Larson Davis SoundAdvisor 831C Reference Manual page 208

Exchange Rate (Q),
Exchange Rate
Factor (q),
Exposure Factor (k)
Far Field
Free Field
Frequency (Hz, rad/
sec)
SoundAdvisor Model 831C
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..
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
6.02
Standard: ANSI S12.19
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). See the definition of Acoustic Near Field 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
no different. See the definition of Geometric Near Field for the advantages of
being in the geometric far field.
A sound field that is free of reflections. This does not mean that the sound is all
coming from one direction as is often assumed, since the source of sound may be
spatially extensive. See the definitions of near and far fields for more detail. This
definition is often used in conjunction with reverberant field.
The rate at which an oscillating signal completes a complete cycle by returning to
the original value. It can be expressed in cycles per second and the value has the

------------------------- -
L = 10Log
eq 10
Exchange Rate
Factor, q
10
4 13.29
5 16.61
20
T
2
t   t d
2
p
T
1
2
p T
o
Exposure Factor, k
1
.75
.60
.50
C-6