A guitar amplifier (or guitar amp) is an electronic amplifier designed to amplify the electrical signal of an electric or acoustic guitar so that it will produce sound through a loudspeaker. Most guitar amplifiers can also modify the instrument's tone by emphasizing or de-emphasizing certain frequencies and adding electronic effects. Vibrations of the strings are "picked up" by a suitable microphone. For electric guitars, strings are all made of metal, and the pickup works by electro-magnetic induction. Acoustic guitars have a normal microphone, designed to convert acoustic vibrations into an electrical signal, but usually they do so from direct contact with the strings (replacing the guitar's bridge) or with the guitar's body, rather than having a membrane like general-purpose microphones.