Chapter One: An Acoustics Primer

7. What is amplitude? | Page 6

Envelopes

The dynamic envelope refers to the amplitude change over time of a sound event (usually a short one, such as an instrumental or synthesized note). While acoustic sounds often exhibit greater complexity, a simplified model of a note consists of four primary phases: an initial attack, defined by the duration required to transition from silence to peak amplitude; a decay phase, during which the amplitude decreases to a steady-state sustain level; and a release phase, representing the time required for the amplitude to return to silence.

Not only do real-world (and complexly synthesized) sounds have more complex overall envelopes, but they often exhibit different envelopes for their respective frequency components.