Larson Davis SoundAdvisor 831C Reference Manual page 218

Time Weighted
Average Sound
Level (TWA,
LTWA(TC))
Time Weighting
Vibration
Wavelength (l)
Wavenumber (k)
SoundAdvisor Model 831C
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.
L
TWA TC
Where TC=T -T and K is the Exchange Rate Factor. It is used for hearing damage
2 1
risk assessment.
Standard: ANSI S12.19
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 measurements.
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 seconds (1500 ms) for the
decay. The reason for the double constant is to allow the very short signal to be
captured and displayed.
The oscillatory movement of a mechanical system (generally taken to be solid). It
is used as a broad descriptor of oscillations.
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.
A number that is related to the wavelength of sound and is used to compare the
size of objects relative to the wavelength or the time delay in sound propagation.
It is related to wavelength through the following equation
where l is the wavelength, c is the sound speed, f is the frequency in Hz, and w is
the radian frequency. It has the dimensions of inverse length.
T

1

-- - 10
= Klog

 
10
T

T
c

- -
=
f
2 2f
----- - ------- -
k = =

c

2
 t    K 
L


p
t d


1

--- -
=
c
C-16