MAKING INDUSTRIAL AUDIOMETRY WORTHWHILE

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MAKING INDUSTRIAL AUDIOMETRY WORTHWHILE

• Robin Howie
• Robin Howie Associates

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CONVENTIONAL TEACHING

• Noise Induced Hearing Loss (NIHL) is a loss in
  sensitivity, primarily at about 4 kHz

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FUNCTIONS OF HEARING

• To detect alarm sounds
• To communicate
• To locate sound sources
• To enjoy sounds

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FUNCTIONS OF HEARING

• Of the above functions, the ability to understand
  speech is probably the most important in our
  society

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SPEECH

• English speech can involve rapid simultaneous
  changes of frequency and intensity

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SPEECH

• The ears ability to distinguish small
  differences in both frequency and intensity
  permits speech to be understood

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SELECTIVITY

• The ear is able to detect frequency differences
  of the order of 1-2 between two frequencies
• It is this ability which allows us to hear a
  solo singer against the background of a full
  orchestra

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SELECTIVITY MECHANISM

• Current theories suggest that the IHC are signal
  detectors which form a positive feedback loop
  with the brain and with the OHC supplying energy,
  so increasing gain and selectivity

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SELECTIVITY

• Noise induced hearing loss reduces both
  sensitivity and frequency selectivity

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SELECTIVITY

• Many persons with sensori-neural hearing loss can
  understand speech well in the quiet but have
  progressively greater difficulty as background
  noise levels increase

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NOISE INDUCED HEARING LOSS

• Persons with noise induced hearing loss often
  have sufficient hearing sensitivity to be aware
  that someone is speaking but may have
  insufficient selectivity to be able to decipher
  speech signals in noisy environments

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AUDIOMETRY

• What is the purpose of audiometry?

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Directive 2003/10/EC

• Article 10 Health surveillance
• 10(2) A worker whose exposure exceeds the upper
  exposure action values 85 dB(A)/140 Pa shall
  have the right to have his/her hearing checked
  ...

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Directive 2003/10/EC

• Article 10 Health surveillance
• 10(2) The objectives of these checks are to
  provide early diagnosis of any loss of hearing
  due to noise, and to preserve the hearing
  function.

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RESOLUTION

• What resolution is required to meet the above
  objectives?

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Hearing check requirements

• To enable the objects of Article 10 to be
  achieved, hearing tests must reliably detect
  early NIHL in individuals caused by exposure
  to 85 dB(A)

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Required audiometric sensitivity

• The objects of early diagnosis and preservation
  of hearing mean that lt 50 of the maximum NIHL at
  critical frequencies must be reliably detected in
  individual subjects

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Required audiometric sensitivity

• Each individual test must therefore allow lt 25
  of the maximum NIHL at critical frequencies to be
  reliably detected

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Required audiometric sensitivity

• Frency Max NIHL Sensitivity
• (kHz) (dB) (dB)
• 1 1.5 -
• 2 3.2 0.8
• 3 5.8 1.5
• 4 6.7 1.5

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Sensitivity of conventional audiometry

• with careful, well-conducted audiometry
  changes of 10 dB between audiograms should be
  regarded as possibly significant.
• the accuracy could be increased two-fold by
  repeating the audiogram four times
• Burns (1973)

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Audiometric technique which achieves the
required sensitivity
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Protocol

• Test only subjects without ear wax, ear
  infection, congestion or recent exposure to high
  noise levels
• Two tests per subject per session
• Test at 1, 2, 3, 4, 6 and 8 kHz only
• Give subjects soft drinks prior to and during the
  test to help clear their Eustachian tubes
• Ensure subjects wear quiet clothing

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Equipment

• Ear inserts rather than ear phones
• Test tones presented in 1dB steps rather than
  2.5 or 5.0 dB steps
• Record increasing SPL thresholds only
• Record seven thresholds at each frequency
  for each ear

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Data analysis

• Analyse the last 5 of 7 increasing SPL thresholds
  only
• Record thresholds on spreadsheets, so enabling
  ANOVA between current and previous
  audiogram(s)
• Analyse results for 4 or 6 kHz dips

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Sensitivity of modified technique

• Differences of 1-2 dB between test sessions
  can be resolved with trained subjects

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Details of modified technique

• Howie, Gardiner and Watt (1998)
• OTO 98 020 on HSE website

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HOWEVER

• The critical consequence of NIHL is loss of
  frequency selectivity rather than loss of
  sensitivity
• Loss of sensitivity is a poor predictor of loss
  of selectivity

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SPEECH AUDIOMETERY

• We should evaluate speech audiometry as a hearing
  surveillance techqique

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SPEECH AUDIOMETERY

• Speech audiometry is a direct measure of the
  generally most important hearing function