1. 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

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. 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

5. Deriving a prescription - hard

6. Overall approach to prescription
Psychoacoustics
Theoretical
predictions
Final formula
Assumptions,
rationale
Compare
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. Loudness perception of speech bands
Normal loudness
Equal loudness

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

12. Feedback suggested that NAL-NL1 was too loud!
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
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. Compression by severe and profound hearing loss
1:1
1.8:1
3:1
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. Increased compression ratio
Output level
NAL-NL1
Children
Adults
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. 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

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

27. 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.

28. 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.

29. 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

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
100 80
Deficit =
= 0.2 60 
Percent Correct
SIIeff 40 20
SIIansi
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 audibility1 m p 30
Sensation level (dB)

38. Variation of m with HL m
1.0 mp ms
0.5
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 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

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

48. 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

49. Deriving a prescription
Using the intelligibility and loudness models

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

51. The two key ingredients
A loudness model
An intelligibility model

52. 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.

53. 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)

54. 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

55. The audiograms, continued

56. 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

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

58. 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

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

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

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

62. “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

63. Thank you for
listening