D-18 824 Reference Manual 11/3/00
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 stan-
dardized 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.
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 loga-
rithm is taken of the ratio of the actual sound pressure to a
reference sound pressure which is 20 MicroPascals (
µ
Pa).
There are various descriptors attached to this level depend-
ing 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
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
⁄
=
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