An ADSR envelope turns a steady drone into a finger pluck, the amplitude shape that separates a sustained organ from a struck piano string.
In the world
A piano note attacks in a few milliseconds, peaks immediately, and decays over seconds. A bowed violin swells slowly. The time-shape of loudness is what your ear uses to recognise the instrument before it even parses the pitch.
An envelope describes how a parameter changes over time. The universal shape is ADSR: Attack (rise time), Decay (fall to sustain), Sustain (held level while the gate is high), Release (fall to zero after gate). Wire the envelope's output into a VCA's CV input and the VCA multiplies every audio sample by the envelope value, sculpting loudness over time.
Short attack, short decay: a pluck. Long attack, long release: a pad. Instant attack, long decay, zero sustain: a strike.
Did you know?
Researchers found that if you remove the first 50 milliseconds of a recorded note, most people cannot tell a piano from a guitar. The attack portion of the envelope carries almost all the information your brain uses to identify an instrument.
Explore
A circular field of glowing pylons traces the ADSR envelope shape - Attack (red), Decay (orange), Sustain (gold), Release (blue). Drag the A/D/S/R sliders to reshape the landscape in real time. Hit PLAY NOTE to send a glowing ball rolling across the surface, hearing the envelope sculpt the sound as it goes.
Notice how the same sine tone goes from a steady drone to a finger pluck just by reshaping its amplitude over time. The pitch has not changed - only the loudness shape.