What is an ADSR envelope?

What is an envelope on a synth?

Press a key on a piano and the note jumps out, then fades away whether you hold the key or not. Bow a violin and the note can creep in and then hold for as long as the bow keeps moving. Much of the difference between those two sounds is in how their volume moves over time, and on a synth the control that sets that movement is the envelope.

An oscillator on its own makes a tone that starts at full volume the instant it's switched on and never changes, which is fine for tuning and not much else. The envelope sits between your finger and the volume. It starts moving when you press a key and moves again when you let go, and the synth's level follows it.

ADSR is the most common kind of envelope, named after its four stages: attack, decay, sustain and release. Three of them are times, measured in milliseconds or seconds, and sustain is the odd one out because it's a level.

Envelopes turn up away from volume too. Point one at a filter and the note's brightness changes over time; point one at pitch and you get the quick drop in pitch at the start of a synth kick drum. This page starts with volume, because that's the one you hear most clearly, and then moves on to the filter.

Reading the lab

The lab above is a small synth with one oscillator, a low-pass filter and two envelopes. You can play it from the keyboard on the screen, from your computer keyboard (the row from A to ; plays the white notes) or with Play riff, which loops a two-bar line in A minor at 120 BPM. The buttons at the top switch between four waveforms, which the oscillator waveforms lesson covers in detail.

The big screen is the amp envelope. The white line is its shape and the four dots are its stages. Drag A sideways to change the attack and R sideways to change the release; D moves the decay and the sustain together, and S moves the sustain on its own.

Behind the white line, the pink shape is the volume of a real note. Each time you change something, the lab renders an A at 220 Hz through the same oscillator, filter and envelope you hear when you play, and draws how loud it is at every moment. The time axis is stretched so a 2 ms attack still gets a few pixels next to a 3 second release, and the labels under each stage give the real times.

While a note plays, a white playhead runs along the envelope and the ENV meter shows where it has got to. The two smaller screens show one cycle of the wave and its harmonics, which is where you can see the filter working. The Right now panel describes what your settings are doing, with numbers measured from the rendered note, and it talks about whichever control you touched last.

The four stages of ADSR

Each stage begins when the one before it finishes, apart from release, which begins whenever you let go of the key.

Attack

Attack is the time a note takes to climb from silence to full volume after you press a key. Short attacks, from about 1 to 10 ms, give a note a hard edge at the front, and that edge is a lot of what makes it sound tight and in time. Plucks, basses, stabs and most leads live here.

From around 20 to 100 ms the click at the front softens, but the note still speaks quickly enough for brass-style leads and string parts that need to stay rhythmic. Above about 200 ms you hear the note fade in, which is pad and swell territory. A 1 second attack on a chord held for a whole bar sounds like a string section easing in; the same attack on a quick riff means most notes are over before they get loud.

In the lab, set the attack to 600 ms. Hold a key and the note takes over half a second to reach full volume, while in the riff the shorter notes let go before they get there.

Decay

Decay is the time the note takes to fall from its peak to the sustain level. It only does anything when the sustain is below 100%, because with the sustain at full there's nowhere lower for the note to fall. Bring the sustain down and the decay turns into an accent on the front of each note: with a 60% sustain, a 100 ms decay gives a short bump at the start and a 1 second decay gives a slow sag.

With the sustain all the way down, the decay becomes the length of the whole note. That's the classic pluck shape, with an instant attack, a decay of 150 to 400 ms and nothing held, so the note dies away by itself the way a plucked string does, even if you keep your finger on the key.

Sustain

Sustain is the level the note holds at for as long as the key stays down, given as a percentage of the peak or in decibels. It has no time of its own. The sustain lasts exactly as long as you hold the note.

At 100% the note stays at full volume, like an organ. In this lab, where sustain is a share of the peak level, 50% settles 6 dB below the peak. Most basses and leads sit somewhere between 60 and 100% so that held notes stay strong, and pads usually stay high too so that chords don't sag, while plucks and anything percussive have a sustain at or near zero. The Right now panel in the lab reports the sustain it measures on the rendered note in decibels below the peak, which is a handy way to connect the two scales.

