An electric guitar is a guitar that uses a pickup to convert the vibration of its strings into electrical impulses. The most common guitar pickup uses the principle of direct electromagnetic induction, but others use piezoelectricity or optical sensors. The signal generated by an electric guitar is too weak to drive a loudspeaker, so it is amplified before sending it to a loudspeaker. Since the output of an electric guitar is an electric signal, the signal may easily be altered using electronic circuits to add "color" to the sound. Often the signal is modified using effects such as reverb and distortion.