Larson Davis System 824, Reference Manual

Page: 511 / 536

11/3/00 D -7
Department of Defense Level
(L
DOD
)
The Average Sound Level calculated in accordance with
Department of Defense Exchange Rate and Threshold Level.
See Average Sound Level
Dose
(See Noise Dose)
Detector
The part of a sound level meter that converts the actual fluc-
tuating sound or vibration signal from the microphone to one
that indicates its amplitude. It first squares the signal, then
averages it in accordance with the time-weighting character-
istic, and then takes the square root. This results in an ampli-
tude described as rms (root-mean-square).
Eight Hour Time-Weighted
Average Sound Level (L
TWA(8)
)
It is the constant sound level that would expose a person to
the same Noise Dose as the actual (unsteady) sound levels.
The equation for it is
NOTE: This definition applies only for a Criterion Duration
of 8 hours.
Standard: ANSI S12.19
Energy Equivalent Sound Level
(L
eq
)
The level of a constant sound over a specific time period that
has the same sound energy as the actual (unsteady) sound
over the same period.
where p is the sound pressure and the Measurement Dura-
tion (specific time period) T=T
2
-T
1
. See Sound Exposure
Level.
Exchange Rate (Q), Exchange
Rate Factor (q), Exposure
Factor (k)
It is defined in ANSI S1.25 as “the change in sound level
corresponding to a doubling or halving of the duration of a
sound level while a constant percentage of criterion expo-
sure is maintained.” The rate and the factors are given in
L
TWA 8
( )
L
c
qLog
10
D
100
---------
 
 
+ =
L
eq
10Log
10
p
2
t
( )
t d
T
1
T
2
∫
p
o
2
T
--------------------------
=