11/3/00 D-21
the actual (unsteady) sound over the same period. If the
Exchange Rate is 3 dB then the TWA is equal to the L
eq
.
where T
c
=T
2
-T
1
and K is the Exchange Rate Factor. It is
used for hearing damage risk assessment.
Standard
: ANSI S12.19
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 mea-
surements.
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 sec-
onds (1500 ms) for the decay. The reason for the double con-
stant is to allow the very short signal to be captured and
displayed.
Vibration
The oscillatory movement of a mechanical system (gener-
ally 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 wave-
L
TWA TC
()
K
log
10
1
T
---
10
L
p
t
()()
K
⁄
td
T
2
∫
=
λ
c
f
--
=
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