An acoustic-electric guitar (also called an electro-acoustic guitar)is an acoustic guitar fitted with a magnetic pickup, a piezoelectric pickup or a microphone. In acoustic-electric nylon string guitars, piezoelectric pickups and microphones are always used because magnetic pickups are not capable of picking up vibrations of non-magnetic materials. The design is distinct from a semi-acoustic guitar, which is an electric guitar but with the addition of sound chambers within the guitar body. Usually, acoustic-electric guitars are fitted with piezoelectric pickups, and hence require a preamplifier incorporated into the guitar body to amplify the signal before it travels to the main guitar amplifier. 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.