A filter removes or emphasises a range of frequencies to shape the tone of a sound. It is how you make something darker or brighter, thinner or fatter, without touching the notes. On a synth the filter is the main tone-shaping stage, and its two headline controls, cutoff and resonance, sit behind a great deal of what people recognise as the synth sound.
The idea applies to any synth, sampler or plugin, in any DAW or a browser instrument. Cutoff, resonance and slope mean the same thing wherever you meet them. None of this is tied to one product.
A filter is a control that removes or emphasises a range of frequencies in a sound, so it changes the tone without changing the notes. Every sound is a stack of frequencies, from low rumble at the bottom to bright air at the top. A filter turns some of those frequencies down, or lifts a few of them up, and the sound gets darker, brighter, thinner or more focused as a result. Nothing is added that was not already there; the filter only decides how much of each frequency you hear.
On a synth the filter is the main tone-shaping stage. It sits after the oscillator that makes the raw waveform, taking that bright, buzzy starting material and carving it into something musical. That is why the filter, more than almost any other control, decides the character of a synth sound. If you want to see where it fits in the wider signal path, our guide on what a synthesizer is lays out the whole chain.
Think of a filter as a tone control with an aim. You point it at a part of the frequency range and tell it to turn that part up or down. The sound that comes out is the same sound, minus or plus the frequencies you chose. A synth filter does this boldly, to shape a whole voice; a mixing tool does it gently, to fix a finished one. The mechanism underneath is the same.
Cutoff and resonance are the two main controls on a filter. The cutoff sets where the filter acts, and the resonance sets how much it emphasises that spot. A third control, the slope, sets how sharply the filter works. Here is what each one does.
The cutoff is the frequency where the filter starts to take effect. On the most common filter, a low-pass, frequencies below the cutoff pass through more or less untouched, and frequencies above it are turned down. Move the cutoff down and more of the top is removed, so the sound gets darker and rounder; move it up and the sound opens out and brightens. On other filter types the cutoff marks a different edge, but it always names the point where the filtering begins. Slowly sweeping the cutoff up or down is one of the most recognisable sounds in electronic music.
Resonance, sometimes labelled Q, is a boost applied in a narrow band right at the cutoff. A little resonance adds a vocal or nasal emphasis and makes the cutoff point stand out. Push it higher and that emphasis grows into a whistle, and on many filters, if you push it far enough, the filter begins to self-oscillate, producing a pure tone of its own even with no sound going in. Resonance is what gives a filter sweep its squelchy, singing character. Used gently it adds focus; used hard it becomes an effect in its own right.
The slope, or steepness, sets how quickly the filter turns down the frequencies past the cutoff, measured in decibels per octave. A gentle slope removes them gradually and sounds subtle and natural; a steep slope cuts them away quickly for a stronger, more obvious effect. The two most common values are 12 dB per octave, which is gentler, and 24 dB per octave, which is steeper. Some filters let you choose between them, often labelled as 2-pole for the 12 dB slope and 4-pole for the 24 dB one.
The type of filter describes which frequencies it removes and which it lets through. There are four you will meet most often. The low-pass is by far the most common, but each has a job it does best:
| Filter type | What it does | Often used for |
|---|---|---|
| Low-pass | Removes the frequencies above the cutoff and lets the lows through. The most common filter by far. | Warming or darkening a sound, taming brightness, the classic synth cutoff sweep |
| High-pass | Removes the frequencies below the cutoff and lets the highs through. The mirror of a low-pass. | Thinning a sound, clearing out low rumble, making room underneath other parts |
| Band-pass | Keeps a band of frequencies in the middle and removes both the lows and the highs. | Telephone and radio voices, thin and focused tones, wah-style effects |
| Notch | Removes a narrow band and lets everything on either side through, the opposite of a band-pass. | Cutting a single problem frequency, hollow phaser-like sweeps |
Many synths put several of these on one filter with a switch, so you can try a low-pass, a high-pass and a band-pass on the same sound and hear which suits it. When a filter offers a resonance control, it usually works across whichever type you pick, so you can add that emphasis at the cutoff on any of them.
You shape a sound with a filter by setting the cutoff for the basic tone and then moving it over time, usually with an envelope or an LFO, so the tone changes as the note plays. A still filter gives you a fixed tone; a moving one gives you the classic synth sound.
A fixed setting is the starting point. Pull the cutoff down for something warm and muffled, or open it up for something bright and present, and add a touch of resonance if you want the cutoff to sing a little. But the reason the filter sits at the heart of synth sound design is that the cutoff can move while a note is playing. Wire an envelope to the cutoff and the tone can snap open at the start of a note and close again as it fades, which is where a great many plucky, vocal and sweeping synth sounds come from. Wire an LFO to the cutoff instead and the tone sweeps back and forth on its own, the wah and wobble you hear in bass and electronic music. This movement, more than any single setting, is what people mean by the classic synth sound.
A mixing EQ works on exactly the same principle as a filter: it turns bands of frequencies up or down. The difference is the aim. A synth filter is built to shape and move the whole voice of an instrument, often boldly and often while it plays. A mixing EQ is built for surgical tone control, small and precise adjustments so a finished sound sits right against everything else in a track. Same tool at heart, pointed at a different problem. Our guide on what EQ is and how to use it covers that side.
A filter clicks into place far faster when you can hear the cutoff open and the resonance sing while a sound is playing. Reading the theory gets you the map; hearing it move is what makes it stick.
ARGISH is a self-playing synthesizer. The filter controls this guide describes, the cutoff and the resonance, are right there on its panel, and because it plays itself you can hear the filter open and close rather than only read about it. Set a mood and a key, build a progression on the chord wheel, and it composes and evolves in the background while you sweep the cutoff and hear the tone move. It runs in your browser, or as a plugin in your DAW: an Audio Unit, a VST3, an AAX for Pro Tools and a standalone app.
You can hear it free in your browser first, no install and no account, then decide.
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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