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Recent development in hearing aid technology

Recent development in hearing aid technology. Lena L N Wong Division of Speech & Hearing Sciences University of Hong Kong. Introduction. Fixed directional microphone How does it work? Benefit Most preferred used environments Limitations Adaptive directional mic How does it work?

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Recent development in hearing aid technology

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  1. Recent development in hearing aid technology Lena L N Wong Division of Speech & Hearing Sciences University of Hong Kong

  2. Introduction • Fixed directional microphone • How does it work? • Benefit • Most preferred used environments • Limitations • Adaptive directional mic • How does it work? • Benefit • Limitations

  3. Directional hearing aids • Dual mic with signals from back mic electronically delayed and subtracted from that of front mic

  4. Benefit of directional mic • 1-16 dB improvement in SNR for 50% intelligibility (e.g., Ricketts, 2000; Valente et al., 2000) • 3-5 dB improvement in real world (e.g., Ricketts et al., 2001) • Useful in about 1/3 of listening environments

  5. Evaluation of directional mic • Older studies: single noise source in relatively non-reverberant environments – not realistic (Compton-Conley, 2004; Ricketts, 2000; Walden et al., 2000) • Real life situations: reverberation at 300 to 1500 ms + varied room size, noise source and distance

  6. Effect of various factors on benefit of dir mic (Chung, 2004; Ricketts, 2000)

  7. Improvement in SNR comparing directional to omnidirectional mics Data from Ricketts (2000) Graph from Fabry (2004)

  8. How much do people use dir mic? (Cord et al., 2002) • Many (23% of new users) do not use dir mode • Dir mode is used ¼ of the time in regular users who switch between modes • Why not use more often? • Can’t remember the difference between programs • Dir mode not advantage over omni • Omni is the default setting • Despite differences in usage, users are just as satisfied with omni and dir modes

  9. Most preferred use (Cord et al., 2002;Surr et al., 2002) • Directional mode if: • Talker is in front • Signal is near • Background noise is non speech • Average size rooms, less reverberation • Omni dir mode if: • Talker is behind or around • No/minimum noise • Other people talking or sounds of nature • Small space (e.g., cars)

  10. Limitations of directional mic • Increased internal mic noise • LF roll-off  compensate to reduce tinny feeling( Ricketts & Henry, 2002)  more noise • Compensation if loss > 40 dB (Ricketts & Henry, 2002) • Provide partial compensation • Use omni mic in quiet • Wind noise (dir mic 20-30 dB more sensitive than omni) • Less sensitive to speech from back • Binaural cues may be affected (Kuk et al., 2002)

  11. Mic noise: omni vs directional Kuk (2000)

  12. Limitations of dir mic • Increased internal mic noise • LF roll-off  compensate to reduce tinny feeling ( Ricketts & Henry, 2002)  more noise • Compensation if loss > 40 dB (Ricketts & Henry, 2002) • Provide partial compensation • Use omni mic in quiet • Wind noise (dir mic 20-30 dB more sensitive than omni) • Less sensitive to speech from back • Binaural cues may be affected (Kuk et al., 2002)

  13. Low frequency roll off in dir aids Thompson (2000)

  14. Limitations of dir mic • Increase internal mic noise • LF roll-off  compensate to reduce tinny feeling ( Ricketts & Henry, 2002)  more noise • Compensation if loss > 40 dB (Ricketts & Henry, 2002) • Provide partial compensation • Use omni mic in quiet • Wind noise (dir mic 20-30 dB more sensitive than omni) (Chung, 2005) • Less sensitive to speech from back • Binaural cues may be affected (Kuk et al., 2002)

  15. Turbulence on the downwind side as wind blows past the head Figure from Dillon, Roe, and Katsch (1999) as appeared in Thompson (2000)

  16. Wind noise: omni vs dir Kuk (2000)

  17. Limitations of dir mic • Increase internal mic noise • LF roll-off  compensate to reduce tinny feeling ( Ricketts & Henry, 2002)  more noise • Compensation if loss > 40 dB (Ricketts & Henry, 2002) • Provide partial compensation • Use omni mic in quiet • Wind noise (dir mic 20-30 dB more sensitive than omni) (Chung, 2005) • Less sensitive to speech from back • Binaural cues may be affected (Kuk et al., 2002)

  18. Front to back ratio From Phonak

  19. Limitations of dir mic • Increase internal mic noise • LF roll-off  compensate to reduce tinny feeling ( Ricketts & Henry, 2002)  more noise • Compensation if loss > 40 dB (Ricketts & Henry, 2002) • Provide partial compensation • Use omni mic in quiet • Wind noise (dir mic 20-30 dB more sensitive than omni) (Chung, 2005) • Less sensitive to speech from back • Binaural cues may be affected (Kuk et al., 2002)

  20. A few caveats (Chung, 2005) • Move away from reflective surface to reduce reverberation • Compensate for LF gain if loss > 40 dB HL; turn off if needed (Ricketts & Henry, 2002) • Use omni in quiet, dir in noise with speech from front • Mic matching to within .02 dB and 1 degree ( adaptive mic matching) • Examine mic for debris

  21. Effect of microphone mismatch Kuk (2000)

  22. Adaptive dir mic • Internal delay altered  vary directional pattern to yield the lowest output • Takes 4 - 10 sec to change from omni to dir, 10 msec - 5 sec to change between polar patterns

  23. Are adaptive directional mic better than fixed direction mic? • Not worse! • Depends on the noise condition: • Better when noise is on the side, from a narrow spatial angle and changing direction (e.g., Valente & Mispagel, 2004; Ricketts et al., 2003) • Same when noise is from a wide spatial angle or multiple noise sources (e.g., Bentler et al., 2004) • Difficult for users to perceive a difference between fixed and adaptive dir mic modes but adaptive dir mic is described more favorably (Surr, 2002)

  24. SNR improvement comparing adaptive dir, fixed dir and omni dir mics Data from Ricketts & Henry (2002) Graph from Fabry (2004)

  25. Limitations with adaptive dir mic • Synchronization between ears may yield the best benefit but not doing so does not degrade performance Payne & Lutman (2002)

  26. Broadband (single polar pattern across frequencies) vs multiband (polar patterns at various frequency bands varied with noise) Fabry (2004)

  27. Limitations • When adjustment of the adaptive algorithm is not fast enough compared to changes in direction of noise • Multiple noise sources particularly when spectra & level of noise sources are similar (Bentler et al., 2003; Bentler et al., 2004)

  28. Second order dir mic • 3 mic • Performance with 3 mic better but not statistically different from 2 mic(Ricketts et al., 2003) • Big LF roll-off 12 dB/octave + high noise • 2 mic < 1000 Hz, 3 mic > 1000 Hz

  29. Directional hearing aids for children? • Head turn to sound source (> age 4 ok) • Reduced low frequency gain in dir mode • Incidental learning (from the back) • Self-monitoring of speech • Safety • Profound loss not useful • Selection of mic directivity • Accuracy of adaptive mic

  30. Which mic system to use? Fabry (2004)

  31. Summary • Helps with speech in front, noise from back with minimal reverberation • Major limitations: mic noise, LF roll off, wind noise, speech from back • Adaptive mic may be preferred; useful in noise from narrow spatial ange • Can be used in older children

  32. Thank you Contact email: LLNWONG@hku.hk

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