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Prescribing hearing aids and the new NAL-NL2 prescription rule

Taiwan, July, 2009. Prescribing hearing aids and the new NAL-NL2 prescription rule. Harvey Dillon National Acoustic Laboratories and The Hearing Cooperative Research Centre. Acknowledgment.

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Prescribing hearing aids and the new NAL-NL2 prescription rule

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  1. Taiwan, July, 2009 Prescribing hearing aids and the new NAL-NL2 prescription rule Harvey Dillon National Acoustic Laboratories and The Hearing Cooperative Research Centre

  2. Acknowledgment • Individual contributions:Gitte Keidser, Teresa Ching, Matthew Flax, Richard Katsch, Karolina Smeds, and Justin Zakis • Data gatherers: Several audiologists at NAL and from Australian Hearing • Sponsors: Bernafon, GN ReSound, Oticon foundation, and Siemens

  3. Using a prescription- easy

  4. Child Measure hearing thresholds (dB HL or dB SPL) Measure individual RECD (or estimate from age) Enter into manufacturer software (hearing aid auto adjusted to approximate prescription) Adjust hearing aid in coupler via computer to better match prescribed coupler gain Adult Measure hearing thresholds (dB HL) Enter into manufacturer software (hearing aid auto adjusted to approximate prescription) Verify with real ear measurement Adjust amplification to better match prescription

  5. Deriving a prescription - hard

  6. Overall approach to prescription Final formula Compare Psychoacoustics Theoretical predictions Assumptions, rationale Speech science Empirical observations

  7. NAL-NL1 • Maximize speech intelligibility, with overall loudness at or below normal • Derived from an optimization procedure combining: • the Speech Intelligibility Index formula (modified) • the 1997 loudness model of Moore and Glasberg • Prescribes gain-frequency responses that • make loudness of speech bands approximately constant across frequency • at medium levels agree with NAL-RP

  8. Post NAL-NL1 • The underlying principles and assumptions have been evaluated • Is the rationale appropriate? • How can we better predict speech intelligibility? • Do new hearing aid users prefer less gain than experienced hearing aid users? • What compression is preferred by hearing aid users with severe to profound hearing loss?

  9. Maximizing speech intelligibility Is this a better rationale than loudness normalization?

  10. Normal loudness Equal loudness Loudness perception of speech bands

  11. NAL-NL1 (speech intelligibility maximisation) vsIHAFF (loudness normalisation) NAL-NL1 preferred (r = 0.65, p = 0.001) Source: Keidser and Grant 2001

  12. Desired loudness Feedback suggested that NAL-NL1 was too loud!

  13. Loudness; adults, medium input level(N = 187)

  14. Loudness, experience

  15. Gain preference over time N = 11 Source: Keidser, O’Brien, Yeend, & McLelland (submitted)

  16. Loudness; adults, low and high input levels Suggest that the compression ratio should be slightly higher, at least for clients with mild and moderate hearing loss

  17. 1:1 1.8:1 3:1 Compression by severe and profound hearing loss Source: Keidser, Dillon, Dyrlund, Carter, and Hartley (2007)

  18. Children Source: Ching 2003

  19. Adults – congenital or acquired?

  20. Loudness; children • An increase of gain • is more likely to lead to greater speech intelligibility at low input levels where speech is most limited by audibility • is less likely to cause noise-induced hearing loss for low input levels than for high input levels • Increase gain for low input levels with a progressive decrease in increased gain with increased input level (i.e. higher CR)

  21. Children Adults Increased compression ratio Output level NAL-NL1 Input level

  22. NAL-NL2: • Keep maximizing speech intelligibility rationale • Change the intelligibility modelling • Prescribe less gain for adults, but more gain for children • Increase the CR for adults and children with mild and moderate hearing loss, while restricting the CR for those with severe/profound hearing loss • Introduce gain adaptation for new hearing aid users, and a gender effect

  23. Any questions at this point?

  24. Predicting speech intelligibility Is our formula for predicting maximum speech intelligibility optimized?

  25. Noise Threshold Audibility: 17 0 16 5 ... ... ... ... x x x x Importance: 0.001 0.002 0.003 0.002 = = = = 0.005 0.032 0.051 0 = 0.30 ... ... ... ... 30 1/3 octave SPL Freq

  26. Speech Intelligibility Index Sum SII = ∑ Ai Ii Importance Audibility But intelligibility gets worse if we make speech too loud!

  27. Speech intelligibility also depends on … Level distortion 1 Level distortion factor 0 140 0 73 Speech level (dB SPL) • Normal hearing people perform poorer at high speech levels

  28. Sentences 100 80 60 Percent Correct Nonsense syllables 40 20 0 0 0.2 0.4 0.6 0.8 1 Speech Intelligibility Index (SII) The transfer function

  29. Observed and Predicted performance Ching, Dillon & Byrne, 1998

  30. Optimizing speech intelligibility • NAL-NL1: Frequency dependent hearing loss desensitization in quiet introduced • Dead regions? (Baer et al., 2002; Moore, 2004) • Desensitization is different in noise? (Turner & Henry, 2002) Effective audibility Sensation level (dB) Source: Ching, Dillon, Katsch & Byrne (2001)

  31. New study • 75 test subjects • Measurements • Hearing threshold levels • Outer hair cell function • click-evoked otoacoustic emissions • Frequency resolution • psychophysical tuning curves • cochlear dead regions – TEN test • Speech perception in quiet and noise • consonants • sentences Source: Ching et al. 2005

  32. Subjects • 20 adults with normal hearing • 55 adults with sensorineural hearing loss • mild to profound • Experienced hearing aid users

  33. Speech perception • Stimuli: Filtered speech • CUNY sentences • VCV syllables • Shaping: • POGO prescription • Conditions: • Quiet at high and low sensation levels • Babble Noise • Headphones: Sennheiser HD25

  34. Audibility and Speech intelligibility – H.I.

  35. Deficit = Sansii - SIIeff SIIeff 100 80 Deficit = 0.6 - 0.4 = 0.2 60  Percent Correct 40 SIIansi 20 0 0 0.2 0.4 0.6 0.8 1 Speech Intelligibility Index (SII)

  36. VCV deficit vs CUNY deficit R=0.77

  37. Intelligibility and audibility m p 1 30 Sensation level (dB)

  38. Variation of m with HL mp ms m 1.0 0.5 0 Hearing Threshold (dB HL)

  39. BKB, VCV and CUNY

  40. Optimizer results: 3 data sets BKB VCV CUNY Q & N

  41. Desensitisation for hearing loss

  42. Should we use anything other than the audiogram to predict speech intelligibility?

  43. Psychoacoustic correlations – 2 kHz

  44. Correlations Age PTC HL OAE Cognit TEN

  45. Multiple regression PTC HL OAE TEN Cognition including HL causes: correlations between age and PTC / OAE / TEN to disappear correlations between cognition and PTC / OAE / TEN to disappear Age

  46. Likely intermediate effects Cognition Mechanical ? PTC OAE Stria Cardio- vascular OHC TEN Noise IHC HL Age

  47. Implications for prescription Pure tone thresholds critical Knowledge of temporal resolution, frequency resolution, dead regions adds relatively little to prediction of intelligibility Age and cognitive ability affect all frequency bands similarly  no effect on gain needed

  48. Deriving a prescription Using the intelligibility and loudness models

  49. The rationale for NAL proceudres Maximize calculated speech intelligibility , but Keep total loudness less than or equal to normal

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