The envelope modifies the loudness contour of a sound. It takes a sound input and shapes it into something you'd hear from an acoustic musical instrument, like a piano or saxophone.

This envelope has two controls: "attack", which is how long it takes to ramp up to maximum volume, and "decay", which is how long it takes to go down to silence again. You can use its third bitSnap[TM] to trigger the envelope from different sources, like the keyboard. [Try it putting the envelope after an oscillator being controlled by a micro sequencer.]