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 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

Using a prescription - easy

Adult Child Measure hearing thresholds (dB HL) (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 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

Deriving a prescription - hard

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

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

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?

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

Loudness perception of speech bands Normal loudness Equal loudness

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

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

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

Loudness, experience

Gain preference over time Source: Keidser, O’Brien, Yeend, & McLelland (submitted)

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

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

Children Source: Ching 2003

Adults – congenital or acquired?

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)

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

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

Any questions at this point?

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

Threshold Audibility: 5 16 17 1/3 octave SPL Freq Importance: 0.001 Noise Audibility: 5 16 17 ... 1/3 octave SPL 30 Freq Importance: 0.001 0.002 0.003 x The basis of the prediction method we use is the Speech intelligibility index, which uses the level of signal that is above threshold to calculate intelligibility. = 0.005 0.032 0.051 ... = 0.30

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

Speech intelligibility also depends on … Level distortion Normal hearing people perform poorer at high speech levels Speech level (dB SPL) Level distortion factor 1 73 140 A modification that must be made is that even people with normal hearing are less able to understand speech as the sound pressure level of the speech increases.

Speech Intelligibility Index (SII) The transfer function 20 40 60 80 100 0.2 0.4 0.6 0.8 1 Speech Intelligibility Index (SII) Percent Correct Nonsense syllables Sentences One the speech intelligibility index is calculated, it can be turned into a predicted percent correct using the transfer function that applies for the particular speech material being used.

Observed and Predicted performance Here are some results obtained in a study we did seven years ago. Each graph shows percent correct versus the sensation level of the speech above threshold. The dots are the average results, and the red lines are the values predicted from the SII method. For this group with a mild flat hearing loss, the prediction method works reasonably well, at least on average, and actually is not too bad for individuals. When hearing loss becomes severe, people do not do nearly as well as we predict, even on average. Ching, Dillon & Byrne, 1998

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)

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

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

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

Audibility and Speech intelligibility – H.I.

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

VCV deficit vs CUNY deficit R=0.77

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

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

BKB, VCV and CUNY

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

Desensitisation for hearing loss

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

Psychoacoustic correlations – 2 kHz

Correlations Age PTC HL OAE Cognit TEN

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

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

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

Deriving a prescription Using the intelligibility and loudness models

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

The two key ingredients A loudness model An intelligibility model

Free field speech level Calculating loudness Loudness model of Moore and Glasberg (2004) Allowance for hearing loss Filtering into auditory bands Excitation level Calculate loudness per band Loudness per band Sum across bands Total loudness External & middle ear Free field speech level Input to cochlea To calculate loudness we used the Moore and Glasberg loudness model. Hearing loss affects these bits of the model.

Deriving optimal gains - step 1 Speech spectrum & level Loudness model Normal loudness Gain-frequency response Amplified speech spectrum Compare Audiogram Intelligibility model Intelligibility achieved Loudness model Loudness (hearing impaired)

The audiograms Rejection criterion : -30<= G <=60 , where G is the slope sum(H(f))/3 <=100 , where f is in the set {0.5, 1, 2} kHz Inverted hearing loss profiles used

The audiograms, continued

Deriving optimal gains - step 1 Audiogram 1 Speech level 1 Optimal gain frequency response Audiogram 1 Speech level 2 Optimal gain frequency response Audiogram 1 Speech level 3 Optimal gain frequency response Audiogram 2 Speech level 1 Optimal gain frequency response 1200 audiograms x 10 speech levels  12,000 gain–frequency responses, each at 20 frequencies from 125 Hz to 10 kHz

The result of step 1 …… Gainf HLf HL3FA f SPL

Deriving optimal gains - step 2 Fit a multi-dimensional equation to the data Gain at frequency f depends on: f, HL at all frequencies, SPL Apply constraints: No compression for speech < 50 dB SPL Low compression ratio for profound loss for fast compression No gain at very, very low frequencies (e.g. 50 Hz) No gain at very, very high frequencies (e.g. 20 kHz) Input SPL Gain 50

Multi-dimensional equation: A neural network H250 H500 H1000 H2000 H8k SPL G250 G500 G1000 G2000 G8k

In summary: This is how NAL-NL2 targets differ from NAL-NL1 targets, and vary with client profile

Preliminary 1) Female new user NAL-NL2 1) Female new user 2) Male experienced user with dead region 3) Child user NAL-NL1 Preliminary

“A challenge for the profession is to devise fitting procedures that are scientifically defensible and the challenge for the individual audiologist is to choose the best procedures from whatever are available” Denis Byrne, 1998

Thank you for listening www.nal.gov.au