An acoustic guitar is a guitar that produces sound acoustically—by transmitting the vibration of the strings to the air—as opposed to relying on electronic amplification (see electric guitar). The sound waves from the strings of an acoustic guitar resonate through the guitar's body, creating sound. This typically involves the use of a sound board and a sound box to strengthen the vibrations of the strings.
The main source of sound in an acoustic guitar is the string, which is plucked or strummed with the finger or with a pick. The string vibrates at a necessary frequency and also create many harmonics at various different frequencies. The frequencies produced can depend on string length, mass, and tension.
Historical and modern acoustic guitars are extremely varied in their design and construction, rather more so than electric guitars. Some of the most important varieties are the classical guitar (nylon-strung), steel-strung acoustic guitar and resophonic guitar.
The classical guitar (also known as concert guitar, classical acoustic, nylon-string guitar, or Spanish guitar) is the member of the guitar family used in classical music. It is a wooden acoustic guitar with strings made of nylon, rather than the metal strings used in acoustic and electric guitars.
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.