ADSR Envelopes

The oscillator gives us the spectral character of a sound. An envelope gives us its shape through time.

Core idea: a waveform by itself simply continues. An ADSR envelope changes its amplitude over time so the waveform can behave like a note, pluck, pad, organ, percussion sound and much more.

1. The simple maths

An amplitude envelope is just a changing gain value. At every moment we multiply the oscillator by the envelope:

output(t) = oscillator(t) × envelope(t)

If the envelope is 1.0, we hear the oscillator at full level. If it is 0.5, the oscillator is half amplitude. If it is 0, we hear silence.

2. What A, D, S and R actually do

A — Attack

Time taken for the signal to rise from silence to its peak after note-on.

0 → 1

D — Decay

Time taken to fall from the attack peak down to the chosen sustain level.

1 → S

S — Sustain

The level held while the note remains on. Sustain is not a time value.

hold at S

R — Release

Time taken for the signal to return to silence after note-off.

current level → 0

3. Interactive ADSR

Master Volume (Safety Capped)

Pitch: 220 Hz

Waveform

Attack: 0.05 s

Decay: 0.30 s

Sustain level: 0.65

Release: 0.70 s

Display sustain/hold time: 1.2 s

Envelope — drag the points or use the sliders

Drag the peak to alter Attack, drag the sustain corner horizontally to alter Decay, drag it vertically to alter Sustain level, and drag the final point to alter Release.

Raw oscillator

Oscillator × ADSR

Ready: press and hold the trigger button or hold the spacebar. While held, the envelope moves through Attack → Decay → Sustain. Releasing starts Release.

4. Why the envelope matters

The same oscillator can behave like completely different instruments simply because its amplitude changes differently through time.

Example	Envelope behaviour	Perceptual result
Organ	Fast attack, little decay, high sustain, fast release	Immediate and held
Pluck	Fast attack, short decay, little/no sustain	Short struck/plucked event
Pad	Slow attack, gentle decay, high sustain, long release	Soft evolving sound
Percussion	Very fast attack, rapid decay, zero sustain	Transient event

5. The next step

Here the envelope controls amplitude. But an envelope is simply a control signal — it can be routed elsewhere.

ADSR → amplifier gain

ADSR → filter cutoff

That gives us the classic synthesiser architecture: oscillator → filter → amplifier, with envelopes shaping both brightness and loudness through time.