D-22 824 Reference Manual 9/29/09
through measurements of sound intensity or sound pressure
around the source. The equation for it is
Sound Speed, (c,) The speed at which sound waves propagate. It is measured
in meters per second. It should not be confused with sound
or particle velocity which relates to the physical motion of
the medium itself.
Spectrum (Frequency
Spectrum)
The amplitude of sound or vibration at various frequencies.
It is given by a set of numbers that describe the amplitude at
each frequency or band of frequencies. It is often prefixed
with a descriptor that identifies it such as sound pressure
spectrum. It is generally expressed as a spectrum level.
Threshold Sound Level (Lt) The A-weighted sound level below which the sound
produces little or no Noise Dose accumulation and may be
disregarded. It is used for hearing damage risk assessment.
Standard: ANSI S1.25
Time Weighted Average Sound
Level (TWA, L
TWA(TC)
)
It is the level of a constant sound over the Criterion
Duration, that would expose a person to the same Noise
Dose as 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
L
w
10log
10
W
W
0
-------
=
WW
0
10
L
w
10
=
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