement in general will result in 30-40% improvement in speech intelligibility Very important for children to hear omni-directionality: safety and learningEvidence based ……Only proven form of signal processing that can improve SNR in a way that leads to improved intelligibility. Benefit decreases as environment becomes more reverberant. Not a fix for poor acoustics, better to try and fix acoustics if possible. Up to 7 dB improvements in SNR have been shown with directional microphones (Valente et al 1995), BTE on average give 4 dB Sentence test: Steepest part of slope improvement in 18% per 1 dB SNR improvement, however for isolated speech sounds can be as little as 3%. Take 10% to be conservative, this means a directional mic that can provide say 3-4 dB improvement in general will result in 30-40% improvement in speech intelligibility Very important for children to hear omni-directionality: safety and learning

    5. Directional microphones enhance the hearing instrument’s 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. Distinguishes between speech sounds by spatial cues. Based on assumption that sounds that are unwanted are generally behind us. Other noise reduction strategies cannot do this as undesired and desired speech has the same spectrum and time structure.Distinguishes between speech sounds by spatial cues. Based on assumption that sounds that are unwanted are generally behind us. Other noise reduction strategies cannot do this as undesired and desired speech has the same spectrum and time structure.

    6. Directional Microphone Sounds coming from the rear enter the rear port first. They are then delayed so that rear sounds that enter the front port impinge on the diaphragm at the same time as sounds from the rear port and hence the diaphragm does not move & sounds (from rear) not picked up. Directional mics more affected by affected by wind noise Only give speech intelligibility improvements if speaker in front of listener. Not much good in cars etcSounds coming from the rear enter the rear port first. They are then delayed so that rear sounds that enter the front port impinge on the diaphragm at the same time as sounds from the rear port and hence the diaphragm does not move & sounds (from rear) not picked up. Directional mics more affected by affected by wind noise Only give speech intelligibility improvements if speaker in front of listener. Not much good in cars etc

    7. 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. More common to find 2 microphone directionality as important users have the option to switch between omni-directional (equally sensitive to sounds from all directions) and directional amplification. The only disadvantages of a simple directional microphone come from the fact that it can’t easily be switch back into an omni-directional microphone. This is particularly a problem in windy places, or when the patent needs to hear sounds equally well no matter what direction they come from. More common to find 2 microphone directionality as important users have the option to switch between omni-directional (equally sensitive to sounds from all directions) and directional amplification. The only disadvantages of a simple directional microphone come from the fact that it can’t easily be switch back into an omni-directional microphone. This is particularly a problem in windy places, or when the patent needs to hear sounds equally well no matter what direction they come from.

    8. 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. By using two microphones, the electronics can combine them to make a directional microphone, or just use one to make an omni-directional microphone. Alternatively, one of the microphones can be a directional microphone and the other an omni-directional microphone. The output from the second microphone is electronically delayed and subtracted from the first microphone output Internal time delays, this is the delay caused by an acoustic damper or resister. Effectively has a low pass filter which passes most of amplified signal but with some delay External time delay is time taken for sounds from outside aid to get from one inlet to the other.By using two microphones, the electronics can combine them to make a directional microphone, or just use one to make an omni-directional microphone. Alternatively, one of the microphones can be a directional microphone and the other an omni-directional microphone. The output from the second microphone is electronically delayed and subtracted from the first microphone output Internal time delays, this is the delay caused by an acoustic damper or resister. Effectively has a low pass filter which passes most of amplified signal but with some delay External time delay is time taken for sounds from outside aid to get from one inlet to the other.

