What is a filter envelope on a synth?
Most synths run their oscillator through a low-pass filter, which lets the low frequencies through and turns the high ones down. Where that starts happening is the cutoff. Set it high and the note is bright; set it low and the note is dark. On its own, though, the cutoff stays wherever you leave it, so every note has the same tone from start to finish.
Real instruments don't behave like that. Pluck a guitar string and the first instant is full of high harmonics, which die away faster than the low ones, so the note gets darker as it rings. A filter envelope lets a synth do the same thing. It's a second envelope, separate from the one that shapes the volume, and it moves the cutoff instead. Every time you press a key it runs again, so every note gets its own sweep.
If you're new to envelopes in general, the ADSR envelopes lesson covers attack, decay, sustain and release on the volume. This page is about pointing an envelope at the filter, and about the cutoff it moves.
What is filter cutoff?
The cutoff is the frequency where a filter starts to remove sound. A low-pass filter passes what's below it and turns down what's above it, and the further above the cutoff a frequency sits, the more it's turned down. The filter in the lab is 3 dB down at the cutoff and falls by 12 dB for every octave above it, which is a gentle slope. Many synth filters are steeper, at 24 dB per octave, and the filter slopes lesson compares them.
On a synth the cutoff decides how much of the oscillator's brightness you hear. A saw wave at 220 Hz has harmonics at 440, 660 and 880 Hz, and they carry on at every multiple of 220 Hz past 10 kHz, so with the cutoff at 12 kHz almost all of them pass and the note is bright and buzzy. Bring it down to 500 Hz and only the first two get through at close to full level. The rest fade fast, and the note sounds as if it's coming from the next room.
The cutoff on its own gives a note a fixed tone. To make the tone move you can turn the knob while you play or automate it, but an envelope or an LFO will do it for you, and of those the envelope is the one that follows each note. For low-pass and high-pass filters as mixing tools, see high-pass and low-pass filters.
Reading the lab
The lab above is a small synth with one oscillator, one low-pass filter and two envelopes. Play it from the keyboard on 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 amp attack and release are locked on this page, and the lesson starts with the amp sustain at 100%, so the volume doesn't move and the changes you hear come from the filter.
On the big screen, the violet dashed line is the filter envelope. It shows how many octaves from the cutoff the filter sits at each moment. The dotted amber line halfway up is the cutoff itself, with 5 octaves above it at the top of the screen and 5 below it at the bottom. The white line is the amp envelope and the pink shape is the volume of a real note rendered through the synth.
While nothing is playing, the spectrum at the bottom shows each harmonic of an A at 220 Hz as a bar, and while you play it follows the live sound. The amber line is the cutoff, and when the filter envelope is on, a violet dashed line marks how far it opens at the start of the note. The Right now panel describes your settings with numbers measured from the rendered note. Its Harmonics count is how many are within 40 dB of the loudest.
Filter envelope controls
Every synth names these a little differently, but the same two ideas turn up on all of them: how far the envelope moves the cutoff, and how long it takes.
Envelope amount
The amount sets how far the envelope pushes the cutoff at its peak. You'll see it labelled Env Amt, Env Mod or Env Depth, and the Minimoog calls it Amount of Contour. Some synths show it as a percentage, some in semitones, and the lab shows it in octaves, where one octave doubles the frequency.
The maths is simple once it's in octaves. With the cutoff at 500 Hz, an amount of 1 octave opens the filter to 1 kHz at the start of each note. Two octaves takes it to 2 kHz and three to 4 kHz, because every extra octave doubles the peak again.
At zero the envelope still runs on every note and moves the cutoff by nothing at all, so that's the first thing to check when a filter envelope seems to do nothing.
Filter decay
The decay is how long the filter takes to close from its peak back down to the cutoff. Short decays, from about 50 to 200 ms, give a quick bright click on the front of each note, which is most of what makes a synth pluck or a stab sound plucked. Somewhere between 300 and 800 ms you hear the brightness drain away as a "yow" or a "wah". Past a second it becomes a slow sweep you can follow by ear, and on a fast riff the next note restarts it before it gets anywhere.
The lab closes the filter in a straight line in octaves, which is how your ears judge pitch. With a 3 octave sweep over 400 ms, the filter is 1.5 octaves above the cutoff after 200 ms. From 500 Hz that's about 1.4 kHz.
How cutoff and amount work together
The cutoff sets where the filter ends up and the amount sets how far above that it starts. Two patches can open to the same frequency and sound nothing alike. A 500 Hz cutoff with 3 octaves of envelope starts at 4 kHz and falls all the way back to 500 Hz. A 2 kHz cutoff with 1 octave also starts at 4 kHz, but only falls to 2 kHz, so the note stays brighter and the sweep is a third of the size.
