Envelope (music)

In music and audio synthesis, an envelope describes how a sound's characteristics change over time. Primarily, it refers to the amplitude (loudness) variation of a sound, but can also be applied to other parameters such as filter cutoff frequency or pitch. Envelopes are a fundamental tool for shaping the timbre and dynamic behavior of synthesized sounds, and are also used to analyze and manipulate naturally occurring sounds.

The most common type of envelope is the ADSR envelope, which stands for Attack, Decay, Sustain, and Release. These four parameters control different phases of the sound's evolution:

• Attack: The time it takes for the sound to reach its maximum amplitude from silence after a note is triggered. A short attack creates a percussive sound, while a long attack creates a softer, swelling sound.
• Decay: The time it takes for the sound to decrease from its peak amplitude (reached during the attack phase) to the sustain level.
• Sustain: The level at which the sound is held for as long as the key is held down. Unlike the other parameters, sustain is a level rather than a time.
• Release: The time it takes for the sound to fade to silence after the key is released.

Variations on the ADSR envelope exist, such as AHDSR (Attack, Hold, Decay, Sustain, Release), which adds a "Hold" phase after the attack, allowing the peak level to be sustained for a specified period. More complex envelopes can have multiple stages or be shaped with curves for more intricate sound design.

Envelopes are crucial for creating realistic and expressive synthesized sounds. By carefully adjusting the envelope parameters, a synthesizer can emulate the dynamic characteristics of acoustic instruments or create entirely new and unique sounds. Different instruments have characteristic envelope shapes; for instance, a piano note has a fast attack and relatively long decay, while a bowed string instrument might have a slower attack and a sustained level that continues until the bow is removed.