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Hearing Aids and Hearing Impairments

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					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
   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
   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 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

    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 < 10ms)

          Small size

          Lower cost

        Signal Processing:
          Biologically inspired/smarter algorithms

          Restore all effects of hearing impairment.
Anatomy of a Hearing Aid

                  Microphone
                  Tone hook
                  Volume control
                  On/off switch
                  Battery compartment
Types of Hearing aids

          Behind The ear
                           In the Ear
          BTE
                           ITE




         In the Canal      Completely in the
                           canal
         ITC
                           CIC
Cochlea-IHC and OHC

                      Organ of corti:
                            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
   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
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.
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
Decreased Temporal Resolution

   Intense sounds mask weaker sounds that
    immediately follow them.
   To overcome this:
       Fast acting compression
       <Problem: Changes the speech cues; decreases
        intelligibility though it increases audibility!>
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
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   Figure 7.2. The response of a healthy
line) and a hearing-impaired person (dashed    basilar membrane (solid line) and one with
line). The abscissa is the sound pressure      deadened outer hair cells (dashed line) to
level of a narrowband sound and the            best-frequency tone at different sound
ordinate is the loudness category applied to   pressure levels (replotted from Ruggero
the signal. VS, very soft; S, soft; C,         and Rich 1991).The slope reduction in the
comfortable; L, loud; VL, very loud; TL, too   mid-level region of the solid line indicates
loud.                                          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
                                               HA wearer adjusts gain, using volume
 hearing-impaired    listener  wearing     a   control, as the level of environment
 compression hearing aid (long dashed line
 with symbol).
                                               changes.
Compressive Amplification

                                                         Slope=1/Compression Ratio
                                                         Imitates compression carried
                                                          out by OHCs
                                                         Fast Acting/Syllabic
                                                          Compression:
                                                             Attack time~5ms
                                                             Release time~60ms

 Figure 7.4. Typical input-output function of a
                                                         Choose release time
 compression hearing aid measured with a pure                To avoid distortion
 tone stimulus at multiple levels. The function
 depicted shows linear operation at low and high             To normalize loudness from
 input levels, and 3 : 1 compression at mid-levels.
 Different compression hearing aids have different
                                                              phoneme to adjacent phoneme
 compression ratios and different levels over which           for syllabic compression
 compression occurs.
Time Constants: Overshoot and Undershoot


                                                               Overshoot:
                                                               •Affects Intelligibility
                                                               •Makes consonants be identified as
                                                               plosives
                                                               Reduce effects:
                                                                                    •Clipping
                                                                                    overshoot
                                                                                    •Delaying gain
                                                               Undershoot:
Figure 7.5. A demonstration of the dynamic behavior of a
compressor. Top: Level of the input signal Middle: Gain that   When release time isn't that large, then
will be applied to the input signal for 3 : 1 compression,
incorporating the dynamics of the attack and release time      forward masking lowers the affect of
constants. Bottom: The level of the output signal,             undershoot
demonstrating overshoot (at 0.05 second) and undershoot (at
0.15 second).
 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
Figure 7.7. Amount of compression applied to music by a             Speech has multiple
wideband compressor (squares) and a multiband compressor
(circles).The compression was measured by comparing the              spectral peaks. Inadequate
peak/root mean square (rms) ratio of the music into and out of
the compressor over different frequency regions. The open            selection of gains.
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.
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.
   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>

				
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