The NAL-NL2 prescription procedure
Gitte Keidser, Harvey Dillon, Teresa Ching, Matthew Flax, Scott Brewer
National Acoustic Laboratories and the HEARing CRC
Adult Hearing Screening, June 2010, Lake Como
TM
TM
creatingsound
sound
value
creating
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www.hearingcrc.org
Overall approach to prescription
Psychoacoustics
Assumptions,
rationale
Theoretical
predictions
Compare
Speech science
Empirical
observations
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Adjust
NAL-NL1
NAL-NL2
Rationale
• Make speech intelligible
• Make loudness comfortable

Prescription affected by other things
–
–
–
–
localization,
tonal quality,
detection of environmental sounds,
naturalness
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Deriving optimal gains
Speech spectrum
& level
Loudness
model
Gain-frequency
response
Normal
loudness
Compare
Intelligibility
achieved
Intelligibility
model
Amplified speech
spectrum
Audiogram
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Loudness
model
Loudness
(hearing
impaired)
Deriving optimal gains
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
24,000 data points
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Multi-dimensional equation
A neural network
H250
H500
H1000
H2000
H8k
G250
G500
G1000
G2000
G8k
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SPL
Deriving optimal gains
New
Speech spectrum
& level
Loudness
model
Gain-frequency
response
Normal
loudness
Compare
Intelligibility
achieved
Intelligibility
model
Amplified speech
spectrum
Audiogram
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Loudness
model
Loudness
(hearing
impaired)
1: Change to SII model
SII
SIIansi =  Ai.Ii.Li
1
SII =  Aeff.Ii.Li
m
p
30
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Sensation level (dB)
1: Change to SII model
• Speech recognition
data collected on 75
adults with varied
degree of hearing loss
45
NL1 (50 dB)
NL1 (65 dB)
NL1 (80 dB)
NL2 (50 dB)
NL2 (65 dB)
NL2 (80 dB)
40
Insertion gain in dB
– new effective audibility
factor in SII model
– Different gainfrequency response
shape
35
30
25
20
15
10
5
0
100
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1000
Frequency in Hz
10000
2: Desired gain
Gain; 187 adults, medium input level
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2: Desired gain
Gain; adults vs children
Output level
Children
NAL-NL1
Adults
Input level
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2: Desired gain
Limiting compression for severe/profound hearing loss
(Fast compression)
60
Insertion gain in dB
50
40
30
20
Fast-acting compression
10
Slow-acting compression
0
100
1000
Frequency in Hz
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10000
3: 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
35
Insertion gain in dB
30
25
20
15
Non-tonal language
10
Tonal language
5
0
100
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1000
Frequency in Hz
10000
Summary
• New features in NAL-NL2
– Different gain-frequency response shape and
higher compression ratios
– Different compression ratios for fast and slow
compressors (severe/profound hearing loss)
– Gender dependent gain
– Age dependent gain
– Language dependent gain (tonal vs non-tonal)
– Gain adaptation for new hearing aid users
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Acknowledgements
For further information:
www.hearingcrc.org
www.nal.gov.au
This research was financially supported by the HEARing CRC
established and supported under the Australian Government’s
Cooperative Research Centres Program
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