    9. Depending on the relationship between the port spacing and the internal delay in the microphone, a whole family of sensitivity patterns can be obtained. Some specific examples are the cardioid, the super-cardioid, the hyper-cardioid, and the Figure-8. Although the three cardioids have nulls in different angles, they have quite similar directivity patterns, and it is not critical which pattern is used. It is also worth remembering that these neat looking patterns are only obtained when the hearing aid is measured suspended away from any obstacles. When it is worn on the head, the head adds its own directivity pattern and a more complex combined pattern emerges.Depending on the relationship between the port spacing and the internal delay in the microphone, a whole family of sensitivity patterns can be obtained. Some specific examples are the cardioid, the super-cardioid, the hyper-cardioid, and the Figure-8. Although the three cardioids have nulls in different angles, they have quite similar directivity patterns, and it is not critical which pattern is used. It is also worth remembering that these neat looking patterns are only obtained when the hearing aid is measured suspended away from any obstacles. When it is worn on the head, the head adds its own directivity pattern and a more complex combined pattern emerges.

    10. Polar sensitivity plots are a useful tool when measuring the various different directional responses. These diagrams consist of two circles, the outer circle illustrating 0dB attenuation and the inner circle 10dB of attenuation. They are a frequency specific measurement. 0 indicates the position of the patients nose. The polar plot indicates the areas around the head where attenuation occurs. To obtain such patterns a single noise source actually moves around the systemPolar sensitivity plots are a useful tool when measuring the various different directional responses. These diagrams consist of two circles, the outer circle illustrating 0dB attenuation and the inner circle 10dB of attenuation. They are a frequency specific measurement. 0 indicates the position of the patients nose. The polar plot indicates the areas around the head where attenuation occurs. To obtain such patterns a single noise source actually moves around the system

    11. 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 They usually use subtractive processing in which waveforms (if using 2 mics) from the rear microphone, which will have been delayed, are subtracted from signals from the front microphone. With dual port directional microphones the subtraction occurs mechanically as sounds from each port press on opposite sides of the diaphragm. Adaptive Arrays = The microphone alters the way signals are combined so that they have minimum sensitivity for sounds coming from the direction of dominant noise source. Noise from the rear is subtracted from the noise from the front to leave a difference signal which will be the what is left from the front signal.They usually use subtractive processing in which waveforms (if using 2 mics) from the rear microphone, which will have been delayed, are subtracted from signals from the front microphone. With dual port directional microphones the subtraction occurs mechanically as sounds from each port press on opposite sides of the diaphragm. Adaptive Arrays = The microphone alters the way signals are combined so that they have minimum sensitivity for sounds coming from the direction of dominant noise source. Noise from the rear is subtracted from the noise from the front to leave a difference signal which will be the what is left from the front signal.

    12. 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. In many circumstances (when noise comes from several directions) the sterrable microphone works no better or worse than an ordinary directional microphone. To alter the directionality you need to just change the time delay of the rear mic/port. The circuit automatically varies the time delay in whichever direction decreases the total power at the output (this will be signal only, as more power when signal + noise). If loud noise then T will settle down to the value that minimizes the microphone sensitivity in the direction of the unwanted noise source.In many circumstances (when noise comes from several directions) the sterrable microphone works no better or worse than an ordinary directional microphone. To alter the directionality you need to just change the time delay of the rear mic/port. The circuit automatically varies the time delay in whichever direction decreases the total power at the output (this will be signal only, as more power when signal + noise). If loud noise then T will settle down to the value that minimizes the microphone sensitivity in the direction of the unwanted noise source.

    13. 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. ie. Panning noise sourcesie. Panning noise sources

    14. 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!

    15. A Word of Caution Increased circuit noise in directional mode Less effective as speaker-listener distance increases Echoes/reverberation Microphone drift Dirt!

    16. 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. Selecting an aid with this feature depends on the maturity of the child. Must be able to competently switch between directional and omni-directional. Possilbly from 7 years onwards!Selecting an aid with this feature depends on the maturity of the child. Must be able to competently switch between directional and omni-directional. Possilbly from 7 years onwards!

    17. Phonak Aero 211, 311 & 413 AZ Provides omnidirectional and fixed directional (supercardiod) microphone performance. Either microphone can be implemented in specific programs. Party noise= directional microphone is the default.Party noise= directional microphone is the default.