Release

Release is the time the note takes to fade to silence after you let go. It's the stage you hear between notes, so it decides whether a part sounds clipped and tight or smooth and joined up.

A bassline usually wants a short release, around 20 to 100 ms, so each note stops cleanly before the next and the low end stays clear. Leads often sit around 100 to 300 ms. Pads use long releases of one to four seconds, so the old chord is still fading when the new one arrives, and a long release on a pluck lets it ring on like a harp.

Set the release right down to 0 and notes can end with a click, because cutting a wave off partway through a cycle makes a sudden jump in the signal. You'll hear it most on low notes and on smooth waves like a sine. A few milliseconds of release are enough to hide it, and that's why the shortest release in the lab is 5 ms.

Release works hand in hand with how long you hold each note. The Gate control in the lab sets how much of each riff note the key is held for. At 50%, an eighth note at 120 BPM is held for 125 ms, which leaves a 125 ms gap before the next one, and that gap is where you hear the release. Set a 1 second release and those gaps fill in.

Letting go before the envelope finishes

The release starts the moment the key comes up, wherever the envelope happens to be. Let go halfway through a 400 ms attack and the note never reaches full volume; it starts fading from about 50%. Let go during the decay and it fades from wherever the decay had got to.

That's why a slow pad sounds weak when you play it in short stabs, and why the same patch can sound loud on held chords and quiet on a fast part. Move the slider to see it happen.

Samplers and drum machines often have a one-shot mode that plays the whole sound however short the key press is, so a quick tap and a long hold come out the same. Most synth envelopes don't work that way unless you ask them to.

Amp envelopes and filter envelopes

Everything so far has been about volume, which is the job of the amp envelope. Nearly every synth has one already wired to the volume, and in most soft synths it's the first envelope in the list.

A filter envelope moves the cutoff of the filter instead. Plucked and struck instruments get darker as they fade, because their high harmonics die away faster than the low ones, so a filter that closes as a note goes on tends to sound natural. Push the envelope amount further and you get the wow of a synth bass or the squelch of an acid line.

In the lab the filter envelope has two controls. Filter env sets how far the filter opens above the cutoff at the start of each note, in octaves, and Filter decay sets how long it takes to close again. With the cutoff at 500 Hz and the amount at 3.5 octaves, each note starts with the filter at about 5.7 kHz and falls back to 500 Hz. The resonance control boosts the frequencies right at the cutoff, so when the envelope sweeps the filter you hear that boost move, which is where the vowel-like sound of a lot of synth basses comes from.

The lab's filter envelope is a simple one that jumps open at the start of each note and closes over the filter decay. Many synths give the filter a full ADSR of its own, and the same four stages apply: a slow filter attack makes a note brighten as it plays, and the filter sustain sets how bright it stays while you hold the key. The filter envelopes lesson goes through those in detail.

Having two envelopes lets volume and brightness follow different shapes. A bass can keep its level with an 80% sustain while its brightness snaps shut in 250 ms, so it sounds punchy at the front and round underneath.

Envelopes for plucks, pads, basses and leads

Most patches fall into a handful of envelope shapes, and once you can picture them you can rough out a sound in a few seconds. The numbers below are starting points chosen so the shape is obvious, and your track decides the rest.

Plucks

Everything happens at the front. The attack is as short as it goes, the sustain sits at zero and the decay sets the length of each note, so on a fast riff 150 ms sounds tight and 400 ms sounds ringy. A filter envelope with a similar decay makes the note darken as it fades, and that's most of what separates a synth pluck from a short beep.

Pads

Pads turn that around. The attack is slow so chords swell in and the sustain stays high so they hold, while a release of a couple of seconds lets one chord overlap the next. If a pad gets lost under the drums, try a shorter attack before you turn it up. A 1.5 second attack on a chord that only lasts a bar means it spends a big part of that bar getting louder.

Basses

A bass wants to get in and out quickly. Keep the attack short for the edge and the sustain high so long notes stay heavy, then set a release under 100 ms so one note doesn't ring into the next. Overlapping notes are more of a problem in the low end than anywhere else, because they pile up into a drone that fights the kick. Lots of synth basses add a quick filter envelope too, so the front of each note is bright and the rest is round.

