Envelopes: ADSR for Amplitude and Filter

An oscillator plays forever at a constant level. An envelope gives each note a shape over time: how fast it starts, how it settles, and how it fades when you let go. Synths usually have at least two, one for amplitude (volume) and one for the filter.

The four stages

Stage	Type	What it controls
Attack	Time	How long the note takes to rise from silence to full level
Decay	Time	How long it takes to fall from full to the sustain level
Sustain	Level	The level held while the key stays down
Release	Time	How long the note takes to fade after the key is released

The important distinction: attack, decay and release are times, while sustain is a level. A sustain of 0 means the note dies away during decay even if you keep holding the key.

Amplitude envelope recipes

Sound	Attack	Decay	Sustain	Release
Pluck / stab	0–5 ms	150–400 ms	0%	100–300 ms
Bass	0–5 ms	200–500 ms	60–100%	30–100 ms
Lead	5–20 ms	200 ms	70–100%	100–300 ms
Pad	300 ms–2 s	1 s	70–100%	1–3 s
Swell / reversed feel	1–4 s	—	100%	500 ms

A few practical notes:

• Attack of exactly 0 can click on some synths. If you hear a tick at note start, raise it to 1–3 ms.
• Release on basses should be short, or notes overlap and the low end smears. A long bass release is one of the most common causes of muddy mixes.
• Pads need long release so chords blend into each other, but too long and every chord change becomes a wash.

The filter envelope

The second envelope modulates the filter cutoff. Its amount (or depth) sets how far it moves the cutoff from its base value. This is where most synth sounds get their character.

The classic move: a low base cutoff, a positive filter envelope with fast attack and a short decay, and low sustain. Each note starts bright and quickly closes down to dark. That single gesture is the difference between a dull saw and a pluck, a squelchy acid bass or a "wow" brass stab.

Worked example: from drone to pluck

1. Saw oscillator, low-pass at 24 dB/oct, cutoff at 200 Hz. It sounds muffled.
2. Filter envelope: attack 0 ms, decay 250 ms, sustain 0%, release 200 ms.
3. Envelope amount: about +50%, so each note opens the cutoff to several kHz.
4. Amplitude envelope: attack 0 ms, decay 400 ms, sustain 0%, release 200 ms.

Now each note snaps open and closes. Shorten the filter decay for a tighter, more percussive pluck; lengthen it for a softer, more piano-like one.

Filter envelope ideas

• Slow attack on the filter (300 ms–1 s) with a held note makes a pad that brightens as it sustains.
• Negative amount closes the filter at note start and opens it as the envelope falls, for reverse-sounding swells.
• Velocity to envelope amount makes harder notes brighter, the same way acoustic instruments behave.

Retrigger and legato

When notes overlap, synths behave differently. In retrigger mode every new note restarts the envelopes from the attack. In legato mode, overlapping notes change pitch without restarting, so a bassline flows smoothly and only the first note of a phrase gets the attack. TB-303-style acid lines rely on legato with slides; plucks and stabs want retrigger.

Envelopes elsewhere

Envelopes can modulate almost anything. A short pitch envelope dropping an octave over 30–80 ms on a sine is a synthesised kick drum. A fast envelope on noise level gives a lead a breathy attack. Once you think of an envelope as "a shape that happens once per note", you will use it everywhere. For shapes that repeat, see LFOs and modulation.