Psychoacoustics of hearing loss and the NALNL2 Prescription Procedure

1
Psychoacoustics of hearing loss and the NAL-NL2
Prescription Procedure

• Harvey Dillon
• Teresa Ching, Gitte Keidser,
• Matt Flax, Richard Katsch Karolina Smeds, Justin
  Zakis Elizabeth Convery, Anna OBrien, Frances
  Lockhart, Emma VanWanrooy, Margot McLelland,
• Ingrid Yeend, Lydia Lai
• NAL
• CRC Hear

2
Talk structure
(?)
Amplification rationales
3
The aim of amplification
4
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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Definition of gain
Real Ear Aided Gain A - F
Real Ear Unaided Gain A - F
Real ear insertion gain A U REAG -
REUG
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Hearing aids amplify , so .

• How much amplification?

Gain
65 dB SPL
Frequency
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Using prescription- easy
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Adult
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
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Infant Fitting Procedure
Behavioural hearing threshold with insert phones
(dB HL)
Electrophysiological hearing threshold with
insert phones (dB nHL)
Measure individual RECD, (or estimate RECD from
age)
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Infant and Child Fitting Procedure
Must be ear-specific hence insert phones
Can be ABR or ASSR, but must be ear-specific and
frequency-specific
Behavioural hearing threshold with insert phones
(dB HL)
Electrophysiological hearing threshold with
insert phones (dB nHL)
Measure individual RECD, (or estimate RECD from
age)

• Estimation includes
• Difference between electrophysiological and
  behavioural thresholds
• Measured or estimated RECD

Initial RECD will use probe in ear canal, or be
estimated
NAL-NL1 uses adult-equivalent hearing level (dB
HL) DSLi/o uses canal dB SPL
Calculate hearing threshold level (adult
equivalent dB HL or dB SPL in ear canal)
Apply prescription to derive coupler gain targets
RECD at time of fitting should use insert phone
coupled to custom earmold
Measurement of the aid in the coupler should use
broad-band test signals (or speech)
Verification of REAG ?
Adjust hearing aid via coupler/programmer to
achieve coupler gain targets
Evaluation !
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Deriving a prescription
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Two rationales for prescription

• Normalize loudness at each frequency
• Maximize speech intelligibility while preventing
  excess total loudness
• ? NAL-NL1 and NAL-NL2

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Loudness normalization
Normal Hearing (average)
Hearing Impaired (individual)
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The rationale for NAL proceudres

• Maximize calculated speech intelligibility ,
• but
• Keep total loudness less than or equal to normal

NAL-NL1 (1999) ? empirical studies
? psychoacoustic studies
? speech intelligibility models
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Deriving optimal gains - step 1
Speech spectrum level
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The audiograms

• Rejection criterion
• -30lt G lt60 , where G is the slope
• sum(H(f))/3 lt100 , where f is in the set 0.5,
  1, 2 kHz

Inverted hearing loss profiles used
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The audiograms, continued
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Deriving optimal gains - step 1
200 audiograms x 6 speech levels ? 1200
gainfrequency responses, each at 20 frequencies
from 125 Hz to 10 kHz
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Diagnostic graphs for derivation
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The result of step 1
Gainf
HLf
HLf2
f
SPL
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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 lt 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)

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Limiting compression ratio
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Multi-dimensional equation

• A neural network

H250
H500
H1000
H2000
H8k
SPL
G250
G500
G1000
G2000
G8k
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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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The two key ingredients

• A loudness model
• An intelligibility model

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Calculating loudness

• Loudness model of Moore and Glasberg (2004)

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Predicting speech intelligibility
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30
1/3 octave SPL
Freq
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Speech Intelligibility Index
Sum

• SII ? Ai Ii

Importance
Audibility
But intelligibility gets worse if we make speech
too loud!
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Speech intelligibility also depends on Level
distortion

• Normal hearing people perform poorer at high
  speech levels

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• SII ? Ai Ii Li

Level distortion factor
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The transfer function
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Observed and Predicted performance
Ching, Dillon Byrne, 1998
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Speech intelligibility modelAllowing for
distortions in hearing loss
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Subjects

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

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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

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Audibility and Speech intelligibility N.H.
May
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Reasons for scatter

• Scores in noise lower than scores in quiet
• Scores for older listeners lower than scores for
  younger listeners
• Scores for some bands consistently lower than
  scores for others

