Acoustic Guitar Construction

Kevin in California wrote: ↑Tue Mar 03, 2026 11:54 pm It looks good Herman
I've always wondered if the back bracing is more about being part of a stable box and less about sound waves and such. The top vibrates....but does the back really vibrate or do the waves "bounce off" so to speak?

It can be either, It is a design choice. If a back is relatively low mass and set to a resonance close to the tops resonant frequency (I set the back 4 semitones above the top), then the back will acoustically couple to the top. This is referred to an active back. The end result is the frequency response of the guitar is more complex and the guitar has a more complex tone. I see it on a guitars spectrum graph, more peaks. I also hear it. It is a design choice because a active back steels some of the string energy to vibrate and the guitar is not quite as loud as it could be with a stiff reflective graph. I am not sure about bounce off but a stiff back does not move nor acoustically couple to the top. Does not steel string energy. Playing unamplified in a larger hall this may be the best choice has the guitar is louder and projects better than the same guitar built with an active back.

Even old Martin Guitars leaned toward an active back. That is why the lower back brace is not as tall has the others. Sure it is wider but stiffness changing proportionately to it width and to the cube with height.

With absolutely no proof - - - - yes. I think the comparison of reflectivity vs. resonance approaches the religious.

peter havriluk wrote: ↑Wed Mar 04, 2026 7:18 pm With absolutely no proof - - - - yes. I think the comparison of reflectivity vs. resonance approaches the religious.

How so, It is just a design choice and it can make a difference either way. I can both see it in a spectrum graph and hear it. I do both depending on the wood I have.