Give each harmonic its own ADSR and upper partials fade first, the differential decay that makes a piano note darken as it rings out.
In the world
On a piano, the upper harmonics of a struck string fade within a second while the fundamental rings for many seconds. That differential decay is why a piano sounds bright at the attack and warm at the tail.
Real acoustic instruments do not have one envelope; they have one envelope per partial. High partials lose energy faster because air damping, string stiffness, and internal friction all attenuate higher frequencies more aggressively. This is the single most important insight of realistic additive synthesis.
Give each of four harmonics its own ADSR, trigger them from the same clock, and set progressively faster decays on the upper partials. The resulting sound darkens as it rings, exactly like a struck bar or plucked string.
Did you know?
The Kawai K5000, released in 1996, was the first affordable synth to give every partial its own envelope - 64 partials, each with a 7-segment envelope. Sound designers spent hours programming a single patch. It flopped commercially but became a cult classic.
Explore
Fireworks burst on the right. Click to launch a new burst. Each firework has 4 layers of particles, each layer representing a partial with its own decay time. Sliders at the bottom control the decay rate per layer - set the upper layers to fast decay and watch the fireworks start bright white, then fade through warm colors as only the low partials survive.