Thoughts on wolf note?

[Alan Carruth]

Changing the body volume is unlikely to alter the 'air' pitch much. Fred Dickens built a 6" deep guitar, and curt it down 1" at a time, re-using the back each time, until he got it as shallow as he dared (something like 2", iirc) and the 'air' pitch only rise by 7%. As always with the guitar, it's not simple.

Before I went to shaving back braces I'd re-do the loading experiment, but this time get some numbers on the resonant pitches as you go. If the back is really beefy you may not have altered the resonances all that much with the added mass, and, as I say, I'm skeptical that making the back looser will help. It's better not to do anything that's hard to undo until you're pretty sure it will work.

If you do decide to shave back brace, removing material in the center of the brace is likely to be more productive than the ends. The brace bends more in the middle. You can conform this with added mass, which will be more effective in the center.

There is always the chance that we've come to a wrong diagnosis. If that's the case it would explain why the usual solutions are not working. Some years ago a guitar I'd built developed a buzz at D on the B string, but only on that string. He'd toured with it for some years, so maybe wear and tear had something to do with it. We spent the better part of a day trying to track down the problem, starting with high frets, of course, with no luck. Re-tuning the string gave the same problem on a different fret; it was pitch linked, and that suggested a 'wolf'. Finally we recorded the played note, and ran a spectrum analysis. It turned out that the 3d, 6th and 9th partials showed 'split' peaks; the string was actually vibrating at two closely spaced frequencies at the same time on the third partial, at about 880 Hz. The bridge was moving up and down enough at that pitch and at the B string position that the string 'thought' it was a different length in the 'vertical' polarity than it was in the 'horizontal' one. Since plucking it gives some of both, it made both pitches, and the buzz was the difference frequency.

Driving the top (hard!) at 880 gave a clear Chladni pattern, with B string location moving a lot. A couple of shavings off the upper end of the upper tone bar were enough to move the node line closer to the B string, and changed the pitch enough to eliminate the problem.

The worst guitar wolf I ever saw happened because the to and back pitches were too close. Again, it came across like fret buzz, but, as near as I can figure it, was actually the back alternately 'storing' and 'releasing' energy at it's own pitch, which was about 7Hz higher than the top pitch. Lowering the top pitch by another 4 Hz by adding some mass at the bridge cleared it up. That was another half day of testing and head scratching to find the issue, and ten minutes to glue a small weight inside to fix it.