Larson Davis SoundAdvisor 831C Reference Manual page 215

Sound Pressure
Sound Pressure
Level (SPL, Lp)
SoundAdvisor Model 831C
Pascal squared- hours. P
reference time of 1 second. It is then put into logarithmic form. It is important to
note that it is not an average since the reference time is not the same as the
integration time.
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
p
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. 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.
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 pressure which is 20 Micro-pascals (
There are various descriptors attached to this level depending on how the actual
sound pressure is processed in the meter:
is the reference pressure of 20 µPa and T is the
0
t
1
2

  e
-- - p
=
rms
T
t
s
1
---------------------- - p
p =
rms

T – T
2
 t   T 
– –
 d
T
2
2

t   t d

1
T
1

Pa).
C-13