Model 820 Technical Reference Manual
C-7
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
=2¼f.
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
cut-
off 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
cut-
off 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 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 unlike band pass
filters. It can be composed of one or more of the following types:
A-Weighting
: A filter that adjusts the levels of a frequency spec-
trum in the same way the human ear does when exposed to low lev-
els of sound. This weighting is most often used for evaluation of
environmental sounds. See table below.