Model 820 Technical Reference Manual
C-17
Weighted Peak:
The peak value of a sound with a meter that has a
frequency weighting other than flat and a peak detector.
Sound Power(W)
The sound power emitted by a sound source. It is measured in
Watts.
Sound Power Level (PWL, L
w
)
The logarithmic form of sound power. It is also expressed by attach-
ment 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 cannot 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 fre-
quency 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
L
w
10log
10
W
W
0
-------
=
W
W
0
10
L
w
10
⁄
=
c
20.05 degC 273
+
=
m sec
⁄
c
49.03 degF 460
+
=
ft sec
⁄
