Larson Davis SoundAdvisor 831C Reference Manual page 204

Free Field
Frequency (Hz, rad/sec)
Frequency Band Pass Filter
Frequency Filter - Weighted
SoundAdvisor Model 831C
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 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.
C-7