A device that affect the sound of an electric instrument or other audio source (such as recorded material) when plugged in to the electrical signal path the instrument or source sends, most often an electric guitar or bass guitar. it can also be used on other instruments or sound sources, like the Rhodes piano, synths or even the human voice. While some effect units transform the sound completely, others just color the sound picture in a minor way. An effects unit consists of one or more electronic devices which tipically contain analog circuitry for processing audio signals, similar to that found in music synthesizers (synthesizer), for example active and passive filters, envelope followers, voltage.controlled oscillators, or digital delays.