According to Wikipedia, "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."
To more fully understand the practical presence of the "howling wolf", and how to reduce its effect, read this article from Strings Magazine. The article focuses on the cello wolf tone, but it applies to all stringed instruments.
Cello wolf tone:
Bass wolf tone:
Here is a video of a certain brand of wolf eliminator, and how it comes to control the vibration.
Not every stringed instrument will produce wolf tones; it depends on the type/cut of the wood and the construction of the instrument.
In summary, wolf tones happen when the resonating frequency of the note being played matches the natural frequency of the wood on the instrument itself.
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