Leads

A lead has to cut through and still feel played, so the attack is short but not always instant. A few milliseconds take the edge off a sine or a soft square without making it feel late. The sustain is high, and the release is long enough that notes don't sound chopped off; for legato lines, something around 200 ms smooths over the gaps.

Presets are starting points

The envelopes in a synth's presets were set by whoever made the preset, for whatever they were playing at the time. Load a pad into a fast track and its slow attack may swallow half of every chord; load a pluck into a slow ballad and the notes may die before the chord changes. After picking a preset, the envelope is often the first thing worth adjusting, before the filter or the effects.

Play the part you've actually written while you do it. Notes that end before they reach full level want a shorter attack. A choppy part usually needs a little more release, and a blurry one needs less.

Envelope times also vary between synths. One synth's 500 ms release can sound longer than another's, because they use different curve shapes and some measure the time to silence while others measure the time to a fraction of the level. In this lab, decay and release are curves that get 98% of the way in the time shown. When you copy a sound from one synth to another, go by ear and use the numbers as a rough guide.

How to set an envelope, step by step

1. Play the real partUse the actual notes at the actual tempo, ideally with the drums playing. An envelope that sounds right on one held note can be wrong in a riff.
2. Set the attack firstStart at 0 and raise it until the front of each note is as soft as you want. Stop as soon as the notes start to feel late.
3. Decide whether the note holdsFor a held sound, set the sustain high. For a pluck, put it at 0 and use the decay to set how long each note rings.
4. Shape the front with the decayWith some sustain, a short decay adds an accent and a long one gives a slow sag after the start.
5. Set the release against the gapsListen between the notes. Lengthen the release until the ends stop sounding cut off, and back off once notes start running into each other.
6. Add a filter envelope if it sounds staticLower the cutoff, then bring up the envelope amount until the start of each note brightens. On a dull patch this often helps more than an effect would.

Common envelope mistakes

A release that muddies the low end

A long release can sound lovely on a bass played on its own and turn to mud under a full mix, because every note is still ringing when the next one starts. Keep bass releases short, and if you want more length, add it with reverb or delay on the parts that can take it.

Clicks at the start or end of notes

A 0 ms attack or release makes the level jump in an instant, and if the wave isn't passing through zero at that moment you hear a click. Raise either one to a couple of milliseconds and the note will still sound instant.

Turning the decay with the sustain at 100%

Decay only moves the level down to the sustain. If the sustain is at full, the decay knob does nothing at all, which is easy to miss when you're turning it and hearing no change.

Slow attacks on fast parts

A pad with a 1 second attack can't keep up with a part that changes chord on every beat. Shorten the attack or write longer notes.

Judging it on a single note

One held note shows you the attack, the decay and the sustain, but you only hear the release between notes. Check every envelope by playing the real part, at its real speed.

Training your ears to hear envelopes

The quiz under the lab plays the riff with hidden settings and asks whether the attack is fast or slow, or whether the release is short or long. The controls are covered and every round is level matched, so the only clue is the shape of the notes. For attack, listen to the very front of each note; for release, listen to the gaps between them. Eight rounds take a couple of minutes, and your best score is saved in your browser.

Other kinds of envelope

ADSR is the most common shape, but there are others. An AR envelope has only attack and release, so the note rises, holds at full while the key is down and falls when you let go. An AD envelope rises and falls on its own and ignores how long you hold the key, which suits drum sounds. Many soft synths add extra stages, such as a delay before the attack or a hold at full volume between the attack and the decay, and some let you bend each stage from a straight line into a curve.

Compressors have attack and release controls too, and the idea is related: how fast something reacts and how fast it lets go. On a compressor they decide how quickly it turns a sound down and back up again, which is covered in compressor attack and release. On a synth they shape the note itself.

An envelope is also one of two common ways to move a control automatically. An LFO moves a control up and down in a repeating cycle, while an envelope moves it once for every note you play, and plenty of sounds use both.

Questions people ask

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