The oscillogram of a percussive tone shown below illustrates the level rising immediately to the top of its range and then decaying. If you drew a box around the upper half of the oscillogram, you could consider it the “envelope” of the sound—an image of the level as a function of time. The role of the envelope generator is to set the shape of this envelope.
The envelope generator usually features four controls—Attack, Decay, Sustain, and Release, commonly abbreviated as ADSR.
Envelope controls
Attack: Sets the time it takes for the signal to rise from an amplitude of 0 to 100% (full amplitude).
Decay: Sets the time it takes for the signal to fall from 100% amplitude to the designated sustain level.
Sustain: Sets the steady amplitude level produced when a key is held down.
Release: Sets the time it takes for the sound to decay from the sustain level to an amplitude of 0 when the key is released.
Note: If a key is released during the attack or decay stage, the sustain phase is usually skipped. A sustain level of 0 produces a piano-like—or percussive—envelope, with no continuous steady level, even when a key is held.