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The effect of advanced signal processing strategies in hearing aids on user performance and preference

21 st Danavox Symposium 31 Aug-2 Sept 2005 Kolding, Denmark. The effect of advanced signal processing strategies in hearing aids on user performance and preference. Gitte Keidser, Lyndal Carter, and Harvey Dillon National Acoustic Laboratories. Introduction.

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The effect of advanced signal processing strategies in hearing aids on user performance and preference

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  1. 21st Danavox Symposium 31 Aug-2 Sept 2005 Kolding, Denmark The effect of advanced signal processing strategies in hearing aids on user performance and preference Gitte Keidser, Lyndal Carter, and Harvey Dillon National Acoustic Laboratories

  2. Introduction • Modern hearing aids contain a DSP computer and are software programmable • complex and multiple manipulations of sound • precise and flexible adjustments • automated adjustments to specific client data • To what extent do the advances in hearing aid technology benefit the hearing aid user? • Focus on clinical implications

  3. Sound quality comparisons • Digital signal processing introduces new forms of distortion in hearing aids, e.g. due to analysis of sound into different frequency regions and subsequent resynthesis • A new processing strategy operating in the time-domain using channel-free signal processing has been introduced • Does the sound quality differ in advanced hearing aids and can objective measurements predict the subjective preference?

  4. Sound quality comparisons • Test devices • Canta 7 (770-D) • Claro 211 dAZ • Senso Diva SD-9M • Symbio 100 • Triano S

  5. Objective measures Poorer than average p = 0.50 Better than average

  6. (Score of 8 shows consistent preference) Subjective measures • Round robin paired comparison preference test • male voice in quiet • female voice in quiet • own voice • male voice in impulse noise • piano music • quiet room p > 0.56

  7. Normal hearing listeners Quiet room

  8. Conclusion • No overall significant difference in sound quality among devices • Devices with less internal noise preferred in quiet surroundings • Normal-hearing listeners preferred devices with less time delay (< 10 msec) for listening to speech in quiet • Recommendation: fit devices with lower internal noise and possibly with shorter processing time

  9. Signal processing and horizontal localisation performance • Interaural time and level difference (ILD and ITD) enables left/right discrimination while monaural spectral cues above 4 kHz enables front/rear discrimination • In linear devices tubing, transducers, and filters cause time delays that may distort ITD, and inadequate amplification above 4 kHz and microphone location on BTE devices distort spectral cues (e.g. Byrne et al., 1992) • In digital hearing aids the signal processing is complex

  10. Signal processing and horizontal localisation performance • Effect of • Multi-channel WDRC • Noise reduction • Directionality

  11. Response patterns N = 16 N = 12 The hearing-impaired subjects produced front/rear confusions in 40% of responses, presumably due to the microphone location on the BTE devices

  12. Effect of WDRC and NR p = 0.09 p = 0.24 • The proportion of front/rear confusions was the same across the four conditions.

  13. Front/rear confusions ignored Effect of directionality p = 0.007 p = 0.00001 Front/rear confusions reduced by 11%, on average, when fitted with the cardioid pair and omni/cardioid combination Microphone-mode mismatch increased left/right errors. Significant bias of perception towards fig8 ear and omni ear

  14. Conclusion • Front/rear confusions are prominent in BTE users • The impact of multi-channel WDRC and noise reduction is considered unimportant • A cardioid characteristic can reduce front/rear confusions • Microphone-mode mismatch increases left/right confusions • Recommendation: Counsel BTE users and clients fitted with adaptive directionality about possible localisation problems

  15. Preference for direct or amplified low-frequency sound • To date the most efficient solution to the occlusion effect is a vent bore or open mould that creates a direct sound path for low-frequency sound • The direct sound path will reduce the potential benefit from directional microphones and noise reduction algorithms (Dillon, 2001) • Do hearing aid users prefer direct or amplified sounds when features such as directionality and noise reduction are enabled?

