In sound and music, an envelope describes how a sound changes over time. For example, a piano key, when struck and held, creates a near-immediate initial sound which gradually decreases in volume to zero. An envelope may relate to elements such as amplitude (volume), frequency (with the use of filters) or pitch.
Envelope generators, which allow users to control the different stages of a sound, are common features of synthesizers, samplers, and other electronic musical instruments. The most common envelope generator is controlled with four parameters: attack, decay, sustain and release (ADSR).