Spartan 821/721 Manual
Glossary
C-18
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
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 threshold level is applied in hearing damage risk assessment. According to the ANSI
S1.25 standard, it refers to the A-weighted sound level below which the dosimeter
produces little or no dose accumulation. For that reason, measured sound below this
level may be disregarded. The threshold should be selected to be within the
measurement range of the instrument which is between 70 dB and 140 dB for the
dosimeter. The OSHA Hearing Conservation threshold is 80 dB; the OSHA Permissible
Exposure Limit threshold is 90 dB.
Time Weighted Average Sound Level (TWA, LTWA(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 Leq.
Where TC=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. The following Detector
speeds settings are available in the 721 or 821:
Lw 10log10
W
W0
--------
=
W
W010
Lw 10
=
c
20.05 degC 273
+
=
m / sec
c
49.03 degF 460
+
=
ft / sec
L
TWA TC
Klog
10
1
T
--- 10
L
p
t
K
t
d
T
1
T
2
=
