An Envelope describes the varying level of a sound signal as it evolves over time, effectively acting as a mold that shapes a raw electronic tone into a recognizable instrument. While a raw oscillator produces a constant and lifeless tone, the envelope generator modulates that tone to give it articulation and dynamics. The most ubiquitous model for this is the ADSR envelope, which stands for Attack, Decay, Sustain, and Release. Attack controls the time taken to reach peak level from zero. Decay sets the time taken to drop from that peak to the sustain level. Sustain is the constant volume level held while the note is physically played or held. Finally, Release determines the fade-out time after the note is stopped. It is important to note that envelopes are not limited to controlling volume; they are frequently used to modulate filter cutoff frequencies or pitch, creating dynamic timbral movement within a sound.