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 and at low frequencies, which depend on[1] the size of the box, the chamber acts like a Helmoltz resonator(see Helmholtz resonance), 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 diff 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[2] (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. The sound is a complex mixture of harmonics that give the guitar its distinctive sound.
| An example of the sounds which an Acoustic Guitar can create through vibration of its strings. This guitar uses steel strings. |
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Amplification
An acoustic guitar can be amplified by using various types of pickups or microphones. The most common type of pickups used for acoustic guitar amplification are piezo and magnetic pickups. Piezo pickups are generally mounted under the bridge saddle of the acoustic guitar and can be plugged into a mixer or amplifier. Magnetic pickups are generally mounted in the sound hole of the acoustic guitar and are very similar to those found in electric guitars.Instruments with larger areas for the guitar top were introduced by Martin in an attempt to create louder volume levels. The popularity of the larger "dreadnought" body size amongst acoustic performers is related to the greater sound volume produced.