Transcript Slide 1

Hearing Aids and Hearing
Impairments
Meena Ramani
02/21/05
Dramatic Decrease In Audibility &
Intelligibility
Original Speech
40dB conductive loss
P. Duchnowski and P. M. Zurek, “Villchur revisited: Another look at automatic gain control
simulation of recruiting hearing loss,” J. Acoust. Soc. Am., vol. 98, no. 6, pp. 3170-3181,
Dec. 1995
Outline
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Facts on Hearing Loss
Hearing Aids
Cochlea-IHC and OHC
Presbycusis
 Decreased Audibility
 Decreased Frequency Resolution
 Decreased Temporal resolution
 Decreased Dynamic Range
Amplification Techniques
 Linear
 Compressive-Single/MultiBand
Facts on Hearing Loss in Adults
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One in every ten (28 million) Americans has hearing loss and the
prevalence of hearing loss increases with age.
While hearing aids can help about 95% (26 million) of them, only 6 million
use hearing aids.
WHY?
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Stigma associated with wearing a Hearing Aid (HA)
Denial about one’s Hearing Loss (HL)
Exorbitant cost (eg. A pair of Widex Senso Diva BTEs cost around $11,000)
Current HAs do not meet user expectations
Hearing Aids- An Engineering perspective
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Area where vast improvement are possible
28 million Hearing Impaired people Huge Market($$$$)
Circuit design and Signal processing personnel
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Circuit design:
 Low power: 1.3V
 Fast acting (delay < 10ms)
 Small size
 Lower cost
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Signal Processing:
 Biologically inspired/smarter algorithms
 Restore all effects of hearing impairment.
Anatomy of a Hearing Aid
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Microphone
Tone hook
Volume control
On/off switch
Battery compartment
Types of Hearing aids
Behind The ear
BTE
In the Canal
ITC
In the Ear
ITE
Completely in the
canal
CIC
Cochlea-IHC and OHC
Organ of corti:
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IHC/OHC
3 times more OHC
Inner Hair Cells (IHC)
 Afferent <to brain>
Outer Hair Cells (OHC)
 Efferent <from brain>
 Sharpen the traveling wave
 Provide an amplification for
soft sounds(40-50 dB SPL)
Damage in OHC/IHC
Sensorineural Hearing Loss
(SNHL)
Presbycusis
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Type of Sensorineural Hearing Loss
HL in aging ears; occurs due to damage in OHCs
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Mild 25-39 dBHL
Moderate 40-68 dBHL
Severe 70-94 dBHL
Problems faced by people with presbycusis:
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Decreased Audibility
Decreased Frequency Resolution
Decreased Temporal resolution
Decreased Dynamic Range
Decreased Audibility
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90% of HI adults loose frequencies between
500Hz-4KHz
HF components of speech (consonants) are
weaker than the LFs.
Loudness dominated by the LFs
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“Speech is loud enough but not clear enough!”
To overcome this:
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HA has to provide more gain at HFs.
Decreased Frequency Resolution
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Asymmetry of traveling wave
Eg. Reverse Audiogram
OHCs do not sharpen the traveling wave.
Decreases the ability to distinguish close
frequencies
Upward spread of masking low frequencies mask
more than high frequencies
Normals and HI: Poor resolution at high intensities
To overcome this:
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HAs less gain at LFs
Try to remove noise before entering HA. Beamforming
Decreased Temporal Resolution
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Intense sounds mask weaker sounds that
immediately follow them.
To overcome this:
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Fast acting compression
<Problem: Changes the speech cues; decreases
intelligibility though it increases audibility!>
Dynamic Range of Hearing
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The practical dynamic range could be said to be from the threshold of
hearing to the threshold of pain
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Sound level measurements in decibels are generally referenced to a
standard threshold of hearing at 1000 Hz for the human ear which can be
stated in terms of sound intensity:
Equal Loudness Contours
Decreased Dynamic Range/Recruitment
SNHL increases threshold of hearing much more than the threshold of
pain; thus decreases the Dynamic Range of the ear.
To overcome this:
HA has to provide Compression; cut down amplification as sound
gets louder.
Decreased Dynamic Range/Recruitment
Figure 7.1. Typical loudness growth
functions for a normal-hearing person (solid
line) and a hearing-impaired person (dashed
line). The abscissa is the sound pressure
level of a narrowband sound and the
ordinate is the loudness category applied to
the signal. VS, very soft; S, soft; C,
comfortable; L, loud; VL, very loud; TL, too
loud.
Figure 7.2. The response of a healthy
basilar membrane (solid line) and one with
deadened outer hair cells (dashed line) to
best-frequency tone at different sound
pressure levels (replotted from Ruggero
and Rich 1991).The slope reduction in the
mid-level region of the solid line indicates
compression; this compression is lost in
the response of the damaged cochlea.
Linear Amplification
Figure 7.3. Loudness growth functions for a
normal-hearing listener (solid line), a
hearing-impaired listener wearing a linear
hearing aid (short dashed line), and a
hearing-impaired
listener
wearing
a
compression hearing aid (long dashed line
with symbol).
HA wearer adjusts gain, using volume
control, as the level of environment
changes.
Compressive Amplification
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Slope=1/Compression Ratio
Imitates compression carried
out by OHCs
Fast Acting/Syllabic
Compression:
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Figure 7.4. Typical input-output function of a
compression hearing aid measured with a pure
tone stimulus at multiple levels. The function
depicted shows linear operation at low and high
input levels, and 3 : 1 compression at mid-levels.
Different compression hearing aids have different
compression ratios and different levels over which
compression occurs.
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Attack time~5ms
Release time~60ms
Choose release time
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To avoid distortion
To normalize loudness from
phoneme to adjacent phoneme
for syllabic compression
Time Constants: Overshoot and Undershoot
Overshoot:
•Affects Intelligibility
•Makes consonants be identified as
plosives
Reduce effects:
•Clipping
overshoot
•Delaying gain
Figure 7.5. A demonstration of the dynamic behavior of a
compressor. Top: Level of the input signal Middle: Gain that
will be applied to the input signal for 3 : 1 compression,
incorporating the dynamics of the attack and release time
constants. Bottom: The level of the output signal,
demonstrating overshoot (at 0.05 second) and undershoot (at
0.15 second).
Undershoot:
When release time isn't that large, then
forward masking lowers the affect of
undershoot
Singleband/Wideband
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Figure 7.7. Amount of compression applied to music by a
wideband compressor (squares) and a multiband compressor
(circles).The compression was measured by comparing the
peak/root mean square (rms) ratio of the music into and out of
the compressor over different frequency regions. The open
symbols on the left show the compression ratio calculated from
the change to the broadband peak/rms ratio. The filled symbols
show the change to the peak/rms ratio in localized frequency
regions.
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Adjust gain across all
frequencies equally
Preserves spectral shape
over short time scales;
speech cues
Choose gain based on
highest level; Spectral peak
Speech has multiple
spectral peaks. Inadequate
selection of gains.
Multiband Compressor
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Normally upto 20 bands are used with varying compression ratio per
band.
Adjust gain/compression in each band independent from other
Change in spectral contrast across bands may cause perceptual
consequences though it restores normal loudness.
STI<Speech Intelligibility Measure> of compressed speech does not
correlate to Listening tests.
With more experience people who use multiband HAs get adjusted
to the change in spectral shape/cues.
Typically vowel perception is not affected as much as consonant
perception
Overamplification occurs at crossover between bands<To avoid this
increase overlap between bands so that gain at a frequency is
controlled by more than 2 bands>