A wolf tone, or simply a "wolf", is a noise that is produced when a note played on a stringed instrument matches the natural resonating frequency of the instrument, producing a tone that is loud and harsh, and basically unwelcomed by most musicians.

Wolf tones are usually only noticed on bowed instruments, most notably the violin and surrounding 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 pitch E and F. The violin is a stringed musical instrument that has four strings tuned a perfect fifth apart. ...

A wolf can be 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 influencs or eliminates the frequency at which the wolf occurs. It is essentially an attenuator that cuts down on the odd reverberation. Recently, one metal worker has engineered the metal tube to be a clip on without the need for a screw or rubber sleeve. These new wolf eliminators also come in different weights depending on how strong or weak the affected wolf note actually is.