The wolf tone arises from impedance-matched bridge-body resonance feedback

Evidence

The impedance of the bridge is normally about 10 times higher than the strings, leading to reflection of 67% of the energy of a pulse initially traveling along the string toward the bridge, as can be easily shown from formula 1.3. The bridge is, of course, firmly affixed to the body, and if the body has a strong resonance coinciding with a low string frequency, one that involves significant rocking of the bridge, the bridge impedance as seen by the string can be drastically lowered, to something much closer to the string impedance. This upsets the usual hierarchy that otherwise makes the bowed string a rather noninteractive drive, as seen by bridge and body. (Of course, the string is very interactive with the bow.) The lowered impedance at the bridge causes a fast dumping of string vibration energy into the body, which can collect enough to send it back to the string through the bridge, in close analogy to resonantly coupled pendula (see section 10.7 and figure 10.6). The wobbly tone that defines the wolf is the result. — Eric J. Heller, Why You Hear What You Hear- An…, 1799

Categorization

• XREF: Connects to the pendula resonance coupling analogy from section 10.7 and to impedance mismatch physics used throughout the book.