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NOISE REDUCTION STRATEGIES

NOISE REDUCTION STRATEGIES. Wave 4 SFR 17/05/04. NOISE REDUCTION STRATEGIES. Multi-microphone technology Noise Reduction Algorithms Expansion/Squelch Multi-channel compression i.e. BILL. What’s The Problem?. Interference from background noise represents the problem

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NOISE REDUCTION STRATEGIES

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  1. NOISE REDUCTION STRATEGIES Wave 4 SFR 17/05/04 MCHAS Modernising Children’s Hearing Aid Services

  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. Effect of decreasing noise Multi-Microphone Technology • Directional amplification is the most effective way to improve signal-to-noise-ratios in hearing aids. Dillon 2001 BKB sentences: Moore, Johnson, Clark & Pluvinage, 1992

  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.

  6. Rear sounds entering 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. Directional Microphone Important information Adapted from Dilion 2001 Dillon 2001

  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.

  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.

  9. 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

  10. 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.

  11. 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.

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

  13. 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.

  14. Phonak Aero 211, 311 & 413 AZ • Provides omnidirectional and fixed directional (supercardiod) microphone performance. Either microphone can be implemented in specific programs.

  15. Danalogic 283D • Provides omnidirectional and fixed directional (hypercardiod) microphone performance. Accessible in all programs.

  16. 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)

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  21. Envelope of speech + noise

  22. Modulation Detection • 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.

  23. 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.

  24. 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.

  25. 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)

  26. 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)

  27. 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)

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

  29. 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

  30. 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

  31. Noise Noise Speech Speech Speech Speech Noise reducedgain reducedgain Noise Speech Speech Speech Speech Speech Noise Danalogic speech/ noise analysis in 14 frequency bands normal gain

  32. Danalogic 283D Noise Reduction • Uses modulation or level fluctuations to identify ‘speech’ from noise. Noise Reduction either ON or OFF

  33. 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. C1 C2 C3 C4 Off - - - - Min min - - min Med med min min med Max max med med max

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

  35. 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.

  36. 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.

  37. 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.

  38. 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.

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

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

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