What is flanging and chorusing?

Flanging and chorusing are rather simple audio effects based on short delays.

Basically, the effects are produced by adding a signal to a delayed version of itself, and varying the delay. The effect is essentially just a sweeping (due to the varying delay) comb filter created by the phase cancellations from summing the delayed and non-delayed signals.

The delays required for a good flanging effect range from virtually nil on one end of the sweep up to about 20 msec at the high end of the sweep.

Chorusing is based on a psychoacoustic effect wherein the ear perceives an exact copy of a signal that is delayed by about 20 to 60 msec to be a second source of sound. Shorter than 20 msec and you get too much phase cancellation and comb filtering (i.e. flanging) and longer than 50 or 60 msec is perceived as an echo rather than a "doubled" signal. When correctly implemented, the effect is to add an apparent extra source of sound or 'chorus'. A commercial chorus device uses several delay taps and typically modulates their length and/or amplitude slightly for a more convincing chorus effect.

The term 'flanging' arises from the way the effect was first produced. The story is that the effect was discovered by accident back in the days before multi-track recording when "overdubbing" actually involved dubbing a previously-made recording (typically 2-track) to another 2-track tape recorder while adding in a new instrumental part or vocal line. An engineer was doing an overdub to two different recorders with a slightly different mix of tape playback to new performance. He was monitoring both tape machines to hear what was actually going to the tape, and his assistant engineer happened to bump the reel on one machine thus causing it to slow down slightly. The change in speed meant that the tape on that machine took slightly longer to travel from the record head to the replay head which introduced a differential delay between the two tape machines. Since the engineer was listening to both machines, when the differential delay appeared he heard a comb filter caused by the phase cancellation, and the notches of the comb shifted in frequency as the differential delay changed. The engineer liked the sound and took the time to experiment on how to reproduce it repeatably. The method he devised for controlling the speed differential (and hence the length of the differential delay) was to drag his thumb on one flange of the tape reel, hence the name 'flanging'