Official Luthiers Forum!

Possibly. Al would know about that more than I do. The frequency that you get the ring+ at is the pitch that you hear when you hold the top up by the upper transverse brace and tap the top where the bridge is going to go. At least that's what I've noticed.

I've noticed that too but I don't know what to do to the braces or top to get lots of "nicely shaped, narrowly defined in pitch" patterns. Usually, the more well defined the ring+ pattern is, the more patterns of high quality that I see. Most of that comes from the top itself. The better the top, the easier it is to get a good ring+ and a bunch of other good patterns.
Here's all of the patterns that I got on that top:

Thanks for those references. And yes, the same Schroeder.

I did a little work with Schroeder diffusers for my master's thesis, actually pretty similar to the studies you are describing. We attempted to measure the scattering from individual Schroeder diffusers as well as more traditional diffusers, then compute hypothetical room impulse responses based on those measurements to simulate concert halls with the different diffusers on the wall. You can then simulate what a hall would sound like using music recorded in an anechoic room (with no reflections) and the measured scattering as essentially a filter on the recordings. I played those for other grad students and surveyed their impression. Although the Schroeder diffusers do scatter sound effectively (as seen in both the math and the measurements), they also color the sound with a certain harshness that was clearly a detractor in the listening tests.

I agree that listening tests (especially with "trained listeners") can be surprisingly effective and conclusive for relating subjective sound quality to objectively measured parameters. Of course my study was a bit further removed from reality, since I had to simulate the "instrument" (the hall), so I think the validity is somewhat questionable. But if the sound is coming from actual instruments, there is a wealth of information there.

Thanks, very nice to see those. Now, can you do some measurements and listening tests with the final instrument?

There is a sense in which the 'ring+' mode of the 'free' plate corresponds with the main monopole mode of the assembled top, but to say they are 'the same' is quite a stretch. It can't, for example, be conclusively demonstrated that a 'well formed' free plate ring+ mode has any effect on the frequency or Q value of the monopole mode of the assembled instrument. If there is an effect (and some of us think there is) it's more subtle.

Keep in mind that you have to mess up a guitar top really badly for it to not have a visible monopole mode in the assembled instrument. I can only think of one guitar in the hundreds I've tested that didn't have a monopole, and it was bad, but there were others that did show monopoles that were also bad. I would hope that most of us are to the point where we're not simply hoping to avoid making a really bad instrument, but rather looking for ways to make already good ones better.

Alas, the guitar is still in the finishing stage so I don't know what it will sound like yet. Once I string it up, I'm going to do and impulse excitation test. I'll try to remember to track down this thread and post the spectrum.