Larson Davis SoundAdvisor 831C Reference Manual page 202

Department of Defense Level (LDOD)
Dose
Detector
Eight Hour Time-Weighted Average Sound Level (L TWA(8))
SoundAdvisor Model 831C
Table C.2 Sound Power Increase
Linear form, non-squared units
Ration of Value to
Reference
1
10
100
200
1,000
10,000
100,000
1000,000
The Average Sound Level calculated in accordance with Department of Defense Exchange Rate
and Threshold Level. See "Average Sound Level (Lavg)".
See "Noise Dose (D)".
The detector converts the actual fluctuating sound from the microphone into a signal that indi-
cates its amplitude. Your choice for the Detector setting (Slow, Fast, or Impulse) determines the
amplitude change rate. The detector first squares the sound signal, then averages it in accordance
with the time-weighting characteristic, and then takes the square root. This results in an ampli-
tude described as rms (root-mean-square).
The Slow and Fast detector responses were originally developed to slow down the movement of
the needle on an electro-mechanical meter, so that the amplitude could be determined. Slow and
Fast detector settings are still useful in modern sound measurement instruments. Slow has a time
constant of 1 second. Fast has a time constant of 1/8th second. The Impulse setting is a non-linear
detector with the rise controlled by a 35 ms time constant followed by a peak hold that has a
decay rate of 2.9 dB/second.
It is the constant sound level that would expose a person to the same Noise Dose as the actual
(unsteady) sound levels. According to the ANSI S12.19 standard, the equation for it is
NOTE: This definition applies only for a Criterion Duration of 8 hours.
Exponential Form
of Ratio
0
10
1
10
2
10
2.3
10
3
10
4
10
5
10
6
10
L = L + qLog
TWA 8  
c
Level form
20•Exponent
0
20
40
46
60
80
100
120
 
D
-------- -
 
10
100
C-5