Moog Music MF-103 12-Stage Phaser - Manual (Page 5)

Figure 3 - Typical spectrum of a musical sound.
A filter, or equalizer, 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 or equalizer 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 vital elements in the music-making 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 three
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’, and (c) a six-stage phaser, which has three dips
Figure 4 - Frequency responses of typical filters: From top, Lowpass Filter; Resonant Filter; and 6-Stage Phaser.