9/29/09 D-19
form. It is important to note that it is not an average since the
reference time is not the same as the integration time.
Sound Pressure The physical characteristic of sound that can be detected by
microphones. Not all pressure signals detected by a
microphone are sound (e.g., wind noise). It is the amplitude
of the oscillating sound pressure and is measured in Pascals
(Pa), Newtons per square meter, which is a metric equivalent
of pounds per square inch. To measure sound, the oscillating
pressure must be separated from the steady (barometric)
pressure with a detector. The detector takes out the steady
pressure so only the oscillating pressure remains. It then
squares the pressure, takes the time average, and then takes
the square root (this is called rms for root-mean square).
There are several ways this can be done.
Moving Average: The averaging process is continually
accepting new data so it is similar to an exponential moving
average. The equation for it is
The sound pressure is squared and multiplied by a
exponential decay factor so that when the time of integration
is near the current time (t) it is essentially undiminished. For
times older (less) than the current time, the value is
diminished and so becomes less important. The rate at which
older data are made less influential is expressed by the
constant T. The larger is it the slower the decay factor
reduces and the slower the response of the system to rapid
changes. These are standardized into three values called
Time Weighting. See the values below.
p
rms
1
T
---
p
2
e
t –T–
î™›d
t
s
t
îš³
=
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