Sustain is the third stage of the ADSR envelope and is the only stage that is not representative of time. In terms of a synthesizer with a keyboard, the sustain is described as the level of volume (amplitude) that the signal remains at for however long the key is being pressed.

Sustain is interconnected with the decay stage, where the decay points out how fast or slow the sound should drop from the maximum volume, the sustain level shows where the decay needs to stop and where the output signal should continue from until it drops off to silence.

An example of a short sustain level would be on drum-based instruments, where the beat of a drum will start to disappear straight away, almost instantaneously. Instruments that are designed for air to pass through them, like the pipe organ, will allow for much longer sustain levels.

There are various ways to affect the level of sustain of some instruments. You are likely to be aware of the sustain pedal underneath a piano, pushing your foot on this pedal will release the dampers from the keys so they can ring out a little longer than usual.

Synthesizers have similar effects and can be achieved by using the latching function, activating this will extend the sustain level to any of the notes that are being played or that are about to be played.