An acoustic guitar is a guitar that uses only acoustic methods to project the sound produced by its strings. The term is a retronym, coined after the advent of electric guitars, which depend on electronic amplification to make their sound audible.
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.