Larson Davis SoundAdvisor 831C Reference Manual page 241

Eight Hour Time-Weighted Average Sound Level (L TWA(8))
Energy Equivalent Sound Level (Leq)
Exchange Rate (Q), Exchange Rate Factor (q), Exposure Factor (k)
Exponential Integration
Far Field
SoundAdvisor Model 831C
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.
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 Duration (specific time period) T=T2-T1. See
"Sound Pressure Level (SPL, Lp)" on page C-15.
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 exposure is maintained."
The rate and the factors are given in the table below.
Table C.3 Exchange Rate
Exchange Rate,
Q
3.01
4
5
6.02
Standard: ANSI S12.19
The alternative to "Linear Integration" for the 831C SLM. Exponential integration shows a long
decay of energy after an impulse. However, it may hide small events or loud impulsive events. For
this reason, this method is not as commonly used as "Linear Integration". Please note that when
performing time history measurements of 10 seconds or less, you must use Linear integration.
There are two types of far fields: the acoustic far field and the geometric far field.
Acoustic Far Field: The distance from a source of sound is greater than an acoustic wavelength. In
the far field, the effect of the type of sound source is negligible. Since the wavelength varies with
frequency (See the definition of Wavelength), the distance will vary with frequency. To be in the
far field for all frequencies measured, the lowest frequency should be chosen for determining the
L = L + qLog
TWA 8  
c

------------------------- -
L = 10Log
eq 10
Exchange Rate
Factor, q
10
13.29
16.61
20
 
D
-------- -
 
10
100
T
2
t   t d
2
p
T
1
2
p T
o
Exposure Factor,
k
1
.75
.60
.50
C-6