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Audibility and Speech intelligibility H.I.
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Deficit Sansii - SIIeff
100
80
Deficit 0.6 - 0.4 0.2
60
?
Percent Correct
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20
SIIansi
0
0
0.2
0.4
0.6
0.8
1
Speech Intelligibility Index (SII)
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VCV deficit vs CUNY deficit
R0.77
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Intelligibility and audibility
1
30
Sensation level (dB)
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Variation of m with HL
m
1.0
0.5
0
Hearing Threshold (dB HL)
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Parameters to optimise
mp
Criterion Minimize error between observed
intelligibility and predicted intelligibility
a1 a2 log (f)
log (f)
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Fitting the data
a0 a1 a2 a3 a4 a5 a6 freq
I(f)
m(f, HL) SPL(f)
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BKB, VCV and CUNY
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Optimizer results 3 data sets
BKB
VCV
CUNY
Q N
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Desensitisation for hearing loss
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Why measure only pure-tone thresholds?
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Other measurements

• Hearing threshold levels
• Outer hair cell function
• click-evoked otoacoustic emissions
• Frequency resolution
• psychophysical tuning curves
• cochlear dead regions TEN test
• Cognitive ability
• Age

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Healthy PTC no dead region
Masker
Signal
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Poor PTC Dead region at 4 kHz
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Otoacoustic emissions

• Transient OAE
• 80 dB SPL
• Non-linear (80 dB / 70 dB algorithm)
• Emission is octave filtered
• 500 Hz, 1 kHz, 2 kHz, 4kHz
• Pressure of filtered emission r2
• Result is coherent emission strength

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• Dead regions

RIP
NAL-NL1 only allows for hearing loss
desensitization on average
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Off-frequency listening TEN test
Basilar membrane vibration
Threshold Equalizing Noise (TEN)
Frequency or position
Based on Moore (2004)
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TEN elevation versus frequency
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Off-frequency listening PTC
Basilar membrane vibration
Frequency or position
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Off-frequency listening
Basilar membrane vibration
Frequency or position
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TEN and PTC (non) agreement
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TEN and PTC (non) agreement
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Cognitive ability

• Visual letter patterns
• Visual digit patterns

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Cognitive word test
Note when a three-letter word completes ...
Practice
t o p a t e n d
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• i

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• t

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• a

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• p

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• i

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• n

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• o

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• a

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• f

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• t

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• tap
• pin
• oaf
• aft

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Psychoacoustic correlations 4 kHz
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Psychoacoustic correlations 2 kHz
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PTC Q factor versus HL (2 kHz)
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Frequency resolution

• Degradation is greater at high than at low
  frequencies, for the same degree of loss

Reason OHCs lost
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Temporal resolution

• Resolution degrades more at the high than at the
  low frequencies, for the same degree of loss

Reason OHCs lost
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TEN elevation versus HL (2 kHz)
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OAE strength versus HL (2 kHz)
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Tuning curve sharpness vs cognition
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Correlations
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Correlations
PTC
Age
HL
OAE
Cognit
TEN
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Multiple regression
including HL causes correlations between
age and PTC / OAE / TEN to disappear
correlations between cognition and PTC / OAE /
TEN to disappear
Age
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Likely intermediate effects
Age
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Can we better predict intelligibility if we use
psychoacoustic results?
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(No Transcript)
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Does knowledge of dead regions help?
Poorer than expected
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VCVs Simple correlations with deficit after
hearing loss correction
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CUNY Simple correlations with deficit after
hearing loss correction
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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

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Why are hearing thresholds so useful?
Speech Perception proficiency
Hearing thresholds
Age
Cognitive ability
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Empirical evidence for prescription of
gain-frequency response
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Overall approach to prescription
Psychoacoustics
Theoretical predictions
Assumptions, rationale
Speech science
Empirical observations
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Speech intelligibility vs Loudness normalization
Laboratory results

• NAL-NL1 preferred over loudness normalization
• NAL-NL1 objectively higher intelligibility in
  noise

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Speech intelligibility vs Loudness normalization
Field test results

• NAL-NL1 significantly preferred over loudness
  normalization
• two-channel better than single channel for
  sloping losses

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Gain adults, medium input level(N 187)
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Gain preference over time
N 11
Source Keidser, OBrien, Yeend, McLelland
(submitted)
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Adjustments to prescription to allow for
experience
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Gain adults, low and high input levels
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Compression ratio preferences severe and
profound hearing loss
Source Keidser, Dillon, Dyrlund, Carter, and
Hartley (2007)
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Children
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Preference rating in real life
17
11
8
3
T Ching, NAL, CRC CIHA INN
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Language ability 12 months after fitting

• Effect of age of fitting p 0.0001
• Effect of hearing loss p lt 0.0001
• Effect of prescription p 0.9

Hearing aids
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Directional microphones for infants and toddlers?
Teresa Ching
Anna OBrien
SNR improvement
Yes ! More to gain than there is to lose.
Ching, OBrien et al, 2009
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RECD in infants (own mold HA2)
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A digression .

