Flanger

A comb filter with a fixed delay gives a fixed set of peaks and notches. A flanger moves the delay, usually between about 1 and 10 milliseconds, with a slow LFO. As the delay shortens, the notches spread apart and move up the spectrum; as it lengthens, they crowd together and move down. On any sound with energy across the spectrum, such as cymbals, noise or a full chord, the moving notches are heard as a sweep. The effect was first made with tape: two machines played the same recording, and pressing on the rim, or flange, of one reel slowed it so that its copy fell a few milliseconds behind.

The tool below flanges a drum loop or a chord. The plot shows where the first few notches are as the delay sweeps, over the last four seconds.

Notches that move

With the delayed copy added to the original, a notch falls at each frequency for which the delay is an odd number of half-cycles, because at those frequencies the copy arrives inverted and cancels the original. The first notch is at one divided by twice the delay, and the rest are at odd multiples of it. A 1 millisecond delay puts the first notch at 500 Hz and the next at 1500 Hz; a 7 millisecond delay puts them at about 71 Hz and 214 Hz, with many more crowded above. Sweeping the delay between those two values moves the whole set of notches up and down together, and that movement is heard as the sweep. Because the notch positions depend on one divided by the delay, the sweep moves fastest at the short end, where a small change in delay moves the notches a long way.

Feedback and through-zero

Feedback sends part of the output back into the delay, so the peaks between the notches grow taller and narrower and the flanger begins to ring at a frequency of one divided by the delay. That ringing, sweeping up and down, is the jet sound. With negative feedback the copy is inverted as well, which swaps the positions of the peaks and the notches. With tape, the delayed machine could be brought back level with the other and past it, so the delay went through zero, which a single delay line cannot do because its delay cannot be less than zero. As the delay approaches zero the first notch rises above the range of hearing, and at zero delay the two copies are identical and the sound is unfiltered for that moment. The notches then return from the top of the range as the delay grows on the other side. Some flangers imitate this by delaying the original slightly as well.

Flanging, chorus and phasing all mix a signal with a moving altered copy. They differ in how the copy is made: chorus uses a delay of around 15 to 30 milliseconds, flanging uses a delay of about 1 to 10 milliseconds, and phasing uses a chain of allpass filters.