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

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

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

6 Evaluation of directional mic Older studies: single noise source in relatively non-reverberant environments – not realistic 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 Real life situations: reverberation at 300 to 1500 ms + varied room size, noise source and distance

7 Effect of various factors on benefit of dir mic Effect of various factors on benefit of dir mic (Chung, 2004; Ricketts, 2000)  Multiple sources of noise  Noise from the front or around  Speech and noise outside critical distance Reverberation min Vent size xCompression  Switching between mic modes

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

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

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

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

12 Mic noise: omni vs directional Kuk (2000)

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

14 Low frequency roll off in dir aids Thompson (2000)

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

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

17 Wind noise: omni vs dir Kuk (2000)

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

19 Front to back ratio From Phonak

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

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

22 Effect of microphone mismatch Kuk (2000)

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

24 Are adaptive directional mic better than fixed direction mic? Not worse! Not worse! Depends on the noise condition: Depends on the noise condition: Better when noise is on the side, from a narrow spatial angle and changing direction 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 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)

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

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

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

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

29 Second order dir mic 3 mic 3 mic Performance with 3 mic better but not statistically different from 2 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 Big LF roll-off 12 dB/octave + high noise 2 mic 1000 Hz 2 mic 1000 Hz

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

31 Which mic system to use? Fabry (2004)

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

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