That's why turning the cutoff on a patch with a filter envelope can feel like it changes two things at once. Raise it and the whole sweep moves up, start included. If you want more snap without a brighter tail, leave the cutoff where it is and raise the amount instead.
A full ADSR on the filter
The lab's filter envelope is the simplest kind: it jumps open the instant a note starts and closes over the decay. The Roland TB-303 works much the same way, with two knobs for its filter envelope, Env Mod and Decay. Most synths give the filter a full ADSR of its own instead, and each stage does the same job it does on the volume, with brightness in place of level.
The filter attack is how long the filter takes to open to its peak. Between 30 and 100 ms, notes brighten as they start, which is a big part of a synth brass sound; longer attacks give a pad that blooms. The decay then brings it down to the sustain, which is how far open the filter stays while you hold the key, as a share of the amount. With the amount at 3 octaves, a 50% sustain holds the filter 1.5 octaves above the cutoff.
The filter release is how long the filter takes to close back to the cutoff after you let go. When the amp release is longer than the filter release, each note darkens as it fades out.
Many synths also let the amount go below zero. A negative amount pulls the cutoff down instead of up, so you set the cutoff high and the envelope closes the filter at the start of each note and lets it open again during the decay. It's less common than a positive envelope, and it gives notes a reversed, sucked-in start. The lab's Filter env knob goes below zero on this page, and the last step uses it.
On soft synths with a modulation matrix you often build the filter envelope yourself. In Serum, for example, ENV 1 is wired to the volume and you drag ENV 2 onto the filter cutoff, and the depth of that connection is the envelope amount.
Resonance and the filter envelope
Resonance boosts a narrow band right at the cutoff. With the filter still, it gives the sound a nasal or vowel-like colour. Once a filter envelope is sweeping the cutoff, that boost sweeps too, and you hear it as a pitch falling with each note.
That falling peak is where the squelch of acid bass comes from. A low cutoff, a big envelope amount, a short decay and a lot of resonance make each note start with a "yow" that drops into the bass. In the lab, a 250 Hz cutoff with 4 octaves of envelope opens to 4 kHz, and at 65% resonance the filter adds a peak of about 12 dB at the cutoff as it falls.
Watch the level when you do this. A resonant peak passing over a loud harmonic can jump out by several decibels, and low notes get louder than high ones as the peak lands on their lower harmonics. The filter resonance lesson goes into the peak itself.
Filter envelope vs amp envelope
The amp envelope shapes how loud a note is and the filter envelope shapes how bright it is. Having both lets those two follow different paths, and the bass and the pluck below both depend on that.
Take a bass. With the amp sustain at 100% the note keeps its full weight for as long as you hold it, while a short filter decay makes only the front of the note bright. You get a clear attack that cuts through the mix and a round, dark body underneath that leaves room for the kick and the vocal. Driving the volume with the same short envelope would make the bass thin and choppy.
For a pluck, the two envelopes work together. The amp sustain goes to zero so the note dies away on its own, and the filter decay is set close to the amp decay so the note darkens as it fades. Set the filter decay much shorter than the amp decay and the note goes dull while it's still loud, which sounds more like a muted string. Set it much longer and the tail stays bright, which can sound artificial.
Key tracking and velocity
Two more settings change how far the filter opens, and most synths have at least one of them near the filter.
Key tracking, sometimes called keyboard follow, moves the cutoff with the note you play. At 0% the cutoff stays put, so high notes have fewer harmonics under the cutoff and sound duller than low ones. At 100% the cutoff rises an octave when you play an octave higher, which keeps the same number of harmonics through the filter on every note. Around 50% is a common middle ground for basses and leads, so the top notes don't turn shrill.
Velocity can scale the envelope amount, so playing a key harder opens the filter further. Acoustic instruments behave like this: hit a piano key harder and the note comes out brighter as well as louder. If your synth has the option, a small amount of velocity on the filter envelope makes your harder notes brighter as well as louder, the way a piano behaves. The lab has neither setting, so try them on your own synth.
