An acoustic guitar is a guitar that uses only acoustic (as opposed to electronic) means to transmit the strings' vibrational energy to the air in order to produce a sound. This typically involves the use of a sound board and a sound box to amplify the vibrations of the string.
The source of sound in an acoustic guitar is the string, which is plucked with the fingers or with a plectrum. The string vibrates at a fundamental frequency but also creates many harmonics at different frequencies. The frequencies produced depend on string length, mass, and tension. The string causes the soundboard and sound box to vibrate, and as these have their own resonances at certain frequencies, they amplify some string harmonics more strongly than others, hence affecting the timbre produced by the instrument.
The acoustic guitar's soundboard, or top, 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 in a process called mechanical impedance matching. The soundboard can move the air much more easily than the string alone, because it is large and flat. This increases the entire system's energy transfer efficiency, and a much louder sound is emitted.
In addition, the acoustic guitar has 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, though some[which?] variants of the acoustic guitar omit this hole, or have holes, like a violin family instrument (a trait found in some electric guitars such as the ES-335 and Lucille models from Gibson). 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.
An example of the sounds which an acoustic guitar can create through vibration of its strings. This guitar uses steel strings.
Classical gut-string guitars had little projection, and so were unable to displace banjos until innovations increased their volume.
Two important innovations were introduced by the American firm, Martin Guitars. First, Martin introduced steel strings. Second, Martin increased the area of the guitar top; the popularity of Martin's larger "dreadnought" body size amongst acoustic performers is related to the greater sound volume produced. These innovations allowed guitars to compete with and often displace the banjos that had previously dominated jazz bands. The steel-strings increased tension on the neck; for stability, Martin reinforced the neck with a steel truss rod, which became standard in later steel-string guitars.
An acoustic guitar can be amplified by using various types of pickups or microphones. However, amplification of acoustic guitars had many problems with audio feedback. In the 1960s, Ovation's parabolic bowls dramatically reduced feedback, allowing greater amplification of acoustic guitars. In the 1970s, Ovation developed thinner sound-boards with carbon-based composites laminating a thin layer of birch, in its Adamas model, which has been viewed as one of the most radical designs in the history of acoustic guitars. The Adamas model dissipated the sound-hole of the traditional soundboard among 22 small sound-holes in the upper chamber of the guitar, yielding greater volume and further reducing feedback during amplification. Another method for reducing feedback is the use of a rubber or plastic disc which fits into the soundhole.
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. A Piezo pickup made by Baldwin was incorporated in the body of Ovation guitars, rather than attached by drilling through the body; the combination of the Piezo pickup and parabolic ("roundback") body helped Ovation to lead the market during the 1970s.Magnetic pickups are generally mounted in the sound hole of the acoustic guitar and are very similar to those found in electric guitars. An acoustic guitar with pickups for electrical amplification is known as an acoustic-electric guitar.
In the 2000s, new types of pickups have been introduced to try to amplify the full sound of these instruments. This includes body sensors such as the Taylor Guitars Expression System®, and systems that include an internal microphone along with the body sensors or under-the-saddle pickups.
A steel strung Yamaha APX700 electric-acoustic guitar
"Taylor Guitars Expression System". Taylor Guitars. Retrieved 2 April 2014.
Carter, Walter (1996). Eiche, Jon, ed. The history of the Ovation guitar. Musical Instruments Series (first ed.). Milwaukee, Wisconsin: Hal Leonard Corporation. pp. 1–128. HL00330187; ISBN 978-0-7935-5876-6; ISBN 0-7935-5876-X (softcover); ISBN 0-7935-5948-0 (hardcover).
Denyer, Ralph (1992). The guitar handbook. Special contributors Isaac Guillory and Alastair M. Crawford; Foreword by Robert Fripp (Fully revised and updated ed.). London and Sydney: Pan Books. ISBN 0-330-32750-X.