Sustain

Sustain is unique among the ADSR stages because it is a level, not a time. It defines the constant value the envelope holds for as long as you keep the note held down, once the initial attack and decay have played out. A high sustain keeps the sound at full strength — appropriate for organs, pads, and anything meant to be held. A sustain of zero means the sound decays completely to silence even while the key is down, which is how plucks, stabs, and percussive tones behave. On a filter envelope, the sustain level sets the brightness the tone rests at during a long note. Balancing decay against sustain is central to shaping a sound's body.