There are numerous theories as to how and why and how a guitar produces its sound. In all types of guitars the sound is produced by the vibration of the strings. However, because the strings can only displace a small amount of air, the volume of the sound needs to be increased in order to be heard. In an acoustic guitar, this is accomplished by using a soundboard and a resonant cavity, the sound box. The body of the guitar is hollow. The vibrating strings drive the soundboard through the bridge, making it vibrate. The soundboard has a larger surface area and thus displaces a larger volume of air, producing a much louder sound than the strings alone. As the soundboard vibrates, sound waves are produced from both the front and back faces.
The sound box provides both a support for the sound board and a resonant cavity and reflector for the sound waves produced on the back face of the soundboard. The air in this cavity resonates with the vibrational modes of the string (see Helmholtz resonance), increasing the volume of the sound again. How you shape the soundboard braces, which play a major role in stabilising the top, will also have an effect on the sound. The back of the guitar will also vibrate to a lesser extent, driven by the air in the cavity. Some sound is ultimately projected through the sound hole (some variants of the acoustic guitar omit this hole, or have f holes, like a violin family instrument). This sound mixes with the sound produced by the front face of the soundboard. The resultant sound is a complex mixture ofharmonics that give the guitar its distinctive sound.