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Musicians and Hearing Aids

Musicians and Hearing Aids. Marshall Chasin AuD, M.Sc., Reg. CASLPO, Aud(C ) Musicians’ Clinics of Canada. Musicians’ Clinics of Canada. Some recent publications…. Canadian Hearing Report, May/June 2007 M. Chasin Hearing Journal, July 2004 M. Chasin Trends in Amplification, 7 , 3, 2004

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Musicians and Hearing Aids

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  1. Musicians and Hearing Aids Marshall Chasin AuD, M.Sc., Reg. CASLPO, Aud(C) Musicians’ Clinics of Canada

  2. Musicians’ Clinics of Canada

  3. Some recent publications… • Canadian Hearing Report, May/June 2007 • M. Chasin • Hearing Journal, July 2004 • M. Chasin • Trends in Amplification, 7, 3, 2004 • M. Chasin and F. Russo • Trends in Amplification, 8, 3 and 4, 2005 • King Chung

  4. Special issue on musicians… • Hearing Review March 2006 • Edited by Marshall Chasin • www.hearingreview.com (archives)

  5. A new book…. • Musicians and Hearing Loss (2008) from Plural Publishing. • Replaces my original Musicians and the Prevention of Hearing Loss (1996), Singular Publishing Group.

  6. …. Two websites… • www.chasin.ca/music • www.musiciansclinics.com

  7. (Five) differences … • Speech vs. Music Spectra • Phonemic vs. phonetic requirements • Differing intensities • Crest factors • (Loudness and intensity)

  8. Five differences … • (1) Speech vs. Music Spectra: • Speech has a relatively uniform spectrum • Human vocal tract source • Long-term speech spectrum “target” • Music has many sources • Highly variable • No “music target”

  9. Five differences … • (2) Phonemic vs. phonetic requirements: • Speech is mostly low-frequency energy and high frequency clarity (AI). • Music perceptual requirements depends on the instrument…. Highly variable. • Violins need to hear the balance between low and high frequencies. • Clarinets only need to hear the lower frequency energy.

  10. Five differences … • (3) Differing intensities: • Speech is 65 dB SPL ± 12 dB • (53 dB SPL to 77 dB SPL) • Shouted speech can be 82 dB SPL • Music can reach 105 dBA; peaks of 120 dBA

  11. Five differences … • (4) Crest factor: (peak – RMS) • Speech has a crest factor of 12 dB • Music has a crest factor of up to 18 dB. • Less damping.

  12. (Five) differences … • (5) Loudness and intensity: • Speech has a well-defined relationship since harmonics are in different critical bands. • Bass instruments have some harmonics within the same critical bandwidth… • Loudness and intensity mismatch with some instruments (intensity increases and no change in loudness) • Bass instruments “suffer” from this mismatch… underplay…. Less low frequency gain needed…

  13. What about hard of hearing musicians … or non-musicians who like to listen to louder music?

  14. Hearing Aids and Musicians 1. Peak input limiting level of most hearing aids limits sound above 85 dB SPL. … great for speech… bad for music. - shouted [a] is about 82 dB SPL peak - music can be >110 dB SPL

  15. An Experiment: • Traditional measures of distortion in the industry involve: • Autocorrelation • Harmonic relationships - total harmonic distortion - intermodulation distortion • Problem: • Works best for linear systems

  16. An Experiment: • Kates (1990, 1991) • Notch filter paradigm (can work for non-linear systems) • Comb filter • With current technology(eg. Adobe Audition), a square notch was removed from the input spectrum • “debris” in the notch is a measure of distortion • An output spectrum with a perfectly square notch is representative of perfect fidelity.

  17. An Experiment: Unfiltered and Filtered Music

  18. An Experiment: • A hearing aid was constructed where the peak input limiting level can be successively reduced from 115 dB SPL, to 105 dB SPL, to 96 dB SPL to 92 dB SPL, … and back to 115 dB SPL. • Acknowledgments: Mead Killion, Russ Tomas, Norm Matzen, Mark Schmidt, Steve Aitken.

  19. Unfiltered and Filtered Music

  20. Two Input Limiting Levels (115 dB and 105 dB SPL)

  21. Two Input Limiting Levels(96 dB and 92 dB SPL)

  22. “Signal/Distortion – 30 dB” ratio (0 dB =best)

  23. Preference Scales • Gabrielsson et al. (1974; 1991) • Cox and Alexander (1983) • Loudness (vs. “faint”) • Fullness (vs. “thin”) • Crispness (vs. “blurred”) • Naturalness (…“as I remember it”) • Overall fidelity (…“dynamics not too narrow”)

  24. Signal/Distortion ratio vs. Preference Scale Totals (n=138)

  25. Therefore ….Peak Input Limiting Level should be at least 105 dB SPL

  26. If you are stuck with a low peak input limiting level …. • Lower volume on stereo or other input and increase gain on aid. • Depending on the manufacturer, you can use an FM system as input. • Use (creative) microphone attenuators (Adhear), or Scotch tape. Tape will provide 7 dB of flat attenuation up to 4000 Hz. (Can also do it electronically- resistive network).

