The Acoustic Envelope describes the variation in volume of a natural sound over time, from its beginning to its end. The envelope of any natural acoustic sound can be divided into three main phases:

1. Attack This is the initial and rapid part of the sound, also called the transient. Think of the beat of a drumstick on a drumhead or the sound of a pick striking a guitar string. This phase is the most important for our brain to recognize the type of instrument and its location in space.

2. Sustain This is the period in which the sound maintains a relatively constant intensity after the initial attack. It's the piano note sounding while you hold down the key, for example. 3. Decay: This is the final phase, where the sound loses energy and disappears after the sound source stops being excited (for example, when the vibration of the strings of a guitar naturally decreases until silence).