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Flanging // is an audio effect produced by mixing two identical signals together, with one signal delayed by a small and gradually changing period, usually smaller than 20 milliseconds. 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.
A short sample followed by two flanging versions.
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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. Note, however, that the Beatles' producer George Martin has disputed this "reel flange" source of the name and said it was not so, instead attributing the term to John Lennon (see Martin's account below).
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. This was one of the problems faced by studio engineers in the sixties and seventies when recording large concept pieces, as explained by Ian Anderson of Jethro Tull when recounting the studio challenges of recording Thick as a Brick.
The development of the classic "flanging" effect is generally attributed to Ken Townsend, an engineer at EMI's Abbey Road Studio, who devised the process in the spring of 1966. Tired of the laborious process of re-recording dual vocal tracks, John Lennon asked Townsend if there was some way for the Beatles to get the sound of double-tracked vocals without actually doing the work. After mulling the problem over, Townsend devised Artificial Double Tracking or "ADT." According to historian Mark Lewisohn, it was Lennon who actually gave the process the name "flanging". Lennon asked Beatles producer George Martin to explain how ADT worked, and Martin answered with the nonsense explanation "Now listen, it's very simple. We take the original image and we split it through a double vibrocated sploshing flange with double negative feedback". Lennon thought Martin was pulling his leg. Martin replied, "Well, let's flange it again and see." From that point on, whenever Lennon wanted ADT he would ask for his voice to be flanged, or call out for "Ken's flanger". According to Lewisohn, "The Beatles' influence was so vast that the term "flanging" is still in use today, more than 20 years on." The first Beatles track to feature flanging was "Tomorrow Never Knows" from Revolver, which was recorded on April 6, 1966. When Revolver was completed and released on August 5, 1966, almost every song on the album had been subjected to flanging.
Others have attributed it to George Chkiantz, an engineer employed at Olympic Studios in Barnes, London. One of the first instances of the sound being used on a commercial pop recording was The Small Faces' 1967 single "Itchycoo Park", recorded at Olympic and engineered by Chkiantz's colleague Glyn Johns.
listen to the tape flanging effect on the 1967 single.
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American music industry veterans David S. Gold and Stan Ross, founders of the renowned Gold Star Studios in Hollywood, claim that they made the first commercial recording to feature the technique - the single "The Big Hurt" by Toni Fisher, which was recorded at Gold Star in late 1959 and became a hit in the U.S. in early 1960, rising to #3 on Billboard magazine's singles chart. Flanging is also heard in the opening of The Ventures' 1962 cover version of The Tornados hit "Telstar", in the context of a simulated rocket launch sound effect.
The first use of the flanging effect in stereo is credited to producer Eddie Kramer, who used the effect in the coda of Jimi Hendrix's "Bold as Love" (1967). Kramer admitted in an 1990s interview that he read BBC Radiophonic Workshop technical journals for ideas and circuit diagrams.
In 1968, the record producer for The Litter, Warren Kendrick, devised a method to precisely control the flanging effect by placing two 15 IPS (inches per second) stereo Ampex tape recorders side-by-side. The take-up reel of recorder A and supply reel of recorder B were disabled, as were channel 2 of recorder A, channel 1 of recorder B and the erase head of recorder B. The tape was fed left-to-right across both recorders and an identical signal was recorded on each channel of the tape, but displaced by approximately 18 inches along the length of the tape. During the recording, a screwdriver was wedged between the tape recorders to make the tape run "uphill" and "downhill." The same configuration was employed during the playback/mixdown to a third recorder. The screwdriver was moved back and forth to cause the two signals to diverge, then converge. The latter technique permits zero point flanging; i.e., the lagging signal crosses over the leading signal and the signals change places.
A similar "jet plane-like" sound effect can also occur naturally in long distance shortwave radio music broadcasts. In this case the varying delays are caused by varying radio wave propagation times and multipath radio interference.
Artificial flanging 
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 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.
"Barber pole" flanging 
Also known as "infinite flanging", this sonic illusion is similar to the Shepard tone effect, and is equivalent to an auditory "barber pole". The sweep of the flanged sound seems to move in only one direction ("up" or "down") infinitely, instead of sweeping back-and-forth. While Shepard tones are created by generating a cascade of tones, fading in and out while sweeping the pitch either up or down, barber pole flanging uses a cascade of multiple delay lines, fading each one into the mix and fading it out as it sweeps to the delay time limit. The effect is available on various hardware and software effect systems.
See also 
- Martin, George; Pearson, William. Summer of Love: The Making of Sgt Pepper. London: Pan Books. p. 82. ISBN 0-330-34210-X.
- Lewisohn, Mark. The Beatles: Recording Sessions (New York: Harmony Books, 1988), page 70.
- Hodgson, Jay (2010). Understanding Records, p.142. ISBN 978-1-4411-5607-5. Interludes between chorus and verse at 0:50-1:07, 1:40-2:05, and 2:20-2:46.
- Diagram by Warren Kendrick - 'K-Tel Reissue CD 10002 (1991)'
- "The Sonic Barber Pole: Shepard's Scale". at cycleback.com.
- Budde, Christian. Barberpole Flanger at kvraudio.com]
- "audio software by oli larkin". Olilarkin.co.uk. Retrieved 2012-08-13.