Larson Davis SoundAdvisor 831C Reference Manual page 250

Sound Pressure Level (SPL, Lp)
SoundAdvisor Model 831C
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 ( 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
L p
where p is the reference sound pressure of 20 µPa. The square of the sound pressure is a power-
0
like quantity that can be expressed in the original form of the level definition
L p
Sound Pressure Level can be converted to sound pressure as follows. If the sound pressure is 1
Pascal, then the sound pressure level is
L = 20log
p 10
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

Pa = 20 10
Energy Average (L ): The value of a steady sound measured over a fixed time period that has the
eq
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-14.
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.
1
----------------------- p
p =
–

rms
T T
2
p rms
------------
= 20log 10
p 0
2
p rms
------------
= 10log 10
2
p 0
1

--------------------- -
= 20log
6 –
10

20 10
6 – 
76.3 20 3.815 6
 
10 = 20 10
T
2

2
t   t d

1
T
1

L p 20
p rms = p 0 10

L p 10
2 2
p rms = p 0 10
  
50000 = 20 4.699
– –
2.185
 
= 20 10 = 20 0.0065
= 94.0dB

= 0.13
C-15