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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 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 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 timedomain 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? Keidser et al. National Acoustic Laboratories, Sydney, Australia Sound quality comparisons Test devices Canta 7 (770-D) Claro 211 dAZ Senso Diva SD-9M Symbio 100 Triano S Keidser et al. National Acoustic Laboratories, Sydney, Australia Objective measures Poorer than average Coherence value Time delay (msec) Harmonic distortion (%) Internal noise (dB) Peak MPO (dB) Standardised normalised values 4 3 p = 0.50 2 1 0 -1 -2 Canta Better than average Claro Diva Symbio Triano Keidser et al. National Acoustic Laboratories, Sydney, Australia Subjective measures -0.5 -1 -1.5 p > 0.56 Triano 0 Symbio 0.5 Diva male voice in quiet female voice in quiet own voice male voice in impulse noise piano music quiet room 10 NH 10 HI 1 Claro 1.5 Canta Round robin paired comparison preference test Avg preference score (Score of 8 shows consistent preference) Keidser et al. 10 8 6 Quiet room 4 2 0 -2 -4 HI NH -6 -8 -10 10 12 14 16 18 20 22 24 26 Internal noise re reference test gain (dB) 6 Normal hearing listeners Avg preference score Avg preference score National Acoustic Laboratories, Sydney, Australia Male voice Female voice 4 2 0 -2 -4 -6 0 2 4 6 8 Time delay (msec) 10 12 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 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 Keidser et al. National Acoustic Laboratories, Sydney, Australia Signal processing and horizontal localisation performance Effect of Multi-channel WDRC Noise reduction Directionality Keidser et al. National Acoustic Laboratories, Sydney, Australia Response patterns Linear amplification, pulsed pink noise Normal hearing, pulsed pink noise 200 200 N = 12 150 100 100 50 50 0 0 -50 -50 -100 -100 -150 -150 -150 -100 -50 0 50 100 Presentation azimuth (degree) 150 200 Response azimuth (degree) Response azimuth (degree) -200 -200 N = 16 150 -200 -200 -150 -100 -50 0 50 100 Presentation azimuth (degree) The hearing-impaired subjects produced front/rear confusions in 40% of responses, presumably due to the microphone location on the BTE devices 150 200 Keidser et al. National Acoustic Laboratories, Sydney, Australia Effect of WDRC and NR p = 0.09 Linear WDRC NR off NR on 75 Total RMS error (degree) Total RMS error (degree) 75 p = 0.24 70 65 60 55 70 65 60 55 50 50 pink noise pink noise + activation noise -The proportion of front/rear confusions was the same across the four conditions. Keidser et al. National Acoustic Laboratories, Sydney, Australia Effect of directionality Front/rear confusions ignored Omni pair Cardioid pair Fig8/Cardioid Omni/Cardioid Total RMS error (degree) 75 70 65 60 55 50 Modified RMS error (degree) p = 0.007 30 28 26 24 Omni pair Cardioid pair Fig8/Cardioid Omni/Cardioid 22 20 p = 0.00001 18 16 14 12 10 pink noise Front/rear confusions reduced by 11%, on average, when fitted with the cardioid pair and omni/cardioid combination pink noise Microphone-mode mismatch increased left/right errors. Significant bias of perception towards fig8 ear and omni ear 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 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 lowfrequency 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? Keidser et al. National Acoustic Laboratories, Sydney, Australia Preference for direct or amplified sound Insertion gain in dB 25 20 N = 22 15 (0 dB, off) (6 dB, off) (12 dB, off) (0 dB, on) (6 dB, on) (12 dB, on) 10 5 0 100 1000 Frequency in Hz 10000 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 Keidser et al. National Acoustic Laboratories, Sydney, Australia Preference for direct or amplified sound Features on 20 20 16 16 No of subjects No of subjects Features off 12 8 4 0 12 8 p = 0.03 4 0 6 12 dB insertion gain at 250 Hz 30 dB HL 34 dB HL 0 0 6 12 dB insertion gain at 250 Hz 28 dB HL 43 dB HL Keidser et al. National Acoustic Laboratories, Sydney, Australia Preference for direct or amplified sound 3.0 250 Hz 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 DI measured on KEM -1.0 -5 0 5 10 15 20 REIG measured on KEMAR (dB) 25 30 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 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? Keidser et al. National Acoustic Laboratories, Sydney, Australia Adaptive paired comparisons HF CR LF CR 1:1 1.8:1 3:1 X 1:1 X X 1.8:1 X X 3:1 X X 21 subjects with moderately severe to severe-profound hearing loss 3 weeks in the field Diaries and exit interview Keidser et al. National Acoustic Laboratories, Sydney, Australia Preferred scheme HF CR LF CR 1:1 1.8:1 3:1 1:1 1 7 3 1.8:1 1 4 4 3:1 1 Keidser et al. National Acoustic Laboratories, Sydney, Australia Ranking order (N varies from 5 to 21 across schemes) 4.5 4.5 p = 0.1 4.0 4.0 3.5 3.5 3.0 3.0 2.5 2.5 2.0 2.0 1.5 1.5 1.0 Ranking score (said prefere Mean Mean±SE Mean±SD 0.5 Ranking score (diary ratings) 1.0 p = 0.004 Mean Mean±SE Mean±SD 0.5 1/1 1/1.8 1/3 1.8/1 1.8/1.8 1.8/3 3/1.8 3/3 Compression scheme (LF CR/HF CR) 1/1 1/1.8 1/3 1.8/1 1.8/1.8 1.8/3 3/1.8 3/3 Compression scheme (LF CR/HF CR) On average, the schemes providing linear amplification in the low frequencies were ranked highest Keidser et al. National Acoustic Laboratories, Sydney, Australia Prediction - Audiometric data? - Onset of loss (congenital = 8 vs. acquired = 13)? - Previous amplification experience (linear = 10 vs. non-linear = 11)? 1.1 1.1 1.0 1.0 0.9 0.9 0.8 0.8 ? 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 Average 1/CR in HF ban Average 1/CR in LF band 0.3 0.2 45 NO NO NO 0.3 50 55 60 65 70 75 80 Average HTL in LF band (dB HL) 85 90 0.2 60 65 70 75 80 85 90 95 Average HTL in HF band (dB HL) 100 105 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 severeprofound loss with (1:1, 2:1). Fine-tuning is essential! 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? National Acoustic Laboratories, Sydney, Australia 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 NALNL1 with 6 dB HF-cut Gain preference measurements @ 3 weeks, 3 months, and 12 months Keidser et al. Keidser et al. National Acoustic Laboratories, Sydney, Australia Preferred - NAL-NL1 prescribed 4FA gain in dB Gain preference @ 3 weeks 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 2.5 dB Inexperienced Experienced (N = 28) (N = 12) Keidser et al. National Acoustic Laboratories, Sydney, Australia Preferred - prescribed 4FA gain in dB Gain preference with 4FA HTL 10 Experienced Inexperienced Linear (Inexperienced) 5 0 -5 r = -0.5, p = 0.006 -10 -15 0 10 20 30 40 50 60 70 4FA HTL in dB HL Difference in gain preference reduced from 2.5 dB to 1.8 dB National Acoustic Laboratories, Sydney, Australia Gain preference over time Preferred - NAL-NL1 prescribed gain in dB 23 inexperienced hearing aid users 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 3 weeks 3 months Keidser et al. 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 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 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 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 Thank you for listening