Moog Music MF-101 Lowpass Filter - Manual (Page 4)

HOW A LOWPASS FILTER WORKS
Let's start with some definitions, so we're all talking the same language.
Sound is a vibration of the air. The speed of vibration is called the frequency. It is
measured in Hertz (Hz). One Hz is one vibration per second. We hear vibrations from 20
Hz to 20,000 Hz.
Musical sounds generally have many frequency components. They're called
harmonics, or overtones, or partials. They are what give the sound its characteristic tone
color, or timbre.
This “spectrum” graph shows the strengths of the harmonics in a sound.
A filter allows some frequencies to pass and cuts off other frequencies. A Lowpass Filter
is a kind of filter that allows all frequencies up to a certain frequency to pass, and cuts
off frequencies above that. That frequency is called the cutoff frequency of the filter.
We'll call it "the cutoff" for short.
A lowpass filter removes the high frequencies from a tone. It makes the tone more
mellow or muted. The lower the cutoff, the more muted the tone sounds. Imagine a
window shade. As it is pulled down, it cuts out the higher light, then the light from the
middle of the window, then finally all the light. A lowpass filter does the same sort of
thing with the sound spectrum.
This “frequency response” graph shows that the gain of a lowpass filter drops off at frequencies above the
cutoff frequency
.
Your MF-101 contains a genuine classic four-pole lowpass filter. This kind of filter was
first patented by Bob Moog in 1968. It is a big part of the 'Moog Sound' of the '60's and
'70's. The effect has been imitated, but never equaled, by many instrument
manufacturers.