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SPTH341: Lecture 3 Classroom Acoustics and Solutions

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The acoustics of the classroom are often considered the forgotten variables in ... Child's Hearing Loss, Classroom Acoustics, HA Distortion. ASHA Guidelines ... – PowerPoint PPT presentation

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Title: SPTH341: Lecture 3 Classroom Acoustics and Solutions


1
SPTH341 Lecture 3Classroom Acoustics and
Solutions
  • Mark C. Flynn, PhD
  • Department of Speech and Language Therapy

2
Acoustics in Classrooms
  • The acoustics of the classroom are often
    considered the forgotten variables in ensuring
    the classroom success of children.

3
Problems in Classrooms
  • Noise
  • Distance
  • Reverberation
  • Signal-to-Noise Ratio (SNR)

4
Noise
  • Children, by nature, are not very quiet.
  • Numerous sources of noise in the classroom.
  • All noise is amplified by the hearing aid.

5
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).

6
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).

7
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8
Reverberation
9
Distance
  • Signal can drop 6dB with every doubling in
    distance.
  • 85dB at the mouth 65dB at 1 meter distance.

10
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11
Signal loss over distance
12
Directional nature of speech
13
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

14
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15
ASHA Guidelines
  • Noise level in an empty classroom 30-35dBA
  • Signal-to-noise level greater than 15dB
  • RT lt 0.4s

16
Noise Control in Schools
  • Types of noise to examine
  • Background
  • Intruding
  • Internally generated
  • Strategies
  • Absorption
  • Vibration
  • Location
  • Noise isolation

17
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

18
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.

19
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20
Noise Propagation
21
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

22
What is FM?
  • A system for transmitting the teachers voice
    straight to the childs ear.
  • Principle is to increase SNR

23
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.

24
FM Systems
  • Personal FM
  • Receiver and headphones
  • telecoil neck loop
  • Direct Audio Input (DAI)
  • BTE-FM
  • Sound Field FM
  • Toteable Sound Field

25
Receiver and Headphones
  • Students listening through headphones
  • Hearing aids are not required

26
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.

27
Inductive Coupling from the Neck loop to Telecoil
  • Hearing aid set to T or MT switch
  • Signal sent to HA via the telecoil

28
Direct Audio Input (DAI)
  • Signal sent directly to the hearing aid.

29
BTE-FM
  • BTE hearing aid with built in FM
  • Great idea in theory.
  • Problems with the quality of the hearing aid.

30
Sound Field FM
31
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32
Toteable Sound Field
33
What is a classroom?
34
Acknowledgements
  • Pictures and graphics in this presentation were
    supplied courtesy of
  • Oticon New Zealand
  • Phonic Ear (USA)
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