Acoustic Guitar

An acoustic guitar is a guitar that uses only an acoustic sound board to help transmitting the strings' vibrational energy to the air in order to produce a sound. The initial timbre and harmonics of the sound in an acoustic guitar are produced by the plucking of the string. The frequencies produced depend on string length, mass, and tension. The soundboard will add various resonant modes due to its own mix of bracing, damping, and undamped resonance.

The acoustic guitar's soundboard also has a strong effect on the loudness of the guitar. No amplification actually occurs in this process, in the sense that no energy is externally added to increase the loudness of the sound (as would be the case with an electronic amplifier). All the energy is provided by the plucking of the string. But without a soundboard, the string would just "cut" through the air without actually moving it much. The soundboard increases the surface of the vibrating area (initially just the strings), in a process called impedance matching. The soundboard has a much easier task to move the air than the string alone, because it is large - it can "push" the air better because the impedance of the soundboard is a little closer to the impedance of the air than the impedance of the strings. This process increases the entire system's transfer efficiency and the energy initially confined to the string now moves much more air: a much louder sound is being emitted.

In addition, the acoustic guitar is a hollow body and an additional coupling and resonance effect increases the efficiency of energy transmission in lower frequencies. The air in a guitar's cavity resonates with the vibrational modes of the string and soundboard. 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. Ultimately, the cavity air vibrations couple to the outside air through the sound hole (some variants of the acoustic guitar omit this hole, or have holes, like a violin family instrument). This coupling is most efficient because here the impedance matching is perfect: it is air pushing air.

There are therefore several sound coupling modes in an acoustic guitar: string to soundboard, soundboard to cavity air, and both soundboard and cavity air, to outside air. The back of the guitar will also vibrate to a lesser extent, driven by the air in the cavity. The function of the entire acoustic system is twofold: to color the sound through the generation and amplification of harmonics, and to maximize the coupling of this energy to the surrounding air - which is ultimately what we perceive as loudness of the produced sound. Improved coupling, however, comes at the expense of decay time, since now the string's energy is more efficiently transmitted. An unamplified guitar (one with no soundboard at all) would have a low volume, but the strings would vibrate much longer, like a tuning fork.

All these complex air coupling interactions, along with the resonant properties of the panels themselves, 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.