An envelope shapes how a sound changes over the life of a single note, from the moment you press a key to after you let go. The classic ADSR envelope has four stages: attack, decay, sustain and release. Three of them are times and one, sustain, is a level. Once you know which is which, the whole thing makes sense.
Applies to almost any synth, sampler or plugin, in any DAW or a browser instrument. Attack, decay, sustain and release mean the same thing wherever you meet them. None of this is tied to one product.
An envelope shapes how one setting changes over the life of a single note, from the instant you press a key to some point after you release it. It is a moving control rather than a fixed value: instead of a sound staying at one volume or one brightness the whole time, an envelope tells that setting how to rise, hold and fall each time a note plays.
The most common job is volume. When you press a key on a real instrument the sound does not appear at full level instantly and then cut off the moment you stop; it swells in, sits for a while, and dies away. An envelope is how a synth reproduces that shape, and it is one of the basic building blocks inside almost any synthesizer or sampler. If you are still getting a feel for how those instruments fit together, our guide on what a synthesizer is sets the scene.
The classic version is called ADSR, after its four stages: attack, decay, sustain and release. Some envelopes have more stages and some have fewer, but ADSR is the shape you will meet most often, so it is worth learning once.
They are the four stages of the envelope, in the order they happen. Attack is the time the sound takes to rise from zero up to its peak after you press the key. Decay is the time it then takes to fall from that peak down to the sustain level. Sustain is the level the sound holds at while you keep the key held down. Release is the time it takes to fall from that level back to zero once you let the key go.
| Stage | Time or level | What it does |
|---|---|---|
| Attack | A time | How long the sound takes to rise from silence up to its peak after the key is pressed. Short is an instant onset; long is a slow fade-in or swell. |
| Decay | A time | How long the sound takes to fall from the peak down to the sustain level. Sets how quickly the initial hit settles into the held part of the note. |
| Sustain | A level | The level the sound holds at for as long as the key stays down. This is a level, not a time. High keeps the note full while it is held; zero means the sound has already faded out by the end of the decay. |
| Release | A time | How long the sound takes to fall from wherever it is back down to zero after the key is let go. Short stops promptly; long lets the note ring on after you release. |
The order follows what happens to a note in time. Press the key and the sound climbs through the attack to its peak, drops through the decay to the sustain level, and stays there for as long as you hold. Let go and the release carries it back down to silence. If the sustain is set to zero there is no held level to rest on, so the sound rises, falls through the decay and is gone while the key is still down, which is exactly what you want for a short, plucky sound.
Sustain is a level; attack, decay and release are times. This is the one point that trips people up, so it is worth stating plainly. Attack, decay and release each set how long a stage takes, measured in milliseconds or seconds. Sustain sets how loud the sound sits while you hold the key down, not how long it lasts.
Put another way, three of the four controls answer the question "how fast", and one answers the question "how high". Turning the sustain up does not make a note last longer; it raises the level the note holds at while the key is down. The note lasts exactly as long as you hold the key, whatever the sustain is set to. Only the release changes what happens after you let go.
A quick way to keep it straight: attack, decay and release move the sound between levels, and sustain is one of those levels, the one it rests on in the middle. The same idea holds when an envelope is steering something other than volume, such as a filter. In that case sustain is still the held level, just measured in brightness or wherever you have pointed the envelope, rather than in loudness.
Sustain does not control time. It is easy to assume the sustain control decides how long a note lasts, because "sustain" means "to hold" in everyday speech. In an ADSR envelope it does not. The note holds for exactly as long as you hold the key, and sustain only sets the level it holds at while you do. How long a sound rings on after you let go is the release, and that one is a time.
Anything whose shape you want to change over the course of a note. The two most common targets are volume and filter cutoff, and on many synths an envelope can be pointed at almost any parameter. Here is what each of those does.
Shaping volume. This is the default use, and it is what decides whether a sound reads as a stab or a wash. A fast attack with a short decay and a low sustain gives a pluck or a stab: the sound appears at once and drops away, like a plucked string or a short mallet hit. A slow attack gives a pad or a swell that breathes in gradually, which is the soft bed our guide on how to make a synth pad is built around. A long release lets the sound ring out and overlap after you release the key, like a note left to decay on a piano with the pedal down.
Shaping a filter. Point an envelope at a low-pass filter's cutoff instead of at volume and it changes the tone over the note rather than the loudness. A fast attack that opens the filter followed by a quick decay back down gives a bright click at the start of each note that then mellows, which is a lot of what makes a bass or a synth pluck feel punchy. A slow attack on the filter makes a sound open up and brighten gradually as it plays.
Almost anything else. On many synths an envelope is a general-purpose modulator you can route to pitch, to an effect amount, to the position in a wavetable, or to how much one oscillator shapes another. The stages still mean the same thing, a rise, a fall to a held level, and a fall back once you let go; only the parameter they are steering has changed.
Most synths give you at least two envelopes: one wired to the volume so the note gets its overall shape, and a second free to modulate the filter or something else. They run at the same time and do not have to match, so you might hold a long, slow volume swell while a faster envelope opens and closes the filter underneath it. Reaching for a separate envelope per job is normal, not a trick.
Reading about attack and release only gets you so far. The fastest way to understand an envelope is to move one control on a sound that is already playing and hear what it does.
This guide is the idea. ARGISH is a self-playing synth where you can shape the envelope and hear the result in real time: pull the attack short and the same patch plays as a stab, stretch it out and it becomes a slow pad that swells in, all while it keeps playing. It is the quickest way to feel what attack, decay, sustain and release actually do. It runs as an Audio Unit, a VST3, an AAX for Pro Tools and a standalone app.
It also runs free in your browser, no install and no account, which is the fastest way to hear a stab turn into a pad as you move a single control.
Everything on this page also works by hand on almost any synth, which is rather the point of writing it down.
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