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The effect of advanced signal processing strategies in hearing aids on user performance and preference Gitte Keidser, Lyndal Carter, and Harvey Dillon.

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Presentation on theme: "The effect of advanced signal processing strategies in hearing aids on user performance and preference Gitte Keidser, Lyndal Carter, and Harvey Dillon."— Presentation transcript:

1 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 21 st Danavox Symposium 31 Aug-2 Sept 2005 Kolding, Denmark

2 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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 National Acoustic Laboratories, Sydney, Australia Keidser et al. Sound quality comparisons Test devices  Canta 7 (770-D)  Claro 211 dAZ  Senso Diva SD-9M  Symbio 100  Triano S

5 National Acoustic Laboratories, Sydney, Australia Keidser et al. Objective measures Better than average Poorer than average p = 0.50

6 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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 (Score of 8 shows consistent preference)

7 National Acoustic Laboratories, Sydney, Australia Keidser et al. Normal hearing listeners Quiet room

8 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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 National Acoustic Laboratories, Sydney, Australia Keidser et al. Signal processing and horizontal localisation performance Effect of  Multi-channel WDRC  Noise reduction  Directionality

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

12 National Acoustic Laboratories, Sydney, Australia Keidser et al. Effect of WDRC and NR -The proportion of front/rear confusions was the same across the four conditions. p = 0.09 p = 0.24

13 National Acoustic Laboratories, Sydney, Australia Keidser et al. Effect of directionality 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 Front/rear confusions ignored p = p =

14 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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 National Acoustic Laboratories, Sydney, Australia Keidser et al. Preference for direct or amplified sound N = 22 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 National Acoustic Laboratories, Sydney, Australia Keidser et al. Preference for direct or amplified sound 30 dB HL 34 dB HL28 dB HL 43 dB HL p = 0.03

18 National Acoustic Laboratories, Sydney, Australia Keidser et al. Preference for direct or amplified sound

19 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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 National Acoustic Laboratories, Sydney, Australia Keidser et al. 21 subjects with moderately severe to severe-profound hearing loss 3 weeks in the field Diaries and exit interview Adaptive paired comparisons X XX X X X X

22 National Acoustic Laboratories, Sydney, Australia Keidser et al. Preferred scheme HF CR LF CR1:11.8:13:1 1: :1144 3:11

23 National Acoustic Laboratories, Sydney, Australia Keidser et al. Ranking order p = 0.1p = On average, the schemes providing linear amplification in the low frequencies were ranked highest (N varies from 5 to 21 across schemes)

24 National Acoustic Laboratories, Sydney, Australia Keidser et al. Prediction - Audiometric data? NO - Onset of loss (congenital = 8 vs. acquired = 13)?NO - Previous amplification experience (linear = 10 vs. non-linear = 11)?NO ?

25 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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 3 weeks, 3 months, and 12 months

28 National Acoustic Laboratories, Sydney, Australia Keidser et al. Gain 3 weeks 2.5 dB Inexperienced Experienced (N = 28) (N = 12)

29 National Acoustic Laboratories, Sydney, Australia Keidser et al. Gain preference with 4FA HTL r = -0.5, p = Difference in gain preference reduced from 2.5 dB to 1.8 dB

30 National Acoustic Laboratories, Sydney, Australia Keidser et al. Gain preference over time 23 inexperienced hearing aid users 3 weeks 3 months

31 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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 National Acoustic Laboratories, Sydney, Australia Keidser et al. 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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