Principles of Flanger
Flanging is an audio effect produced by mixing two identical audio signals together, with one signal delayed by a small and gradually changing period, usually smaller than 20 milliseconds (kept below the threshold of echo perception). This produces a swept comb filter effect: peaks and notches are produced in the resultant frequency spectrum, related to each other in a linear harmonic series. Varying the time delay causes these to sweep up and down the frequency spectrum. A flanger is an effects unit dedicated to creating this sound effect.
Part of the output signal is usually fed back to the input (a "re-circulating delay line"), producing a resonance effect which further enhances the intensity of the peaks and troughs. The phase of the fed-back signal is sometimes inverted, producing another variation on the flanging sound.
The flanging effect has been described as a kind of whoosh passing subtly through the sound. The effect is also compared to the sound of a jet passing overhead, in which the direct signal and ground reflection arrive at a varying relative delay. If flanging is done rapidly enough, an audible Doppler shift is introduced which approximates the Leslie effect commonly used for organs.
The name "flanging" comes from the original method of creation. Originally, a signal would be recorded to two tape machines simultaneously. The playback-head output from these two recorders was then mixed together onto a third recorder. In this form, minute differences in the motor speeds of each machine would result in a phasing effect when the signals were combined. The "flange" effect originated when an engineer would literally put a finger on the flange, or rim of one of the tape reels so that the machine was slowed down, slipping out of sync. by tiny degrees. A listener would hear a "drainpipe" sweeping effect as shifting sum-and-difference harmonics were created. When the operator removed his finger the tape sped up again, making the effect sweep back in the other direction.
Alternatively, the track could be recorded to two matching tape decks first, then replayed simultaneously with both decks closely in sync. With this method, slowing down one deck by pressing the tape reel flange would "sweep" the flange effect in one direction, but when released the playback of that deck would remain slightly behind the other, and the effect would not sweep back. Instead, pressing the flange of the other deck would sweep the effect back in the other direction as the tape position of the decks move toward being in sync again.
Older recording hardware could suffer from flanging as an undesired side effect when recording very long tracks. As the weight of the tape built up on one reel, the pressure on the capstans could cause flanging during mixdown or dubbing.
In the 1970s, advances in solid-state electronics made the flanging effect possible using integrated circuit technology. Solid-state flanging devices fall into two categories: analog and digital. One of the most famous flanger pedals is the Electro Harmonix Electric Mistress. The flanging effect in most newer digital flangers relies on Digital Signal Processing (DSP) technology. Flanging can also be accomplished using computer software.
Note that the original tape-flanging effect sounds a little different from the later electronic and software re-creations. Not only is the tape-flanging signal time-delayed, but the response characteristics at different frequencies of the magnetic tape and tape heads inevitably introduced some phase shifts into the signals as well. Thus, while the peaks and troughs of the comb filter are more or less in a linear harmonic series, there is a significant amount of non-linear behaviour too, causing the timbre of tape-flanging to sound more like a combination of what came to be known as flanging and phasing.