Update your browser to view this website correctly. Update my browser now
| | Attack | Decay | Sustain | Release | | | Key | on | | off | | |
When an acoustic musical instrument produces sound, the loudness and spectral content of the sound change over time in ways that vary from instrument to instrument. The "attack" and "decay" of a sound have a great effect on the instrument's sonic character.  Sound synthesis techniques often employ an envelope generator that controls a sound's parameters at any point in its duration. Most often this is an "ADSR" (Attack Decay Sustain Release) envelope, which may be applied to overall amplitude control, filter frequency, etc. The envelope may be a discrete circuit or module, or implemented in software. The contour of an ADSR envelope is specified using four parameters:
An early implementation of ADSR can be found on the Hammond Novachord in 1938 (which predates the first Moog synthesizer by over 25 years). A seven-position rotary knob set preset ADS parameter for all 72 notes; a footpedal controlled release time.  The notion of ADSR was specified by Vladimir Ussachevsky (then head of the Columbia-Princeton Electronic Music Center) in 1965 while suggesting improvements for Bob Moog's pioneering work on synthesizers, although the earlier notations of parameter were (T1, T2, Esus, T3), then these were simplified to current form (Attack time, Decay time, Sustain level, Release time) by ARP. 
Some electronic musical instruments allow the ADSR envelope to be inverted, which results in opposite behavior compared to the normal ADSR envelope. During the attack phase, the modulated sound parameter fades from the maximum amplitude to zero then, during the decay phase, rises to the value specified by the sustain parameter. After the key has been released the sound parameter rises from sustain amplitude back to maximum amplitude.
8-step envelope on Casio CZ series
A common variation of the ADSR on some synthesizers, such as the Korg MS-20, was ADSHR (attack, decay, sustain, hold, release). By adding a "hold" parameter, the system allowed notes to be held at the sustain level for a fixed length of time before decaying. The General Instruments AY-3-8912 sound chip included a hold time parameter only; the sustain level was not programmable. Another common variation in the same vein is the AHDSR (attack, hold, decay, sustain, release) envelope, in which the "hold" parameter controls how long the envelope stays at full volume before entering the decay phase. Multiple attack, decay and release settings may be found on more sophisticated models.
Certain synthesizers also allow for a delay parameter before the attack. Modern synthesizers like the Dave Smith Instruments "Dave Smith (engineer)") Prophet '08have DADSR (delay, attack, decay, sustain, release) envelopes. The delay setting determines the length of silence between hitting a note and the attack. Some software synthesizers, such as Image-Line's 3xOSC (included for free with their DAW FL Studio) have DAHDSR (delay, attack, hold, decay, sustain, release) envelopes.