I have seen many types of bracing for guitar, for example:
How do they work? What do the lines mean? Will they affect the tone?
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The lines illustrate the bracing... Special made wooden braces glued on the top and bottom of the guitar, inside the body of the guitar.
Without bracing, the construction would not be strong enough for the tension of the strings and various forces/impacts from everyday useage.
The bracing has an effect on how the top and bottom of the guitar vibrate in responce to vibrations from the strings through the bridge and nut. It therefor has an effect on how the guitar distorts the sound from the strings through the natural amplification process of the body of the guitar.
The challenge is to construct the bracing in a way that strengthens the construction adequately and distorts the sound in a desired way.
The question of what is a desired distortion of the sound, is subject to individual opinion, and many guitar players prefer not to limit themselves to a single type of sound, and therefore may use different guitars with different bracing along with all the other charactaristics that can be chosen for a guitar.
The challenge of optimizing the bracing, with regard to the constructural strength of the guitar and the sound of the guitar, is more often tackled through trial and error, rather than computer modeling and other modern engineering methods.