Larson Davis System 824, Technical Reference Manual

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9/29/09 D -23
Time Weighting The response speed of the detector in a sound level meter.
There are several speeds used.
Slow : The time constant is 1 second (1000 ms). This is the
slowest and is commonly used in environmental noise
measurements.
Fast : The time constant is 1/8 second (125 ms). This is a less
commonly used weighting but will detect changes in sound
level more rapidly.
Impulse : The time constant is 35ms for the rise and 1.5
seconds (1500 ms) for the decay. The reason for the double
constant is to allow the very short signal to be captured and
displayed.
Vibration The oscillatory movement of a mechanical system
(generally taken to be solid). It is used as a broad descriptor
of oscillations.
Wavelength (l) The distance between peaks of a propagating wave with a
well defined frequency. It is related to the frequency through
the following equation
where c is the sound speed and f is the frequency in Hz. It
has the dimensions of length.
Wavenumber (k) A number that is related to the wavelength of sound and is
used to compare the size of objects relative to the
L
TWA TC
( )
K log
10
1
T
--- 10
L
p
t
( ) ( )
K
⁄
t d
T
1
T
2
∫
⎝ ⎠
⎜ ⎟
⎜ ⎟
⎛ ⎞
=
λ
c
f
--
=