The intensity of a sound is generally not a static quantity, but changes over the duration of the sound. This change in the intensity of a sound over time is called an ENVELOPE. Thus, at the beginning of a sound, the intensity does not change in an instant from silence to peak level; rather, it requires a certain amount of time, called the ATTACK TIME (or simply, the ATTACK), to do so. Similarly, at the end of a sound, the intensity does not suddenly plummet to silence. The amount of time required for the intensity of the sound to diminish to nothing is called the DECAY TIME (or simply, the DECAY), or the RELEASE.

See ENVELOPE from Barry Truax's Handbook for Acoustic Ecology.