Sustain is the third of the four segments in an Attack Decay Sustain Release (ADSR). The sustain portion of the ADSR envelope begins when the attack and decay portions have run their course, and continues until the key is released. The sustain control is used to determine the level at which the envelope will remain. While the attack, decay, and release controls are rate or time controls, the sustain control is a level control.
Drum percussion instruments are usually the instruments with the shortest sustain; a drum beat begins to fade almost instantly. Instruments with the highest possible sustain often involve the passage of air, such as brass or the pipe organ, which have theoretically unlimited sustain. Feedback created using the Larsen effect also provides virtually unlimited sustain, but only to electronically amplified instruments. Resonating instruments such as the piano and guitar also have fairly long sustain for string instruments.
Many factors affect sustain for a given instrument. For example, sustain in guitars is determined by factors including body construction (hollow versus solid), body woods, neck woods, the placing of strings (through the body or atop the body), string gauges and pickup design.
There are musical devices used to increase sustain, known as "Sustainers", such as the patented Fernandes Sustainer guitar system and the EBow, which can be used to produce virtually infinite sustain.
There are many factors that affect the level of sustain in guitars. The most obvious ones are the quality of the production or manufacturing and the quality of the wood and other material. Larger string diameters have also been shown to increase sustain.