SoundAdvisor Model 831C
C-6
Energy Equivalent Sound Level (Leq)
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-13.
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 exposure is maintained.”
The rate and the factors are given in the table below.
Standard: ANSI S12.19
Exponential Integration
One of 2 integration methods used on 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
. Please note that when performing time history measurements of 10 ms or
less, you must use Linear integration.
You can locate this setting by selecting any menu
Setup Manager
General.
Far Field
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
distance. For example, if the lowest frequency is 20 Hz, the wavelength at normal temperatures is
near 56 ft. (17 m); at 1000 Hz, the wavelength is near 1.1 ft. (1/3 m).
for the
advantages of being in the acoustic far field.
Geometric Far Field
: The distance from a source of sound is greater than the largest dimension of
the sound source. In the far field, the effect of source geometry is negligible. Sound sources often
have a variety of specific sources within them, such as exhaust and intake noise. When in the far
field, the sources have all merged into one, so that measurements made even further away will be
for the advantages of being in the geometric far field.
L
eq
10
Log
10
p
2
t
t
d
T
1
T
2
p
o
2
T
--------------------------
=
Table C.3 Exchange Rate
Exchange Rate,
Q
Exchange Rate
Factor, q
Exposure Factor,
k
3.01
10
1
4
13.29
.75
5
16.61
.60
6.02
20
.50
