A wolf tone, or simply a "wolf", is produced when a played note matches the natural resonating frequency of the body of a musical instrument, producing a sustaining sympathetic artificial overtone that amplifies and expands the frequencies of the original note, frequently accompanied by an oscillating beating (due to the uneven frequencies between the natural note and artificial overtone) which may be likened to the howling of the animal. A similar phenomenon is the beating produced by a wolf interval, which is usually the interval between E♭ and G♯ of the various non-circulating temperaments.
Wolf tones are usually only noticed on bowed instruments, most notably the violin family, since the tones produced are played for much longer periods, and thus are easier to hear. Frequently, the wolf is present on or in between the pitches E and F♯ on the cello, and around G♯ on the double bass. A wolf can be reduced or eliminated with a piece of equipment called a wolf tone eliminator. This is a metal tube and mounting screw with an interior rubber sleeve, that fits around the offending string below the bridge. Different placements of this tube along the string influence or eliminate the frequency at which the wolf occurs. It is essentially an attenuator that slightly shifts the natural frequency of the string (and/or instrument body) cutting down on the reverberation.
An older device on cellos was a fifth string that could be tuned to the wolf frequency; fingering an octave above or below also attenuates the effect somewhat, as does the trick of squeezing with the knees. 
While it has been said[by whom?] that Lou Harrison wrote a piece (evidently reworked as the second movement of the Suite for Cello and Harp) that exploited the wolf specific to Seymour Barab's new cello, there is no clear evidence that this occurred.
- Dünnwald, H. (1979). "Versuche zur Entstehung des Wolfs bei Violininstrumenten". Acustica 41 (4): 238–45.
- Firth, Ian M. (1973). "The wolf in the cello". The Journal of the Acoustical Society of America 53 (2): 457. doi:10.1121/1.1913343.
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