  16. N = 22 Preference for direct or amplified sound • HTL at 500 Hz ranged from 12 to 65 dB HL • Fitted vent size ranged from open to 1.5 mm (vent effects were compensated for in two responses) • Field evaluation of 4 weeks

  17. p = 0.03 30 dB HL 34 dB HL 28 dB HL 43 dB HL Preference for direct or amplified sound

  18. Preference for direct or amplified sound

  19. Conclusion • Generally, there was a strong preference for direct sound to amplified sound, even with features such as directionality and noise reduction enabled • Recommendation: only compensate for vent effects to reach a target insertion gain of 3 dB or above rather than provide sufficient gain to achieve effective operation of hearing aid features

  20. Compression parameters for severe to profound hearing loss • Intuitively we would fit severe and profound hearing loss with low compression thresholds and high compression ratios in multiple channels; a combination that has proved to adversely affect speech recognition (Souza, 2002) • When fitted with moderate compression parameters, people with severe to profound hearing loss generally prefer WDRC to linear amplification (Ringdahl et al., 2000; Barker et al., 2001) • What compression ratios in the low and high frequencies are preferred by hearing aid users with severe to profound hearing loss?

  21. X X X X X X X Adaptive paired comparisons • 21 subjects with moderately severe to severe-profound hearing loss • 3 weeks in the field • Diaries and exit interview

  22. Preferred scheme

  23. On average, the schemes providing linear amplification in the low frequencies were ranked highest Ranking order (N varies from 5 to 21 across schemes) p = 0.1 p = 0.004

  24. ? Prediction • Audiometric data? NO • Onset of loss (congenital = 8 vs. acquired = 13)? NO • Previous amplification experience (linear = 10 vs. non-linear = 11)? NO

  25. Conclusion • Predominant preference for compression ratios between 1:1 and 2:1, with a preference for a higher ratio in the high than in the low frequencies • Recommendation: Fit moderately severe loss with (1.5:1, 2:1) and fit severe-profound loss with (1:1, 2:1). Fine-tuning is essential!

  26. NAL-NL1 and gain adaptation • General belief that new hearing aid users prefer less gain than experienced users and that new users will acclimatise to more gain over time • No support in the literature (on average 2 dB difference in preferred gain), but adaptation managers are introduced in fitting software (Convery et al., 2005) • Do new users prefer less gain than experienced users overall, in the low, or in the high frequencies?

  27. Study design in brief • 60 new and 25 experienced (>3 years) hearing aid users fitted with the same type of device • NAL-NL1, NAL-NL1 with 6 dB LF-cut, and NAL-NL1 with 6 dB HF-cut • Gain preference measurements @ 3 weeks, 3 months, and 12 months

  28. 2.5 dB Gain preference @ 3 weeks Inexperienced Experienced (N = 28) (N = 12)

  29. Difference in gain preference reduced from 2.5 dB to 1.8 dB Gain preference with 4FA HTL r = -0.5, p = 0.006

  30. Gain preference over time 23 inexperienced hearing aid users 3 weeks 3 months

  31. Conclusion • Little evidence to support that new hearing aid users prefer significantly less gain than experienced users – at least when the hearing loss ranges from mild to moderate • Recommendation: don’t use adaptation managers with NAL-NL1 • Data from this study will form part of the revisions made in NAL-NL2

  32. Summary • Avoid fitting digital aids with high level of internal noise and possibly long processing delays • Be aware that BTE users may have great difficulty discriminating between sounds coming from the front and the rear and that adaptive directionality may affect left/right discrimination • Remember that a microphone characteristic with different sensitivity to sounds coming from front and rear may enhance front/rear discrimination in BTE users

  33. Summary continued • Don’t compensate for vent effects when fitting clients with directionality and noise reduction except to reach target gain of 3 dB or above • Don’t assume that a hearing aid user with a severe/profound loss can’t benefit from WDRC but fit this population with ratios in the range 1:1 to 2:1 and provide sufficient support to facilitate fine-tuning • Don’t use adaptation managers when fitting new hearing aid users with the NAL-NL1 target, however, some fine-tuning may be needed

  34. Many thanks to • Tom Scheller from Bernafon, • Ole Dyrlund and Gary Gow from GN Resound, • Volkmar Hamacher, Kristin Rohrseitz, Joseff Chalupper, and Matthias Froehlich from Siemens Instruments, and • Anna O’Brien, Heidi Silberstein, Elizabeth Convery, Lisa and David Hartley, Margot McLelland, and Ingrid Yeend from NAL • Several audiologists from Australian Hearing

  35. Thank you for listening

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