An Acoustic-electric guitar is an acoustic guitar

Acoustic guitar

An acoustic guitar is a guitar that uses only an acoustic sound board. The air in this cavity resonates with the vibrational modes of the string and 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...

 fitted with pickups, a microphone

Microphone

A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...

 or transducers. In acoustic-electric guitars, the transducers and microphones are always used because conventional pickups are not capable of picking up vibrations of non-magnetic materials. The design is distinct from a semi-acoustic guitar

Semi-acoustic guitar

A semi-acoustic guitar or hollow-body electric is a type of electric guitar with both a sound box and one or more electric pickups. This is not the same as an electric acoustic guitar, which is an acoustic guitar with the addition of pickups or other means of amplification, either added by the...

, which is an electric guitar

Electric guitar

An electric guitar is a guitar that uses the principle of direct electromagnetic induction to convert vibrations of its metal strings into electric audio signals. The signal generated by an electric guitar is too weak to drive a loudspeaker, so it is amplified before sending it to a loudspeaker...

 but with the addition of sound chambers within the guitar body. Usually, acoustic-electric guitars are fitted with piezoelectric

Piezoelectricity

Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...

 pickups, and hence require a preamplifier

Preamplifier

A preamplifier is an electronic amplifier that prepares a small electrical signal for further amplification or processing. A preamplifier is often placed close to the sensor to reduce the effects of noise and interference. It is used to boost the signal strength to drive the cable to the main...

 incorporated into the guitar body to amplify the signal before it travels to the main guitar amplifier

Guitar amplifier

A guitar amplifier is an electronic amplifier designed to make the signal of an electric or acoustic guitar louder so that it will produce sound through a loudspeaker...

. These preamps may also come with tone controls of varying types, but usually equalizers with up to six frequency bands are used. They are also referred to as a "plug-in acoustic guitar", due to their ability to simply "plug in" to a speaker system without the need for microphones.

They are commonly used in folk

Folk music

Folk music is an English term encompassing both traditional folk music and contemporary folk music. The term originated in the 19th century. Traditional folk music has been defined in several ways: as music transmitted by mouth, as music of the lower classes, and as music with unknown composers....

 and sometimes classical music

Classical music

Classical music is the art music produced in, or rooted in, the traditions of Western liturgical and secular music, encompassing a broad period from roughly the 11th century to present times...

 as they possess the sound of an acoustic guitar but more volume if plugged into a speaker system.

The first acoustic-electric guitar was the Ovation Roundback developed by Charles H. Kaman in 1966, built from fiberglass

Fiberglass

Glass fiber is a material consisting of numerous extremely fine fibers of glass.Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the invention of finer machine tooling...

 and based on his research as an aerospace engineer into the vibration of helicopter rotor blades. He added electrical pickups to provide the option of amplification. The Kaman Music Corporation

Kaman Music Corporation

Kaman Music Corporation is a part of the Kaman Corporation founded by Charles Kaman. Kaman is also a guitarist and he came to explore the use of composite materials technologies to guitar building. He and his engineers created the round-backed, composite-body Ovation guitar which revolutionized...

, built on the success of the Roundback, became one of the largest music instrument manufacturers in the United States. They can be small.