Envelope

In sound and music, an envelope describes how a sound changes over time. Envelope normally relates to the amplitude (volume), but it may also involve elements such as filters (frequencies) 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 are common features of synthesizers, samplers, and other electronic musical instruments.

The most common kind of envelope generator has four stages: attack, decay, sustain, and release (ADSR).

• Attack is the time taken for initial run-up of level from nil to peak, beginning when the key is pressed.
• Decay is the time taken for the subsequent run down from the attack level to the designated sustain level.
• Sustain is the level during the main sequence of the sound's duration, until the key is released.
• Release is the time taken for the level to decay from the sustain level to zero after the key is released.

While, attack, decay, and release refer to time, sustain refers to level.