SPTH341: Lecture 3 Classroom Acoustics and Solutions

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SPTH341 Lecture 3Classroom Acoustics and
Solutions

• Mark C. Flynn, PhD
• Department of Speech and Language Therapy

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Acoustics in Classrooms

• The acoustics of the classroom are often
  considered the forgotten variables in ensuring
  the classroom success of children.

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Problems in Classrooms

• Noise
• Distance
• Reverberation
• Signal-to-Noise Ratio (SNR)

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Noise

• Children, by nature, are not very quiet.
• Numerous sources of noise in the classroom.
• All noise is amplified by the hearing aid.

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Noise

• Background noise has the effect of overlapping
  some of the speech to make it inaudible or
  masked.
• Listeners have difficulty understanding speech
  and communicating in situations where the level
  of background noise is loud enough that it
  interferes with the speech signal (Dirks, Morgan
  Dubno, 1982 Helfer, 1991).

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Reverberation

• RT Sounds can overlap each other (overlap
  masking) and smear the internal energy within
  each sound occurs (self-masking).
• Studies of small amounts of RT (lt0.6s) found a
  significant decrease in perception of speech with
  increased RT (Harris Swenson, 1990 Nabelek
  Pickett, 1974 Nabelek Robinette, 1978).

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Reverberation
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Distance

• Signal can drop 6dB with every doubling in
  distance.
• 85dB at the mouth 65dB at 1 meter distance.

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Signal loss over distance
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Directional nature of speech
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S/N Ratio Requirements

• Normal hearing children require 5dB.
• Hearing-Impaired children require 20dB.
• Why the difference? 3 Distortions Childs
  Hearing Loss, Classroom Acoustics, HA Distortion

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ASHA Guidelines

• Noise level in an empty classroom 30-35dBA
• Signal-to-noise level greater than 15dB
• RT lt 0.4s

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Noise Control in Schools

• Types of noise to examine
• Background
• Intruding
• Internally generated
• Strategies
• Absorption
• Vibration
• Location
• Noise isolation

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Strategies

• Keep noise away from the school
• Reduce noise entering the classroom
• Reduce level at originating point
• Reduce level within noise pathways to classrooms
• Isolate and seal classrooms

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Noise and Reverb. Control within the Classroom

• Watch the frequency response.
• Watch reflection times, especially
  Hass/precedence effect.
• A small (lt.03s) amount of reverberation is good,
  any increase on that will result in decreased
  perception of speech.

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Noise Propagation
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Measuring Sound

• Background noise
• Sound level meter (instant reading).
• Noise dosimeter (noise can be measured over
  time).
• Reverberation Time (RT)
• Time it takes for sound to decay 60dB

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What is FM?

• A system for transmitting the teachers voice
  straight to the childs ear.
• Principle is to increase SNR

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The Secret of FM

• Distance from the teachers mouth to the students
  ear is 15 cm.
• Signal to Noise Ratio 30 dB, so speech from
  the teacher is clear.
• Reverberation is minimised as the teachers voice
  is transmitted directly.

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FM Systems

• Personal FM
• Receiver and headphones
• telecoil neck loop
• Direct Audio Input (DAI)
• BTE-FM
• Sound Field FM
• Toteable Sound Field

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Receiver and Headphones

• Students listening through headphones
• Hearing aids are not required

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Hearing Aids and FM

• HA Provide Amplification FM Increases S/N Ratio.
• Assumes Child has an optimally fitted hearing
  aid.
• FM system is not to make up for a poor hearing
  aid fitting.

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Inductive Coupling from the Neck loop to Telecoil

• Hearing aid set to T or MT switch
• Signal sent to HA via the telecoil

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Direct Audio Input (DAI)

• Signal sent directly to the hearing aid.

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BTE-FM

• BTE hearing aid with built in FM
• Great idea in theory.
• Problems with the quality of the hearing aid.

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Sound Field FM
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Toteable Sound Field
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What is a classroom?
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Acknowledgements

• Pictures and graphics in this presentation were
  supplied courtesy of
• Oticon New Zealand
• Phonic Ear (USA)