DEVELOPMENT OF NAL-NL2
Harvey Dillon, Gitte Keidser, Teresa Ching,
Matt Flax, Scott Brewer
The HEARing CRC & The National Acoustic Laboratories
TM
TM
creatingsound
sound
value
creating
value
www.hearingcrc.org
Prescribe hearing aids to:
• 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 - step 1
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)
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
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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
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Overall prescription approach
Psychoacoustics
Assumptions,
rationale
Theoretical
predictions
Compare
Speech science
Empirical
observations
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Adjust
Final
formula
Limiting compression ratio
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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
The two key ingredients
Speech spectrum &
level
Loudness
model
Gain-frequency
response
Normal
loudness
Compare
Intelligibility
achieved
Intelligibility
model
Amplified speech
spectrum
Audiogram
creating sound valueTM
Loudness
model
Loudness
(hearing
impaired)
Psychoacoustics
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Why are hearing thresholds so useful?
Frequency
selectivity
Temporal
resolution
Speech
Perception
proficiency
Hearing
thresholds
Central auditory
processing
Age
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Other
Cognitive ability
BKB, VCV and CUNY
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Factors affecting prescription
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Gain; adults, medium input level
(N = 187)
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Gain for adults:
low & high input levels
Smeds et al. 2006
Zakis et al. 2007
Preferred gain deviation from
NAL-NL1 re gain preferred at 65
dB SPL in dB
1.5
1
0.5
0
-0.5
-1
-1.5
-2
50
Suggest that the
compression ratio should
be slightly higher,
at least for clients with
mild and moderate
80
hearing loss
Input level in dB SPL
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Binaural loudness correction
Gain variation (dB)
7
6
5
4
3
2
1
0
0
20
40
60
80
Input level (dB)
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100
120
Empirical evidence:
variations from NAL-NL1
Output level
Children
NAL-NL1
Adults
Input level
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Adults – congenital or acquired?
Preferred gain deviation
from NAL-RP (dB)
0
-2
-4
-6
-8
-10
-12
Congenital
(N=15)
Acquired
(N=28)
-14
LFA
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HFA
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
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Tonal versus non-tonal language
Insertion gain in dB
40
NAL-NL1
35
Male, exp, non-tonal
30
25
Male, exp, tonal
20
15
10
5
0
100
1000
Frequency in Hz
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10000
Example audiogram: moderate sloping
Frequency (Hz)
250
500
Hearing threshold (dB HL)
0
20
40
60
80
100
120
1k
2k
4k
8k
Insertion Gain (dB)
125
40
35
30
25
20
15
10
5
0
100
50 dB
65 dB
80 dB
1000
Frequency (Hz)
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10000
Example audiogram: flat 60
Frequency (Hz)
250
500
Hearing threshold (dB HL)
0
20
40
60
80
100
120
1k
2k
4k
8k
Insertion Gain (dB)
125
40
35
30
25
20
15
10
5
0
100
50 dB
65 dB
80 dB
1000
Frequency (Hz)
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10000
Example audiogram: steeply sloping
Frequency (Hz)
250
500
Hearing threshold (dB HL)
0
20
40
60
80
100
120
1k
2k
4k
8k
Insertion Gain (dB)
125
40
35
30
25
20
15
10
5
0
100
50 dB
65 dB
80 dB
1000
Frequency (Hz)
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10000
Example audiogram: extreme ski-slope
Frequency (Hz)
125
250
500
1k
2k
4k
8k
Hearing threshold (dB HL)
20
40
60
80
100
120
Insertion Gain (dB)
35
0
30
50 dB
25
20
65 dB
15
80 dB
10
5
0
100
1000
Frequency (Hz)
creating sound valueTM
10000
Example audiogram: reverse sloping
Frequency (Hz)
125
250
500
1k
2k
4k
8k
Hearing threshold (dB HL)
20
40
60
80
100
120
Insertion Gain (dB)
30
0
25
20
50 dB
65 dB
15
10
80 dB
5
0
100
1000
Frequency (Hz)
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10000
Variables in NAL-NL2
Age
Vent
Comp speed
Language
RECD
Bi-uni
WBCT
Tube
N
Depth
Gender
CR
BWC
Experience
RECD
REUG
UCT
REDD
REIG
Transducer
Aid type
I/O
MLE
REAG
AC
CG
AC'
ABG
ESG
BC'
RESR
BC
SSPL2cc
ESCD
Blue = User i/p
Grey = internal variable
Red = effect of saturation
Dash-dot = alternatives
Green = stored data
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SSPLES
Limiting
type
“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
creating sound valueTM
Acknowledgements
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
creating sound valueTM
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