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Old 04-08-2020, 08:56 PM
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Quote:
Originally Posted by Alan Carruth View Post
pbla4024 asked:
"What do you call acoustic center then? "

At the risk of seeming to state a tautology, it's the best place to drive the top.

On a guitar it will generally be the spot on the top that gives the strongest 'tap tone'. Usually this will be the place where the 'main top' monopole resonance shows the highest amplitude when driven. Since that's the mode that probably produces most of the actual output of the instrument that makes some sense.

It's something of a self-fulfilling prophecy, though. On a completed instrument the bridge is there; a large mass concentration that's either going to be on or near a node line, or at the antinode of a mode. Even without the bridge in place the acoustic center is usually reasonably well defined, since the bracing is designed to provide the necessary stiffness at that point.

I'll note that it's not anything like a mathematical point: it's more like a low-Q resonant peak that's spread out a bit. It does seem to be more tightly defined on 'better' instruments.

I have weak math chops, which is one reason I tend to rely on experiments and verbal descriptions. This is also a complex system that is not well understood or defined in many respects, so that there are probably limits as to the utility of a mathematical description.
I have been reading and learning whenever Alan (and John) talk about guitar construction. I too am mathematically challenged (well at least getting through calculus and farther) but get physics concepts pretty easily. I never really thought of it as putting the bridge at the acoustic center, but then there is no imperative for the acoustic center to be at the center of the lower bout once you view the bracing as a part of the whole. And then you stick in the hole, which shifts the acoustic center back when the bracing was shifting it forward.


I think my head is starting to hurt.
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