roomacoustics

Room Acoustics

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I. Room Acoustics

A. Noise
B. Reverberation
C. Distance

A. Noise
(that pesky stuff!)

Noise levels are measured with a sound level meter.
There are 3 weightings on the meter which vary in the amount of low frequency reduction.
The weighting which is most like the response of the ear is the dBA scale.

The primary energy in background noise is low frequency.

Sources of noise in the classroom include

Internal

furniture
A/C or heater
kids talking
equipment in room
lights
COMD students snoring
TA lecture

External

gym
music room
hallway
road
airport
machinery

Optimal Noise Level
for Hard of Hearing class = 30-35 dBA, yet the typical level is = 60 dBA

Optimal S/N Ratio
The optimal S/N ratio= +30, while the typical S/N ratio= 0 to +5

Big Important Empirical Stuff
Finitzo-Heiber study mentioned in readings showed effects of noise and reverberation on speech recognition scores with Normal Hearing and

Hard of Hearing Children 8-12 years old .

In a soundbooth: No Noise, Little Reverberation
Norm= 95% H of H= 83%

With RT= .4
Norm H of H
S/N Ratio of +12: 83% 60%
S/N Ratio of 0: 48% 28%

With RT= 1.2
Norm H of H
S/N Ratio of +12: 70% 41%
S/N Ratio of 0: 30% 11%

B. Reverberation

Sound in a room travels until it reaches a partition or room barrier and then it is reflected.
The level of reflection depends on the absorption of the sound by the surface.
The higher the absorption, the less sound is reflected.

General Definition:

Reverberation time (RT) reflects the decay of energy of a sound after its offset.

Specific Definition:

The RT is the time for the SPL to decrease by 60 dB after the sound stops.

Typical Values of RT

auditoriums - 1.5 secs
classrooms - .4 secs

-Ideally, classrooms for the hearing-impaired would have reverberation time close to zero.

-The effect of reverberation on speech recognition is a smearing of temporal cues in speech. The silent intervals between phrases and syllables are filled in with reflected energy.
-Reverberation may also cause greater masking of weak consonants following intense vowels.

C. Distance

In general:
Sound decreases w/ increased distance.
With every doubling of distance (w/ 0 RT) there's a 6 dB decrease in SPL. (Inverse Square Law)

Sound Fields:

Near Field: .1 -.7 meters from speaker.
*Inverse square law holds for near field (doubling of distance decreases 6 dB).

2. Far Field .8-2 meters from speaker
*Inverse square law does not hold for far field.
(SPL approximately the same in far field for a broad band noise)

Important:
Because of Near Field effects you do not want to hold FM mic six inches from speaker for behavioral testing. Better to measure at one meter. Less variability in signal level due to movements of microphones.

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