SoundAdvisor Model 831C
C-13
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
).
This is called the sound exposure. It’s 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 refer
-
ence 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 import
-
ant. 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 3 values called Time Weighting. See the values below.
Fixed Average
: The averaging process is over a fixed time period. The equation for it is
The sound pressure is squared and averaged over a fixed time period. Unlike the moving average,
the sound pressures in all time intervals are equally weighted.
Sound Pressure Level (SPL, Lp)
The logarithmic form of sound pressure. It is also expressed by attachment of the word decibel to
the number. The logarithm is taken of the ratio of the actual sound pressure to a reference sound
SEL
10
Log
10
p
2
t
t
d
T
1
T
2
p
0
2
T
-----------------------------
=
p
rms
1
T
---
p
2
e
t
–
T
–
d
t
s
t
=
p
rms
1
T
2
T
1
–
-----------------------
p
2
t
t
d
T
1
T
2
=
