Larson Davis Spartan 821, Справочное руководство

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Spartan 821/721 Manual Glossary C-6
Energy Equivalent Sound Level (L
eq
)
See Equivalent Continuous Sound Level (L
eq
)
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
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). To learn more about the advantages of being in
the acoustic far field, see Acoustic Near 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. To learn more about
the advantages of being in the geometric far field, see Geometric Near Field: .
Free Field
A sound field that is free of reflections. This does not necessarily mean that the sound is
all coming from one direction, as is often assumed. 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.
Frequency (Hz, rad/sec)
The rate at which an oscillating signal accomplishes a complete cycle by returning to the
original value. It can be expressed in cycles per second, which uses the unit symbol “Hz”
(Hertz), or in radians per second, which has no unit symbol. The Greek letter “
ω
” and the
letter “f” are used for universal descriptors. The two expressions are related through the
equation

=2
π
f.
TABLE C.1 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