Dave Smith Instruments Sequential Prophet-6 Operation Manual page 35

To give you a real-world example, the main difference between the
sound of the wind and the sound of a snare drum is that they have very
different volume envelopes. Otherwise, they are essentially both white
noise. Wind has a relatively slow attack, a long sustain, and a long decay
and release. A snare drum has a sharp attack, no sustain, and virtually no
decay or release. But again, they are both fundamentally white noise.
AMPLIFIER ENVELOPE
VELOCITY
ENV AMOUNT ATTACK DECAY SUSTAIN RELEASE
Amplifier envelope
Time
A typical four-stage, ADSR envelope shape
Sets the attack time of the envelope. The higher the setting, the
Attack:
slower the attack time and the longer it takes for a sound to reach its full
volume. Pads typically have softer (longer) attacks. Percussive sounds
have sharper (shorter) attacks.
Sets the decay time of the envelope. After a sound reaches its full
Decay:
volume at its attack stage, controls how quickly the sound transi-
decay
tions to the level set with the control. The higher the setting, the
sustain
longer the decay. Percussive sounds, such as synth bass, typically have
shorter decays.
Sets the sustain level of the envelope. The higher the setting,
Sustain:
the louder the sustained portion of the sound will be. The sound will stay
at this level for as long as a note is held on the keyboard.
Prophet-6 Operation Manual Amplifier Envelope 25