In acoustic guitars, string vibration is transmitted through the bridge and saddle to the body via sound board. The sound board, typically made of tonewoods such as spruce or cedar. Timbers for tone woods are chosen for both strength and ability to tranfer mechanical energy from the strings to the air within the guitar body. Sound which is further shaped by the characteristics of the guitar body's resonant cavity.
In electric guitars, transducers known as pickups convert string vibration to an electric signal, which in turn is amplified and fed to speakers, which vibrate the air to produce the sound we hear. Nevertheless, the body of the electric guitar body still performs a role in shaping the resultant tonal signature.
The body of the acoustic instrument is thought to be a major determinant of the overall sound quality. The guitar top, or soundboard, is a finely crafted and engineered element made of tonewoods such as spruce and red cedar. This thin, often only 2 or 3mm thick, piece of wood, strengthened by differing types of internal bracing, is considered by many luthiers to be the dominant factor in determining the sound quality of a guitar. The majority of the instrument's sound is heard through the vibration of the guitar top as the energy of the vibrating strings is transferred to it.
Body size, shape and style has changed over time, 19th century guitars, now known as salon guitars, were smaller than modern instruments. Differing patterns of internal bracing have been used over time by luthiers, (Torres, Hauser, Ramirez, Fleta, and C.F. Martin being among the most influential designers of their time). Bracing not only strengthens the top against potential collapse due to the stress exerted by the tensioned strings, but also affects the resonance characteristics of the top. The back and sides are made out of a variety of timbers such as mahogany, Indian rosewood and highly regarded Brazilian rosewood (Dalbergia nigra). Each one is primarily chosen for their aesthetic effect and can be decorated with inlays and purfling.
The body of an acoustic guitar has a sound hole through which sound is projected. The sound hole is usually a round hole in the top of the guitar under the strings. Air inside the body vibrates as the guitar top and body is vibrated by the strings, and the response of the air cavity at different frequencies is characterised, like the rest of the guitar body, by a number of resonance modes at which it responds more strongly.
Instruments with larger areas for the guitar top were introduced by Martin in an attempt to create louder volume levels(although tone quality will also be affected by materials used in guitar top size and design). The popularity of the Dreadnought body size amongst acoustic performers is related to the greater sound volume produced.