An acoustic guitar is a guitar that uses only acoustic methods to project the sound produced by its strings. It is a retronym, coined after the advent of electric guitars, which depend on electronic amplification to make their sound audible.
Sound production Edit
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. 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 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 of harmonics that give the guitar its distinctive sound.
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. The function of the entire acoustic system is to maximize intensity of sound, but since total energy remains constant, this comes at the expense of decay time. 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. This is because a damped harmonic oscillator decays exponentially, with a mean life inversely proportional to the damping. When the strings are driving the larger soundboard and sound box, the damping is much higher.
AmplificationEditAn 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.
Various shapes determine the way the soundboard vibrates. The thinner and lighter the soundboard (less mass), the louder the sound. However, there are practical limitations to how thin the soundboard can be made without breaking. Braces are used inside the guitar to provide strength and resilience. The mass and position of these braces have consequences for the range of frequencies reproduced. During the wave cycle, different regions of the soundboard may be moving in different directions, depending on the sound frequency. Different configurations of bracing and different shapes of soundboard produce different vibration patterns, giving subtle variations in the range of sounds produced.
The materials and shape of the guitar produce a complex series of damping, resonating, and phase cancelling or reinforcing effects. The range of factors determine the overall acoustic qualities or timbre of the instrument. Artisan luthiers tap potential pieces of wood to determine their acoustic resonance, but this is usually not done for mass-produced instruments. Different timber species have different tones and careful selection of timber is required when designing and making an instrument. Guitars have been made with steel soundboards and resonators, and some experiments have been conducted with novel materials including aluminium and plastics. Even the hardness or viscoelasticity of the glues and varnishes can have a dramatic effect on the sound, damping or resonating some or all frequencies. Quality instruments are made with hard glues and lacquers which have less damping on the transmission of vibrations around the structure of the instrument. Most people prefer the sound of wooden instruments, although the steel resonator guitar has found favor in some genres, like blues. Sitka spruce is traditionally the favored material for the soundboard because of its high strength-to-weight ratio and stiffness. In recent years King William pine has been found to produce very good results.