Larson Davis SoundAdvisor 831C Reference Manual page 209

Frequency Band
Pass Filter
Frequency Filter -
Weighted
SoundAdvisor Model 831C
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 expressions are related
through the expression
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.
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 spectrum in the same
way the human ear does when exposed to low levels of sound. This weighting is
most often used for evaluation of environmental sounds. See table below.
B-Weighting: A filter that adjusts the levels of a frequency spectrum in the same
way the human ear does when exposed to higher levels of sound. This weighting
is seldom used. See table below.
C-Weighting: A filter that adjusts the levels of a frequency spectrum in the same
way the human ear does when exposed to high levels of sound. This weighting is
most often used for evaluation of equipment sounds. See table below.
Flat-Weighting: A filter that does not adjust the levels of a frequency spectrum. It
is sometimes an alternative selection for the frequency-weighting selection.
Z-Weighting: Similar to a flat-weighting curve, this is a bandpass filter with a
passband from 10 Hz to 20 kHz.

=2Pf.
C-7