Acoustic shock an overview

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Acoustic shock - an overview

• Harvey Dillon, Michael Fisher
• National Acoustic Laboratories
• CRC for Cochlear Implant and Hearing Aid
  Innovation
• ISA, Melbourne
• March, 2002

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Overview

• Acoustic shock
• What is it?
• Who does it affect?
• What sounds cause it?
• Why does damage occur?
• How do we prevent it?

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What is acoustic shock ?
Loud sound
Adverse physical, psychological, or auditory
effects
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What is acoustic shock ?

• Any temporary or permanent disturbance of the
  functioning of the ear, or of the nervous system,
  which may be caused to the user of a telephone
  earphone by a sudden sharp rise in the acoustic
  pressure produced by it.
• (International Telecommunications Union
• European Transmission Standards Institute)

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Who is at risk ?
Telephone call centre workers
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Reports of Acoustic Shock

• Kristensen (2000)
• Denmark
• concentration / memory
• panic / fear
• earache
• nausea
• tinnitus
• tiredness
• headaches
• stress
• dizziness
• neck and shoulder pain
• quality of life

• Hinke Brask (1999, 2001)
• Denmark
• No incidents in one call centre, but 20/90 in
  another
• stress
• neck and shoulder pain
• smoking

• RNID / TUC (1999)
• United Kingdom
• pain
• tinnitus
• hearing impairment

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Acoustic shock in Australia (Milhinch, 2001)
103 cases investigated
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Symptoms last from seconds to years
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What sounds cause acoustic shock ?

• High-pitched, high level sounds
• Acoustic feedback
• Fax tones
• Alarm signals
• Whistle blowing
• 2.3 to 3.4 kHz 80 to 120 dB SPL
• Same as alarm signals!

Screeches
Shrieks
Squarks
Howls
High-pitched tones
Acoustic incidents
Spikes
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How is the person damaged by acoustic shock?

• Not accumulated damage (c.f. 85 dB A SPL)
• Not instantaneous direct damage from the sound
  (because phone output lt 140 dB SPL peak)
• Linked to acoustic startle (Patuzzi)

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The startle response

• Muscle response - upper limbs, shoulders, neck,
  eye and ear (the stapedius muscle and the tensor
  tympani muscle)

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Middle ear muscles
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The startle response

• Muscle response - upper limbs, shoulders, neck,
  eye and ear (the stapedius muscle and the tensor
  tympani muscle)
• Emotional state affects magnitude and threshold

Reflex threshold
Startle
Shriek
Reflex magnitude
Reflex latency
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One result abnormal loudness perception
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Loud but OK
Slightly Loud
Comfortable
Slightly Soft
Soft
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How do we prevent acoustic shock ?

• 1. Limit sound levels
• How low does it need to be ?
• Effect on intelligibility

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Effect of noise and limiting
Noise 60 dB A Limiting level 80 dB SPL at
eardrum
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SII 0.6 gives 90 correct for low-context words
in sentences
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How do we prevent acoustic shock ?

• 1. Limit sound levels
• How low does it need to be ?
• Effect on intelligibility
• Too low creates stress !
• 2. Selectively suppress troublesome sounds
• 3. Reduce stress levels
• General work stress
• Confidence in equipment
• 4. Reduce room noise levels

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The future

• Acoustic shock as a problem

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For more information ..

• www.nal.gov.au - short general article
• Milhinch, J (2001). Acoustic Shock Injury.
• Patuzzi, R. A book !
• www.xxx.xxxx.xxx - CD rom Proceedings of Risking
  Acoustic Shock seminar.

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..Thats it from me.
the physiologist, the audiologist
the engineer ...
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Requirements for intelligibility

• Intelligibility can be calculated (Speech
  Intelligibility Index)

Low Frequencies Removed
High Frequencies Removed
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The End
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