The Envelope of Sound

In sound and music, an envelope describes how a sound changes over time.

There are four stages: attack, decay, sustain, and release.

As an example, here are the four stages with respect to pressing a piano key:

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

These four stages also apply to the way most dynamics processors such as expanders and compressors operate.

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