SoundAdvisor Model 831C
C-14
pressure which is 20 Micro-pascals (
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
Sound Pressure Level can be converted to sound pressure as follows. If the sound pressure is 1
Pascal, then the sound pressure level is
Calibrators often use a level of 94 dB so they generate a sound pressure of 1 Pascal.
If the sound pressure level = 76.3 dB, then the sound pressure is
Energy Average (L
eq
)
: The value of a steady sound measured over a fixed time period that has the
same sound energy as the actual time varying sound over the same period. This descriptor is
widely used. It is a fixed average. See
“Sound Pressure” on page C-13.
Impulse
: The value of an impulsive sound. The reading will depend on the time-weighting
applied.
Unweighted Peak
: The peak value of a sound with a meter that has flat frequency weighting and
a peak detector.
Weighted Peak
: The peak value of a sound with a meter that has a frequency weighting other
than flat and a peak detector.
Sound Power (W)
The sound power emitted by a sound source. It is measured in Watts.
Sound Power Level (PWL, Lw)
The logarithmic form of sound power. It is also expressed by attachment of the word decibel to
the number. The logarithm is taken of the ratio of the actual sound power to a reference sound
power, which is 1 pico-watt. Sound power level cannot be measured directly, but can only be
deduced through measurements of sound intensity or sound pressure around the source. The
equation for it is
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
=
L
p
20log
10
1
20
10
6
–
----------------------
20log
10
50000
20 4.699
94.0
dB
=
=
=
=
Pa
20
10
6
–
10
76.3 20
20
10
3.815
6
–
20
10
2.185
–
20 0.0065
0.13
=
=
=
=
=
L
w
10log
10
W
W
0
--------
=
W
W
0
10
L
w
10
=
