Larson Davis System 824, Техническое справочное руководство

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D-10 824 Reference Manual 9/29/09
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
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
frequency 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
frequency, 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
frequency, 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
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 bandwidth 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 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 Frequency 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