Download Auditory encoding of speech features: Effects of hearing impairment

Auditory encoding of speech
features: Effects of hearing
impairment
C315/A6: A Faulkner. Jan 30 2004
Readings:
Auditory processing of Speech (Faulkner 2003 chapter)
Chapter 16 (by Revoile) in Pickett “The acoustics of speech
communication” 1999.
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The role of audibility
Psychoacoustic effects of sensori-neural
(cochlear) hearing loss
• Much of the impact of hearing loss is
thought of in terms of AUDIBILITY
• HOW much of the information in speech is
audible?
• Loss of sensitivity (most often at higher
frequencies)
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Words recognised from simple sentences in quiet by aided
hearing impaired adults as a function of average hearing loss at
0.5, 1 and 2 kHz. Circles are scores from individual subjects
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(After Boothroyd, 1990)
Raised thresholds
Reduction of dynamic range
Abnormal loudness growth
LIMITS AUDIBILITY OF INFORMATION
– Over frequency
– Over intensity
• Impaired frequency selectivity typical in
frequency regions with significant sensitivity loss
• Consider the audible area of frequency and
intensity in relation to the range of
frequencies and intensities in speech
– Due to damaged OHCs
– LIMITS ANALYSIS OF INFORMATION
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Speech range re audibility
The blue area represents the dynamic
range of speech over frequency
relative to the normal threshold of
hearing.
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Frequency importance
weightings: AI
I (2000 Hz)
The red curves represent the range of
audible levels over frequency for a
typical moderate sloping hearing loss.
Intelligibility is reduced when not
all of the speech range is audible.
W (2000 Hz)
Audible portion can be increased
with a hearing aid
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A is the Articulation Index (predicted
intelligibility). A is determined by adding up
W x I over frequency bands, where I is the
band importance weight and W is the
proportion of a 30 dB dynamic range of speech
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in that band that is audible.
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AI predictions
AI predicts intelligibility rather well for mild and moderate hearing losses.
But not for severe and profound losses – here the effects of audibility are
not enough to explain limits to speech recognition
Caveat to audibility accounts
• Good predictions of speech intelligibility
from audibility hold only for mild to
moderate hearing losses
• Complete restoration of audibility with
more severe losses cannot restore
intelligibility
• And these predictions only hold for speech
in the quiet
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Coding spectral shape
• 1st key property of auditory coding – The
cochlea acts like a bank of band-pass filters
Impaired frequency selectivity
– Each filter has a place (position on basilar
membrane)
– At basal places band-pass filters have high
frequency pass-bands
– At apical places band-pass filters have low
frequency pass-bands
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Auditory filtering in normal
hearing
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Auditory filtering in impaired
hearing
35 dB loss at 2 kHz
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2
Normal compared to impaired excitation patterns
Impaired frequency selectivity
• Broadened auditory filters lose detail in the
auditory representation of the spectrum of speech
leading to …
– a reduced ability to resolve spectral detail (for example,
where formants are).
– lessened ability to separate speech from noise - reduced
signal to noise ratio at filter output.
• Because hearing loss typically affects higher
frequencies, selectivity is also typically more
impaired at higher frequencies
Impaired excitation pattern – auditory place coding of
spectrum retains much of the formant structure in quiet
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Listening to speech with reduced
frequency selectivity
Normal compared to impaired excitation patterns
Acoustic spectrum: Speech-to-Noise ratio is +6 dB – Noise fills valleys in the spectrum
Both noise and reduced selectivity affect a range of speech tasks
Normal excitation pattern retains much of formant structure in noise
Impaired excitation pattern - shows reduced formant structure as noise spills into filters
tuned to formant frequencies
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Speech sound confusions
threshold
Threshold
Discomfort
Consonant place cues completely unavailable
Frequent confusions of consonant place: Severe hearing loss
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3
Effects of reduced spectral
resolution
Stimulus
Consonant confusions with reduced spectral information
Response
(4 bands)
l
m
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q
k
v
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c
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cY
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sR
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l m
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1
2
20
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1 22
9
1
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2
1
1
1
q
k
v
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3
8
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1
5 12
1
c
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f
cY
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10
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27
S
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30
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7 20
3 24
1
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29
1
1
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1
1
27
27
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• Even though all frequencies are AUDIBLE
loss of spectral resolution affects speech
intelligibility
• Primarily through loss of consonant place of
articulation cues and vowel formant cues
3
5 15
1 23
1
1
29
2 19
1
1
8
1
30
3
1
7
20
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Summary: 1
Summary: 2
• Three main consequences of cochlear
hearing loss
• Reduced audibility limits speech
recognition
– Reduced audibility: unable to hear the quieter
parts of speech.
– Loudness recruitment: dynamic range of speech
exceeds dynamic range of hearing
– Reduced frequency selectivity: spectral
smearing of speech signal.
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• Broadened auditory filters
– Poorer resolution of spectral features of speech
in excitation pattern – especially in noise
– Fewer harmonics resolved, limiting the
usefulness of temporal coding of frequency
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