Filter envelope settings for common sounds
These are starting points chosen so the filter envelope is obvious. Amounts are given in octaves; if your synth uses a percentage, start in the middle and adjust by ear.
| Sound | Cutoff | Amount | Filter envelope | Resonance |
|---|---|---|---|---|
| Pluck | 300 – 800 Hz | 2 – 4 oct | Instant attack, 150 – 400 ms decay, no sustain | Low |
| Punchy bass | 150 – 400 Hz | 2 – 3 oct | Instant attack, 100 – 300 ms decay, low sustain | Low to medium |
| Acid bass | 150 – 400 Hz | 3 – 5 oct | Instant attack, 100 – 300 ms decay | High |
| Brass | 500 Hz – 1.5 kHz | 1 – 2 oct | 30 – 100 ms attack, medium decay, sustain around half | Low |
| Pad | 800 Hz – 2 kHz | 0.5 – 1.5 oct | Slow attack or slow decay, over a second | Low |
| Lead | 1 – 3 kHz | 1 – 2 oct | Instant attack, 300 – 800 ms decay | Low to medium |
How to set a filter envelope, step by step
- Turn the amount to zeroStart with the filter envelope out of the way, so you can hear the cutoff on its own.
- Set the cutoff for the darkest the note should getPlay the real part and lower the cutoff until the body of each note has the tone you want. This is where the filter will settle, so it sets the colour of the sound.
- Bring the amount upRaise it until the start of each note has the brightness you want. Two or three octaves is a good first try.
- Set the decay against the tempoShorten it for a snap on fast parts and lengthen it for a sweep on slow ones. If the next note always arrives before the filter has closed, either that's the sound you want or the decay is too long.
- Add sustain and release if your synth has themSustain keeps held notes brighter, and the release decides how the tone changes as each note fades out.
- Add resonance lastA little brings out the sweep. Check the level on low notes as you raise it.
Common filter envelope mistakes
A cutoff that's already wide open
If the cutoff sits at 10 kHz, there's barely any brightness left for the envelope to add, and turning up the amount seems to do nothing. A filter envelope needs a filter that starts mostly closed, so bring the cutoff down to a few hundred hertz before you judge it.
A front so bright it clicks
A very short filter decay with a big amount puts a sharp, bright tick on every note. On a bass played on its own that can sound exciting, and under drums it can turn into a clicky layer that clashes with the hi-hats. Lower the amount or lengthen the decay a little until it sits.
Turning the decay with the amount at zero
The filter decay knob only does something once the envelope is connected. With the amount at zero, you can turn it from end to end and hear nothing change.
Forgetting that legato doesn't retrigger
Many mono synths have a legato mode where overlapping notes don't restart the envelopes. Play a smooth line in that mode and only the first note gets its filter sweep. If the sweep keeps disappearing, leave a small gap between notes or switch the envelope to retrigger on every note.
Judging the sweep on one note
A long filter decay can sound lovely on a held note and do almost nothing in a busy riff, because every note restarts it. Set the filter envelope while you play the real part at its real tempo.
Training your ears
The quiz under the lab plays the riff with a hidden filter setting. A round either asks whether the cutoff is bright or dark, 6 kHz against 600 Hz, or whether the filter envelope is snappy or slow, 120 ms against 900 ms with 3 octaves of amount. The volume never changes and every round is level matched, so the filter is the only difference. For the cutoff, listen to how much buzz is left on top of each note; for the decay, listen to how long each note stays bright after it starts. Eight rounds take a couple of minutes, and your best score is saved in your browser.
Once that feels easy, set an amount and a decay in the lab without looking at the numbers, play the riff, then guess the decay before you check. Telling 100 ms from 300 ms is a good next target.
Questions people ask
What does filter envelope amount do?
It sets how far the envelope moves the cutoff at its peak. With the cutoff at 500 Hz, an amount of 1 octave opens the filter to 1 kHz, and 3 octaves opens it to 4 kHz. At zero the envelope still runs but moves nothing.
Why does my filter envelope do nothing?
Usually the cutoff is already so high that the filter is open before the envelope starts, or the amount is at zero. Lower the cutoff to a few hundred hertz, then turn the amount up.
Should the filter envelope match the amp envelope?
It doesn't have to. A bass with a full amp sustain and a short filter decay keeps its weight while the bright front snaps away, which you can't get with one envelope driving both.
What is a negative filter envelope?
An amount below zero, which many synths allow. The envelope pulls the cutoff down instead of up, so with a slow attack the note gets darker as it starts and then opens up again during the decay.
What is filter key tracking?
A setting that moves the cutoff with the note you play. At 100% the cutoff rises an octave when you play an octave higher, so high notes come out as bright as low ones; at 0% high notes sound duller through the same filter.
Is a filter envelope the same as automation?
Automation moves the cutoff along the song timeline, once, wherever you draw it. A filter envelope restarts on every note you play, so each note gets the same movement. You can use both at once, automating the cutoff slowly over a build while the envelope adds a sweep to each note.