Hearing Aids and Hearing Impairments

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Hearing Aids and Hearing Impairments

• Meena Ramani
• 02/21/05

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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
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Outline

• 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

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Facts on Hearing Loss in Adults

• 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?
• Stigma associated with wearing a Hearing Aid (HA)
• Denial about ones Hearing Loss (HL)
• Exorbitant cost (eg. A pair of Widex Senso Diva
  BTEs cost around 11,000)
• Current HAs do not meet user expectations

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Hearing Aids- An Engineering perspective

• Area where vast improvement are possible
• 28 million Hearing Impaired people ?Huge
  Market()
• Circuit design and Signal processing personnel
• Circuit design
• Low power 1.3V
• Fast acting (delay lt 10ms)
• Small size
• Lower cost
• Signal Processing
• Biologically inspired/smarter algorithms
• Restore all effects of hearing impairment.

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Anatomy of a Hearing Aid

• Microphone
• Tone hook
• Volume control
• On/off switch
• Battery compartment

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Types of Hearing aids
Behind The ear BTE
In the Ear ITE
Completely in the canal CIC
In the Canal ITC
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Cochlea-IHC and OHC

• Organ of corti
• IHC/OHC
• 3 times more OHC
• Inner Hair Cells (IHC)
• Afferent ltto braingt
• Outer Hair Cells (OHC)
• Efferent ltfrom braingt
• Sharpen the traveling wave
• Provide an amplification for soft sounds(40-50 dB
  SPL)
• Damage in OHC/IHC ?Sensorineural Hearing Loss
  (SNHL)

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Presbycusis

• Type of Sensorineural Hearing Loss
• HL in aging ears occurs due to damage in OHCs
• Mild 25-39 dBHL
• Moderate   40-68 dBHL
• Severe 70-94 dBHL
• Problems faced by people with presbycusis
• Decreased Audibility
• Decreased Frequency Resolution
• Decreased Temporal resolution
• Decreased Dynamic Range

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Decreased Audibility

• 90 of HI adults loose frequencies between
  500Hz-4KHz
• HF components of speech (consonants) are weaker
  than the LFs.
• Loudness dominated by the LFs
• Speech is loud enough but not clear enough!
• To overcome this
• HA has to provide more gain at HFs.

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Decreased Frequency Resolution

• 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
• HAs less gain at LFs
• Try to remove noise before entering HA.
  Beamforming

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Decreased Temporal Resolution

• Intense sounds mask weaker sounds that
  immediately follow them.
• To overcome this
• Fast acting compression
• ltProblem Changes the speech cues decreases
  intelligibility though it increases audibility!gt

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Dynamic Range of Hearing

• The practical dynamic range could be said to be
  from the threshold of hearing to the threshold of
  pain
• 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
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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.
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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.
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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.
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Compressive Amplification

• Slope1/Compression Ratio
• Imitates compression carried out by OHCs
• Fast Acting/Syllabic Compression
• Attack time5ms
• Release time60ms
• Choose release time
• To avoid distortion
• To normalize loudness from phoneme to adjacent
  phoneme for syllabic compression

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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Time Constants Overshoot and Undershoot

• Overshoot
• Affects Intelligibility
• Makes consonants be identified as plosives
• Reduce effects
• Clipping overshoot
• Delaying gain
• Undershoot
• When release time isn't that large, then forward
  masking lowers the affect of undershoot

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).
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Singleband/Wideband

• 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.

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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Multiband Compressor

• 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.
• STIltSpeech Intelligibility Measuregt 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
  bandsltTo avoid this increase overlap between
  bands so that gain at a frequency is controlled
  by more than 2 bandsgt