Larson Davis SoundAdvisor 831C Reference Manual page 242

Free Field
Frequency (Hz, rad/sec)
Frequency Band Pass Filter
Frequency Weighting Filter
SoundAdvisor Model 831C
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). "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. "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 defini-
tions of near and far fields for more detail. This definition is often used in conjunction with rever-
berant 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 unit symbol Hz (Hertz) added
and the letter f is used for a universal descriptor. It can also be expressed in radians per second,
which has no symbol, and the Greek letter w is used for a universal descriptor. The two expres-
sions are related through the expression =2Pf.
The part of certain sound level meters that divides the frequency spectrum on the sound or vibra-
tion into a part that is unchanged and a part that is filtered out. It can be composed of one or
more of the following types:
Low Pass: A frequency filter that permits signals to pass through that have frequencies below a
certain fixed frequency, called a cutoff frequency. It is used to discriminate against higher fre-
quencies.
High Pass: A frequency filter that permits signals to pass through that have frequencies above a
certain fixed frequency, called a cutoff frequency. It is used to discriminate against lower frequen-
cies.
Bandpass: A frequency filter that permits signals to pass through that have frequencies above a
certain fixed frequency, called a lower cutoff frequency, and below a certain fixed frequency,
called an upper cutoff frequency. The difference between the two cutoff frequencies is called the
bandwidth. It is used to discriminate against both lower and higher frequencies so it passes only a
band of frequencies.
Octave band: A bandpass frequency filter that permits signals to pass through that have a band-
width based on octaves. An octave is a doubling of frequency so the upper cutoff frequency is
twice the lower cutoff frequency. This filter is often further subdivided in 1/3 and 1/12 octaves (3
and 12 bands per octave) for finer frequency resolution. Instruments with these filters have a suf-
ficient number of them to cover the usual range of frequencies encountered in sound and vibra-
tion measurements. The frequency chosen to describe the band is the center frequency. For more
detail, see Table C.4 in Frequency Weighting Filter.
The frequency filter used to adjust the amplitude of the frequency spectrum of the sound or vibra-
tion to account for the relative loudness perceived by the human ear.
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