The envelope of a sound displays how the level of a sound wave changes over time.
The envelope of a wave helps establish the sound’s unique individual quality; it has a significant influence on how we interpret sound.
The envelope of a sound can be measured in four ways:
1. Attack – The attack is the portion of the envelope that represents the time taken for the amplitude to reach its maximum level. Essentially it is the initial build-up of a sound.
Sound with an obvious attack
2. Decay – The decay is the progressive reduction in amplitude of a sound over time. The decay phase starts as soon as the attack phase has reached its peak. In the decay phase, the signal level drops until it reaches the sustain level.
Sound with short decay (and long sustain)
3. Sustain – The sustain is the period of time during which the sound is held before it begins to fade out. Many instruments do not contain a sustain phase.
Sound with long sustain (and no attach or delay)
4. Release – The release is the final fade or reduction in amplitude over time.
Sound with obvious long release
On synthesizers, the ADSR envelope is usually controlled via a set of control knobs or sliders.
Every sound is different. Percussion sounds start suddenly, then decay and release quickly as no more energy is being applied to sustain the sound.
A bowed string, on the other hand, may build up with a slow attack, sustain for a short period and then release.
Most natural sounds decay the higher frequencies faster than the lower frequencies because high-frequency energy is dissipated more rapidly than low-frequency energy.
The concept of hearing envelopes relies upon the root-mean-square values (RMS) values of amplitude and not peak to peak amplitudes.
High peaks in the signal will not necessarily make an instrument sound loud unless the amplitude is sustained for a period of time.
Short peaks tend to contribute to the character (timbre) of the sound rather than the loudness.