Sustain refers to the duration a sound is held after the initial attack and decay phases, and it is one of the key stages in a synthesizer’s ADSR envelope (Attack, Decay, Sustain, Release). When a note is played on a synth, the sustain level determines how long the sound remains at a consistent volume while the key is being held down. Unlike attack and decay, which are measured in time, sustain is a level—essentially the steady volume that a sound maintains until the key is released. For instance, if the sustain is set high, the note will continue to sound loudly; if it’s set low, the sound will drop to a softer volume.

The concept of sustain is essential for shaping how a sound behaves over time. In musical contexts, sustain allows for the emulation of acoustic instruments like strings or organs, where notes can be held continuously at a certain volume. It is also useful for creating pads, drones, or long atmospheric sounds. Adjusting the sustain level can drastically change the character of a sound—keeping it high can give a sense of continuity or smoothness, while lowering it can make the sound more percussive or short-lived. By creatively manipulating sustain within an ADSR envelope, musicians can achieve greater expressiveness and variety in their synthesizer patches.