Flanging is an audio process that combines two copies of the same signal , with the second delayed slightly, to produce a swirling effect. The process originated before digital effect boxes and computer editing were available. The effect, invented in the early 1950s by Les Paul and later used by artists such as Jimi Hendrix and The Beatles, was originally created using two tape recorders.
Here's how the nondigital process worked: While the original sound was being played from Tape Recorder #1, a second copy of the same audio material was played back from Tape Recorder #2. This process alone creates a hollow sound caused by the slight irregularities in the phase relationship of the audio waveforms. To get the flanging effect, the speed of the second recording was altered slightly. This was done most often by pressing a finger lightly on the tape reel's "flange", the large metal circle that surrounds and contains the tape on its hub. This created a time delay in addition to the phase differences, making the effect more pronounced.
Today, digital simulations of the process have replaced the flanging effect that was created using reel-to-reel tape recorders. The basic concept remains the same. The software or hardware device delays a copy of the source audio, but instead uses a low frequency oscillator (LFO) to vary the speed of the copy's playback. (The oscillator moves in the range of 1-20 cycles per seconds to get the effect.) Feeding the processed signal back into the device to be processed again can get a more intense effect.
Guitarist Les Paul invented flanging. He and Mary Ford first made it popular in the early 1950s. Les Paul also invented the solid body electric guitar and many sound techiques in use today. The first completely digital electronic flanging unit was the Delta Lab Research CompuEffectron, introduced in the 1970s.