    18. Danalogic 163D Provides omnidirectional and fixed directional (hypercardiod) microphone performance. Accessible in all programs. Slightly more sensitive to sounds from behind than phonak super-cardioid.Slightly more sensitive to sounds from behind than phonak super-cardioid.

    19. Danalogic 283D Provides omnidirectional and fixed directional (hypercardiod) microphone performance. Accessible in all programs. Slightly more sensitive to sounds from behind than phonak super-cardioid.Slightly more sensitive to sounds from behind than phonak super-cardioid.

    20. 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)

    21. 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.

    22. 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. Omni-dirctional Mic will pick up everything: sometimes be ‘noise’ only or ‘speech’ only but more commonly it is a combination of the two. The best way to improve speech intelligibility in background noise is to improve the signal to noise ratio, and hence make the signal more audible. No strong evidence to support claim that noise reduction algorithms improve speech intelligibility in noise, especially if noise is speaker babble.Omni-dirctional Mic will pick up everything: sometimes be ‘noise’ only or ‘speech’ only but more commonly it is a combination of the two. The best way to improve speech intelligibility in background noise is to improve the signal to noise ratio, and hence make the signal more audible. No strong evidence to support claim that noise reduction algorithms improve speech intelligibility in noise, especially if noise is speaker babble.

    23. Modulation Detection Underlying algorithms may vary between manufacturer’s 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.

    24. 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.

    25. Envelope of speech + noise

    26. 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 channel’s output is characteristic of speech the gain in that channel remains unaltered. Modulation Detection The hearing aid will sample the incoming signal over a period of approximately 0.5s. The hearing aid will sample the incoming signal over a period of approximately 0.5s.

    27. 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.

    28. 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.

    29. 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.

    30. What’s Wrong with this Picture? (Starkey laboratories, 2003)

    31. Modulation Detection: Advantages 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)

    32. 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)

    33. Synchrony Detection: Advantages & 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)

    34. MCHAS Modernising Children’s Hearing Aid Services The Future of Noise Reduction? One company have recently launched a system that combines the two……??

    35. 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

    36. 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 Remember phonak Aero has 15 channels, therefore signal/noise detection occurs across quite a number of narrow band frequencies.Remember phonak Aero has 15 channels, therefore signal/noise detection occurs across quite a number of narrow band frequencies.

    38. Danalogic 283D Noise Reduction Speech and noise maybe identified by taking a measure of modulation frequency…………….. Or modulation depth (difference between peak and troughs of signal). Speech normally has a modulation index of at least 12 dB, an aid like the danalogic would therefore not attenuate signals that had a modulation index of at least 12 dB. Signals that had a low modulation index would be attenuated by 6 dB or 9 dB. Speech and noise maybe identified by taking a measure of modulation frequency…………….. Or modulation depth (difference between peak and troughs of signal). Speech normally has a modulation index of at least 12 dB, an aid like the danalogic would therefore not attenuate signals that had a modulation index of at least 12 dB. Signals that had a low modulation index would be attenuated by 6 dB or 9 dB.

    39. Prisma 2M & 2D SP Can be deactivated. Reduction of gain can be altered to ‘min=12 dB’, ‘med=18 dB’ and ‘max=24 dB’. Defaults to min. Gain reduction is prioritised in channel 1 & 4. Prioritised to ensure there is not too much of a drop in gain over mid frequencies where more important speech info occurs?Prioritised to ensure there is not too much of a drop in gain over mid frequencies where more important speech info occurs?

    40. Starkey Strata 312 Off = 0dB reduction Min = Up to a 10dB channel gain reduction (default) Max = Up to a 20dB channel gain reduction

    41. 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.

    42. Expansion/Squelch Expansion or squelch occurs when the compression ratio is less than 1:1, 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.

    43. 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. Will have different consequences when testing aids in the field……..Will have different consequences when testing aids in the field……..

    44. 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.

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

    46. MCHAS Modernising Children’s Hearing Aid Services Questions?

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