Larson Davis 824 - Sound Pressure Level (SPL, Lp

D-20 824 Reference Manual 9/29/09
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, L
p
) 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 MicroPascals (
 Pa).
There are various descriptors attached to this level
depending on how the actual sound pressure is processed in
the meter:
Instantaneous: The time varying reading on a meter face on
in a meter output due to changes in the sound pressure. The
reading will depend on the time-weighting applied.
The fundamental relationship between the two is logarithmic
where p
0
is the reference sound pressure of 20 µPa. The
square of the sound pressure is a power-like quantity that
can be expressed in the original form of the level definition
p
rms
1
T
2
T
1
–
-----------------------
p
2
ttd
T
1
T
2

=
L
p
20log
10
p
rms
p
0
----------
=
p
rms
p
0
10
L
p
20
=
L
p
10log
10
p
rms
2
p
0
2
----------
=
p
rms
2
p
0
2
10
L
p
10
=