1. DEVELOPMENT OF NAL-NL2 Harvey Dillon, Gitte Keidser, Teresa Ching,
Matt Flax, Scott Brewer
The HEARing CRC & The National Acoustic Laboratories
creating sound valueTM

2. Prescribe hearing aids to:
Make speech intelligible
Make loudness comfortable
Prescription affected by other things
localization,
tonal quality,
detection of environmental sounds,
naturalness.
We know how to model intelligibility and loudness, but not the other goals.
Intelligibility and loudness are also arguably more important than the other goals.
Consequently, the derivation process is based on intelligibility and loudness.

3. 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 bottom left loop acts to adjust gain frequency response (upwards) so that intelligibility is maximized.
The bottom right loop acts to adjust gain (downwards) so that loudness is no greater than that perceived by a normal hearer.

4. 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
Some of the audiograms used to derive the optimum gains.

5. 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
200 audiograms x 6 speech levels  1200 gain–frequency responses, each at 20 frequencies from 125 Hz to 10 kHz
The optimal gains that resulted from each audiogram for each speech level.

6. Overall prescription approach
Psychoacoustics
Compare
Adjust
Theoretical
predictions
Final
formula
Assumptions,
rationale
The theoretical predictions are then compared with empirical findings obtained over the last decade and are adjusted when there is a conflict.
Speech science
Empirical
observations

7. Limiting compression ratio
As an example, compression ratio should not be allowed to become too high, especially for fast-acting compression, or intelligibility is reduced. For normal hearing no compression is needed, and for profound hearing loss, compression ratio should not exceed 1.5 in the low frequencies.

8. Multi-dimensional equation
A neural network
H250
H500
H1000
H2000
H8k
SPL
A neural network, made up of successive layers of perceptrons is used to calculate the gain for any arbitrary audiogram and speech level. The coefficients within each layer of the neural network are trained by using the gains that were obtained from the theoretical derivation, as adjusted by the comparison with empirical findings.
G250
G500
G1000
G2000
G8k

9. The two key ingredients
Compare
Speech spectrum & level
Gain-frequency response
Amplified speech spectrum
Loudness model
Normal loudness
Loudness (hearing impaired)
Audiogram
Intelligibility model
Intelligibility achieved
We need a loudness model and an intelligibility model.

10. Psychoacoustics
We have studied a lot of psychoacoustics, (dead regions, tuning curves, otoacoustic emissions) but there is not time to talk about in this presentation.

11. Why are hearing thresholds so useful?
Frequency
selectivity
Temporal
resolution
Speech
Perception
proficiency
Hearing
thresholds
Central auditory
processing
Other
Although super-threshold abilities like frequency selectivity and temporal resolution are very important, they are partly predicted by hearing thresholds, so it is feasible to have a prescriptive formula based on hearing thresholds alone.
Age
Cognitive ability

12. BKB, VCV and CUNY
The key result of the modified speech intelligibility index model is how the maximum amount of information available to people (when it is amplified to be audible) varies with the degree of hearing loss, on average.

13. Factors affecting prescription
Experimental findings, relative to NAL-NL1, allow us to fine tune the NAL-NL2 prescription to agree with empirical findings.

14. Gain; adults, medium input level (N = 187)
On average, adults prefer less gain than prescribed by NAL-NL1.
Males prefer more gain than females.
For those with a moderate or severe loss, experienced aid wearers prefer more than new aid wearers.

15. Gain; adults, medium input level (N = 187)

16. Gain for adults: low & high input levels
Suggest that the compression ratio should be slightly higher,
at least for clients with mild and moderate hearing loss
People prefer higher compression ratios than prescribed by NAL-NL1.
For low input levels, they turn the gain up more than NAL-NL1 prescribes.
For high input levels, they turn the gain down more than NAL-NL1 prescribes.

17. Binaural loudness correction
Listening with two ears provides more loudness than listening with one.
Less gain is therefore used for bilateral fittings, especially at higher input levels.

18. Empirical evidence: variations from NAL-NL1
Output level
NAL-NL1
Children
Adults
Compared to NAL-NL1, NAL-NL2 provides more gain at low input levels for children, but less gain at high input levels for adults.
Input level

19. Adults – congenital or acquired?
The difference in gain preferences between children and adults is just that, rather than a difference between those with congenital hearing loss and those with adventitious hearing loss.

20. Effect of language
Gain at each frequency depends on importance of each frequency
Low frequencies more important in tonal languages
Two versions of NAL-NL2
Tonal languages
Non-tonal languages
Tonal languages (like Mandarin) convey more information with the pitch of speech than do non-tonal languages (like English). Consequently, the low frequencies are given more gain for tonal languages than for non-tonal languages.

21. Tonal versus non-tonal language
The gain differences between tonal and non-tonal languages are, however, small, and we are currently gathering more information on the frequency importance function for tonal languages.

22. Example audiogram: moderate sloping
Frequency (Hz)
250
125
500 1k 2k 4k 8k 20 40 60 80
100
120
50 dB
65 dB
Hearing threshold (dB HL)
80 dB
Example of the insertion gain prescription for a moderate sloping loss.

23. Example audiogram: flat 60
Frequency (Hz)
250
125
500 1k 2k 4k 8k 20 40 60 80
100
120
50 dB
65 dB
Hearing threshold (dB HL)
80 dB

24. Example audiogram: steeply sloping
Frequency (Hz)
250
125
500 1k 2k 4k 8k 20 40 60 80
100
120
50 dB
65 dB
Hearing threshold (dB HL)
80 dB

25. Example audiogram: extreme ski-slope
Frequency (Hz)
250
125
500 1k 2k 4k 8k 20 40 60 80
100
120
50 dB
65 dB
Hearing threshold (dB HL)
80 dB

26. Example audiogram: reverse sloping
Frequency (Hz)
250
125
500 1k 2k 4k 8k 20 40 60 80
100
120
50 dB
65 dB
Hearing threshold (dB HL)
80 dB

27. Variables in NAL-NL2 Age Vent Tube Comp speed WBCT N Language Depth
Gender
RECD
Bi-uni CR
RECD
BWC
Experience
REUG
Aid type
UCT
I/O
REDD
MLE
Transducer
REIG
REAG AC CG
AC'
ABG
ESG
BC'
RESR
ESCD BC
SSPL2cc
This is the way that the various quantities come together within the NAL-NL2 software.
Blue = User i/p
Grey = internal variable
Red = effect of saturation
Dash-dot = alternatives
Green = stored data
SSPLES
Limiting
type

28. “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”
A quote from Denis.
Denis Byrne, 1998

29. Thanks for listening Acknowledgements www.hearingcrc.org
This research was financially supported by the HEARing CRC established and supported under the Australian Government’s Cooperative Research Centres Program
creating sound valueTM