Reverb units simulate sounds produced in an echo chamber by creating a large number of echoes that gradually fade or "decay". A plate reverb system uses an electromechanical transducer to create vibrations in a plate of metal. Spring reverb systems, which are often used in guitar amplifiers, use a transducer to create vibrations in a spring. Digital reverb effects use various signal processing algorithms to create the reverb effect, often by using multiple feedback delay circuits.