When it comes to choice of wood to use in classical guitar construction there are two main schools of thought (three if you include flamenco guitars—perhaps I’ll do a separate post on that later). They both mandate the use of spruce or cedar for the soundboard, but apart from that they are almost opposites. Surprisingly they both make playable and excellent sounding guitars, thus demonstrating that, like life, guitar acoustics are more complicated than you might expect!
The traditional approach decrees that the back and sides of the guitar should be light—and therefore thin—and responsive. Rosewood (particularly Brazilian or Indian) fulfils this requirement admirably and has the further merit of looking good. It also has the disadvantage that it has become an endangered species. When you play a good guitar built in this way its whole body vibrates and contributes to the sound. The neck, by the way, should also be a dense hardwood, but not too dense. Mahogany (often the Cedrela variety) is a good choice.
But there’s another school of thought that starts from the principle that it’s not the job of the guitar body, apart from the soundboard itself, to vibrate. In doing so, it is argued, energy that could go into producing a louder sound is wasted in causing the whole of the guitar to vibrate. The soundboard is the component that makes the sound, so only the soundboard should vibrate.
Proponents of the second approach (pioneered by Greg Smallman) try to ensure that all the energy goes into vibrating the soundboard, which is therefore made as light as possible, with a thickness of around a millimeter and lattice bracing. The rest of the guitar is made as rigid as is practicable, by laminating the sides and back, and using a particularly dense wood such as maple or walnut for the neck. The back and sides typically have three layers glued together, sometimes keeping rosewood for the outer layer for the sake of appearances, wth perhaps ash for the middle layer and mahogany for the inner layer. Other woods are also used, either for their cosmetic properties or, experimentally, for their acoustics.