Spartan 821/721 Manual Glossary C-16
Sound Exposure Level (SEL, LE)
The total sound energy in a specific time period usually expressed in decibels. The
following equation shows that the sound pressure is squared and integrated over a
specific period of time (t
2
-t
1
).
The Sound Exposure Level is usually expressed in Pascals squared-seconds or Pascals
squared-hours. P
0
is the reference pressure of 20 µPa, and “t” is the reference time of 1
second. It is then put into logarithmic form. It’s important to note that this is not an
average since the reference time is not the same as the integration time.
Sound Pressure
Sound Pressure is 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).
Sound Pressure is the amplitude of the oscillating sound pressure and is measured in
Pascals (Pa), or Newtons per square meter, which is a metric equivalent of pounds per
square inch. To measure sound, a sound level meter uses a detector to separate the
oscillating pressure from the steady (barometric) pressure. The sound level meter then
squares the pressure, takes the time average, and takes the square root (this is called
rms for root-mean square). This method is one of several ways to mathematically
measure sound.
Moving Average: The averaging process is continually accepting new data so it is similar
to an exponential moving average. In the equation, 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 it is, 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. For more information on these values, see Time Weighting
.
Fixed Average: The averaging process is over a fixed time period. The equation for it is
SEL 10 l og
10
p
2
t td
T
1
T
2
P
2
o
t
-----------------------
dB
=
p
rms
1
T
---
p
2
e
t – T–
d
t
s
t
=
p
rms
1
T
2
T
1
–
-----------------------
p
2
t td
T
1
T
2
=
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