A sound's envelope is its amplitude contour over time, and that shape is a primary cue for instrument identity

Every sound — synthesized or acoustic — has an envelope: the overall contour traced by its amplitude over time, the curve of how loudness rises, holds, and falls. Each instrument has a characteristic profile: a plucked string peaks immediately then decays and releases automatically; a bowed violin swells slowly, sustains, and releases gradually; a piano key rises, decays to a held level, then releases on key-up. This contour is a primary cue the ear uses to identify instrument character, so much so that two sounds at the same pitch can seem to be different instruments purely because their envelopes differ. Early synthesizers produced constant-amplitude tones that sounded mechanical and test-tone-like; adding an amplitude envelope was the single most important step toward naturalistic synthesis, and even a coarse piece-wise approximation dramatically improves perceived naturalness. On an acoustic instrument the physics fixes the envelope; in synthesis the user sets each stage explicitly. Grasping envelope as a property of all sounds — not just synths — is the conceptual prerequisite for designing your own.

Examples

A 220 Hz sine at constant amplitude sounds like a test tone; the same oscillator through an ADSR (fast attack, medium decay, ~70% sustain, ~300 ms release) sounds like a plucked or held note. A plucked guitar string: fast attack, medium decay to a sustaining resonance, gentle release. A bowed cello vs. a handclap at the same pitch read as different instruments almost entirely because of their contrasting amplitude contours.

Assessment

Describe the envelope of a handclap and of a bowed cello purely as amplitude contour over time, and explain why that difference alone can make them sound like different instruments at identical pitch. Then set attack=0ms, decay=200ms, sustain=0%, release=0ms on any synth, say what instrument it resembles, and describe how raising sustain to 80% changes the character.