The sound is a complex mixture of harmonics that give the guitar its distinctive sound. And the air in this cavity resonates with the vibrational modes of the string and at low frequencies, which depend on the size of the box, the chamber acts like a Helmholtz resonator, increasing or decreasing the volume of the sound again depending on whether the air in the box is moving in phase or out of phase with the strings. When in phase, the sound is increased by about 3 decibels and when in opposing phase, it is decreased about 3 decibels. As a Helmholtz resonator, the air at the opening is vibrating in or out of phase with the air in the box and in or out of phase with the strings.
These resonance interactions attenuate or amplify the sound at different frequencies, boosting or damping various harmonic tones. Also, the air in the box is coupled to the resonance of the top plate. Together, which cause further interactions. 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. All these complex air coupling interactions, along with the resonant properties of the panels, are a key reason that different guitars will have different tonal qualities.
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