Larson Davis System 824, Reference Manual

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11/3/00 D -9
in cycles per second and the value has the unit symbol Hz
(Hertz) added and the letter f is used for a universal descrip-
tor. It can also be expressed in radians per second, which has
no symbol, and the greek letter
ω
is used for a universal
descriptor. The two expressions are related through the
expression
ω
=2
Π
f.
Frequency Band Pass Filter
The part of certain sound level meters that divides the fre-
quency spectrum on the sound or vibration 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 fre-
quency, called a cutoff frequency. It is used to discriminate
against higher frequencies.
High Pass: A frequency filter that permits signals to pass
through that have frequencies above a certain fixed fre-
quency, called a cutoff frequency. It is used to discriminate
against lower frequencies.
Bandpass: A frequency filter that permits signals to pass
through that have frequencies above a certain fixed fre-
quency, called a lower cutoff frequency, and below a certain
fixed frequency, called an upper cutoff frequency. The differ-
ence between the two cutoff frequencies is called the band-
width. 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 sig-
nals to pass through that have a bandwidth based on octaves.
An octave is a doubling of frequency so the upper cutoff fre-
quency 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 sufficient number of them to cover
the usual range of frequencies encountered in sound and
vibration measurements.The frequency chosen to describe
the band is that of the center frequency. Note table in Fre-
quency Filter - Frequency Weighting.
Frequency Filter - Weighted
A special frequency filter that adjusts the amplitude of all
parts of the frequency spectrum of the sound or vibration