Moog MIDI MuRF MF-105M Usage Instructions page 6

In general, bright sounds have lots of strong overtones, while darker,
mellower sounds have fewer (and weaker) overtones.
A filter is a signal-modifying device that colors a sound by
emphasizing some parts of the audio spectrum and attenuating (cutting
down) other parts. In general, a filter has a quality of its own which is
superimposed on the tone color of the original sound. Some types of
filters (like the bass and treble controls on your sound system) have
subtle, gentle effects on a sound's timbre. Other types of filters have
stronger and more dramatic effects, and are frequently used as elements
in the music-making and sound design process. Strong filters include
phasers, flangers, and wah-type resonant filters.
A graph showing what a filter does is called the filter's frequency
response. The horizontal axis is frequency. The vertical axis is the filter's
gain. A gain of "1" (unity) means that, at that frequency, the output of the
filter is just as strong as the input. A gain of less than unity means that the
filter's output is attenuated at that frequency, while a gain of greater than
unity means that the output is actually greater than the input.
Figure 4 shows examples of the
frequency response characteristics
of two common types of filters:
(a) a lowpass filter, which passes
frequencies without attenuation up
to a so-called 'cutoff frequency',
and attenuates the frequencies
above cutoff; (b) a resonant filter,
which emphasizes frequencies
around the filter's 'center
frequency'.
Both of these filter types are
widely used in electronic music.
Each of them has its own distinct
sound, a large part of which is
directly related to the shape of its frequency response graph. The first
type is the same as in the moogerfooger MF-101 lowpass filter, as well as
the lowest filter in the MIDI MuRF's BASS voicing; the second type is
in the top 7 bands of the MIDI MuRF's BASS voicing and all 8 bands of
the MIDS voicing.
Figure 4 - Filter Frequency Response
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