A synthesis envelope is a control curve that shapes a parameter of the generated signal over time

In sound synthesis, an envelope is a time-varying control signal applied to a parameter of a synthesizer — most commonly amplitude, filter cutoff, or oscillator frequency. Unlike the signal-analysis meaning (outlining the peaks of a waveform), the synthesis meaning is generative: the envelope is a data source that tells the synth how that parameter should behave across the duration of a note. A typical amplitude envelope makes a sound rise, hold, decay, and fade; without it, a sustained tone at constant volume sounds mechanical and unnatural. Only the non-negative part of the curve is used for control.

Examples

An ADSR envelope applied to a 220 Hz sine: the amplitude starts at zero, rises to full during attack, falls to sustain level during decay, holds there, then fades to zero on release — audibly like a piano keypress.

Assessment

Distinguish the signal-analysis meaning of envelope (outlining a waveform’s extremes) from the synthesis meaning (a control-data source). Give one parameter an envelope typically controls.