MX

234 Audio effects
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This effect is a result of the speed of the sound which first increases and then
decreases, thus also changing the sound pitch. The wave length also changes
relative to the frequency. If there were a second siren at your location, an
oscillation would develop between both sounds (just like when two instruments
are out of tune).
Chorus also splits the signal into at least two components: a direct sound and
an effects portion, and there are multiple effects components in CORVEX.
The Doppler effect is created by a short signal delay from the effect. For most
equipment, this delay is within the range of 10-30 ms (as in this one), this
means that it is short enough to be perceived as an "echo". The times would
also be similarly short if you duplicated a guitar track, for instance. A short
delay in the mix already sounds "duplicated", but this is not authentic. This is
where the above-mentioned "out-of-tune" effect comes in: the pitch of the
effect signal is slightly modulated by gently "drifting" forward and backward in
the delay curve. The result is a floating effect where the speed is influenced by
drifting.
The "Flange" effect is similar to that of the chorus, but does have a different
technical and historical background. It resulted by chance: someone (various
sources say John Lennon) slowed down one of two running interconnected
tape machines in a studio with his hand. The result was a rather brief delay of
the second signal compared to the first, brought about cancellations within the
frequency spectrum, leading to a so-called comb filter effect (the sum of both
signals creates "peaks" and "dips" in the spectrum that look familiar to the
teeth of a comb).
Flanging is basically a chorus effect with a low delay time (less than 10 ms).
"Release" or doubling of signals is not the main focus in this case; more
creative deformation of the frequency response is what we are interested in.
A complete flange effect will definitely require feedback: the flange portion is
returned to the input to increase the effect. People often refer to this as the "jet
effect" since it resembles a jet on take-off.