  27. Reducing output (not gain)

  28. A quick way to determine the peak input limiting level • In a test box, set the stimulus level to 100 dB. (may need to move the reference mic). • Set the gain of the aid to about 5-10 dB, and the OSLP90 is set >110dB . • (The output is less than the OSPL-90) • If distortion is >10%, then this is “front end” clipping and the peak input limiting level is too low. • Ref: Chasin, 2006, Hearing Journal.

  29. Music is intense so… • A useful hearing aid fitting for listening to and playing loud music, even for those with a moderate to severe loss, is a non-occluding BTE fitting. • At higher input levels, less gain and output is required and all that may be needed is 15-20 dB of mid and high frequency gain…

  30. A non-occluding BTE mod’n… • Non-occluding BTE provide gain above 1000 Hz and do not occlude the ear canal. • Useful for those with a high frequency loss • BUT still has a front end peak limiting problem…

  31. A non-occluding BTE mod’n… • SO…. We can use a desensitized microphone. • Use a high frequency emphasis (-6 dB low frequency roll-off) microphone. • Same frequency response but less front end distortion.

  32. Hearing Aids and Musicians 2. One channel is best. - relationship between low frequency fundamental and higher frequency harmonics is maintained (similar CR). - may not be useful in some bass-heavy noisy situations (modification of multi- band?) (L. Revit)

  33. Hearing Aids and Musicians … Two channels may be better (?) with one below 500 Hz set to have a higher CR than the higher frequency channel (CR = 1.7:1). This will cut the low frequency gain for all bass sounds.

  34. Hearing Aids and Musicians(hearing aids and music) 3. Compression issue. - speech has a crest factor of 12 dB. - music has a crest factor of 18-20 dB. …. Detector should be set 5 dB higher? Depends whether the compression system is based on RMS or peak.

  35. WDRC? • WDRC with a higher compression threshold (if it uses a peak detector) may be quite useful…?? • Most of damage for mild to moderate loss is OHC… WDRC?

  36. Crest factor and output • Music has an 18-20 dB crest factor • Speech has a 12 dB crest factor • Therefore the output (OSPL90) of a “music program” in a hearing aid should be 6 dB lower than the program for “speech in quiet”. • Also, given similar compression characteristics, the gain should also be 6 dB less intense.

  37. K-AMP or “K-AMP-like” is the best hearing aid for music… • High input limiting level or “equivalent” • One channel • Appropriate compression • WDRC

  38. Digital Hearing Aids and Music • Greater dynamic range than 96 dB? (need to be able to transduce inputs >110 dB SPL). Perhaps 20 dB SPL to 116 dB SPL?? • 16 bit limitation? • A/D converter limitations? • … other solutions?

  39. Music and a flat response… • I have always “assumed” that a flat frequency response was needed in order to maintain the balance of the low-frequency fundamental energy with the higher frequency harmonic energy…. • … This may be secondary…

  40. Music and a flat response… • A flat frequency response means a better transient response. • More important for music than for speech. • Speech has some transients such as [t]… • Some forms of music have many transients.

  41. Frequency Transpostion? • Decreasing high-frequencies by about a fifth (eg. G to C) may be useful • Like changing from a violin to a viola… • Examples: Sonovation (non-linear) Widex Inteo Audibility extender (linear)

  42. Noise and Feedback Reduction • Disable these functions for music??

  43. A few slides on feedback reduction techniques… • In general, all approaches use a • Fixed slow acting component for static noise • Adaptive fast acting component for dynamic noises.

  44. A few slides on feedback reduction techniques… • Three major approaches: • (1) adaptive gain reduction • eg. Phonak Perseo, Phonak Verve, (Widex Diva/Inteo) • (2) adaptive notch filters • eg. Siemens Triano, Unitron Liaison • (3) phase cancellation • eg. Oticon Syncro/Safron, Resound Canta, Bernafon Symbio, Phonak Savia/Verve, (Widex Diva/Inteo), Unitron Indigo, ...

  45. A few slides on feedback reduction techniques… • Negative issues for each approach: • Gain reduction: undesirable gain reduction • Notch filter: Frequency-hopping artifact resulting in a “blurry sound”. • Phase cancellation: Chirping- there is no feedback signal for the generated signal to cancel.

  46. Six feedback reduction techniques and music… • For music…. Since a feedback reduction system may confuse the pure tones of music (and of microwaves, etc) with feedback… • (1) Limit the frequency range that activates the feedback reduction • Oticon Syncro and Safron (>1.5 kHz) • Siemens Triano (>2 kHz)

  47. Six feedback reduction techniques and music… • (2) Long signal detection (10 sec) with pattern recognition AND a slow adaptive time constant for filter… • … short signals are not cancelled. • (Siemens Acuris/Centra, Bernafon Symbio, Oticon Syncro & Delta, Unitron Indigo, ….)

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