• At what age do you implant children?

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Language at 6 and 12 months after implant
Effect of implant age p lt 0.001
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Language skills at 3 yrs
Effect of age of implant p 0.02
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Empirical evidence variations from NAL-NL1
Output level
NAL-NL1
Input level
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Adults congenital or acquired?
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Binaural loudness summation

• Loudness adds across ears
• Total loudness loudnessleft loudness right
• Binaural loudness 2.(monaural loudness)

Loudness doubling Low levels 3 dB High levels
10 dB
Data for hearing impaired Low levels 3-4
dB High levels 5-10 dB
Implication Binaural gain correction increases
with input level
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Binaural loudness summation

• For symmetrical loss correction from 3 dB to 8
  dB
• For asymmetical loss less

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Prescribing amplification features
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(No Transcript)
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Amplification / tone controls Feedback cancelling
Intelligibility in quiet
Convenience
Comfort quality
Clinicians
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Intelligibility in quiet
Wireless (e.g. FM) Directional microphones
Intelligibility in noise
Convenience
Comfort quality
Clinicians
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Intelligibility in noise
Intelligibility in quiet
Transient noise suppression Adaptive
noise suppression Multi-program Open
fittings Expansion Bilateral feedback
control Active occlusion reduction Bilateral auto
control Echo reduction
Convenience
Comfort quality
Clinicians
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Intelligibility in noise
Intelligibility in quiet
Comfort quality
Transient noise suppression Adaptive
noise suppression Multi-program Open
fittings Expansion Bilateral feedback
control Active occlusion reduction Bilateral auto
control Echo reduction
Convenience
Clinicians
120
Intelligibility in noise
Intelligibility in quiet
Bilateral manual control Phone interface Auto-tele
coil Rechargeable battery Alerting tones and
messages
Comfort quality
Convenience
Clinicians
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Intelligibility in noise
Intelligibility in quiet
Convenience
Bilateral manual control Phone interface Auto-tele
coil Rechargeable battery Alerting tones and
messages
Comfort quality
Clinicians
Data logging Integrated RECD
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Intelligibility in noise
Intelligibility in quiet
Comfort quality
Convenience
Data logging Integrated RECD
Clinicians
123
Intelligibility in noise
Intelligibility in quiet
ADRO WDRC Auto-program
Comfort quality
Convenience
Auto-gain adaptation Trainability
Clinicians
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Intelligibility in noise
Intelligibility in quiet
Amplification / tone controls Feedback cancelling
Wireless (e.g. FM) Directional microphones
Comfort quality
Convenience
ADRO WDRC Auto-program
Adaptive noise suppression Multi-program Open
fittings Expansion Bilateral feedback
control Active occlusion reduction Bilateral auto
control Echo reduction
Bilateral manual control Phone interface Auto-tele
coil Rechargeable battery Alerting tones and
messages
Implantable hearing aids
Auto-gain adaptation Trainability
Data logging Integrated RECD
Clinicians
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• Coming real soon .
• NAL-NL2
• Demo

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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

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Thanks for listening

• www.nal.gov.au

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Not being used
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In the land of prescriptions.
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VCV deficit vs tuning curve sharpness
Uncorrected for HL
Dead regions marked
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VCV deficit vs tuning curve sharpness
Corrected for HL
Dead regions marked
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CUNY deficit vs PTC
Corrected for hearing loss
Uncorrected for HL
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Yes, a little speech deficit increases as
frequency selectivity gets broader
But not once we fully build HL into the SII
prediction
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4. Experience
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Combined gender-experience effect
Female new users prefer, on average, 3.7 dB less
gain than male experienced users
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Sharply tuned
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Psychophysical tuning curve and cochlear dead
region 4 kHz
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BKB
VCV
CUNY
Q
N
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Psychoacoustical tuning curve Q10
Fc
Q Fc / BW Q10 1000/340 3.4
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Cognition words vs digits