All of this Chladni stuff is a work in progress, and quite empirical at the moment. Obviously the way the top and back vibrate before you glue them down should have something to do with the way the guitar works when it's done. Just as obviously, there are so many things that are changed by the act of gluing those plates down that tracing out the cause and effect of it all is far from trivial. We're chipping away at it, slowly, but it's not as if this was Homeland Security: funding's a problem.
FWIW, my own experience suggests that it's the mode shapes that matter most. In particular, you'd like to see some of the 'ring' type modes 'closed'. On the top the one that seems to have the greatest effect is the 'ring-and-a-half' or 'ring+' mode; the one that has a (you hope) closed ring in the lower bout, and a curved line across the upper bout roughly along the shoulder brace. There actually seem to be two or more different modes that can become the 'ring+', deepending on how the bracing is done and so forth, but whichever one it is, getting it to 'close' seems to be the key.
By 'closed' I mean that the ring is a complete one, with no gaps. It may not be round, or even very smooth, although, again, smoother rings seem to work better. In some cases the ring in the lower bout will have one or more 'parenthesis' with it: you can see )O( or )O, with the parenthesis joined to the ring, and that's OK.
Once you get that, you can look higher in frequency to see if there are more modes with 'closed' rings. There will be 2-ring and 3-ring modes as you go up. The best guitars I've made, at least according to the folks that have tried them and heard them, tend to have a lot of these modes, and they are well formed (symmetric, and smooth), and very active (narrow node lines) with high Q values (narrow bandwidth).
Many moons ago Oliver Rogers, who just passed on, did a long computer modeling study of the 'free' plate modes of the violin that enabled him to publish 'mode influence surface maps'. These told you where to take off wood to alter specific mode frequencies. The problem for us is that guitar plates are much less standardized than violins; even small changes in shape, wood, or bracing patterns alter the way things work. The only way to get really good at it is to make a lot of one design, and then you're only really good at that one design.
Again, FWIW, the braces below the bridge on the top seem to have the greatest effect. You can reduce the lower tone bar to almost nothing at the end. On my tops making the finger braces too low tends to get me into trouble. Other than that, about all you can do is try something and see what happens.
In the beginning you may find that nothing happens, no matter what you try. That's probably because the braces are too tall: they so dominate the stiffness that the top can't influence the modes at all. Once you get to a better balance between top and brace stiffness things will start to happen; sometimes 'way too fast. Take a few shavings at a time, and be patient. Once you find something that works, do some more of that. When it stops working try something else.
Sometimes the guitar will be balanced on the head of a pin, so to speak; one or two shavings in the wrong place can really alter the tone. Again, take it easy.
|
