D-20 824 Reference Manual 11/3/00
Sound Power Level (PWL, L
w
)
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 can-
not be measured directly, but can only be deduced 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 pro-
duces little or no Noise Dose accumulation and may be dis-
regarded. 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 Dura-
tion, that would expose a person to the same Noise Dose as
L
w
10log
10
W
W
0
-------
=
WW
0
10
L
w
10
⁄
=
c 20.05 degC 273+= msec
⁄
c 49.03 degF 460+=
ft sec
⁄
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