In sound and music, an envelope describes how a sound changes over time. It may relate to elements such as amplitude (volume), frequencies (with the use of filters) or pitch. For example, a piano key, when struck and held, creates a near-immediate initial sound which gradually decreases in volume to zero.
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 form of envelope generator is controlled with four parameters: attack, decay, sustain and release (ADSR).