Filter Cutoff and Resonance Explained: How Synth Filters Work

A synth filter turns down part of a sound’s frequency range, and cutoff sets where that starts. On a low-pass filter, the most common kind, turning cutoff down removes the high overtones first, so the sound goes from bright and buzzy to warm and then muffled. Resonance boosts a narrow band right at the cutoff point, which adds a vocal, whistling edge that gets sharper the higher you turn it.

What does cutoff do on a synth?

Almost every sound is a stack of frequencies. Play A2 on a sawtooth wave and you get the note itself at 110 Hz plus overtones at 220, 330 and 440 Hz and every multiple above, up past the top of human hearing at about 20 kHz. Those upper overtones are what make a saw sound bright. A filter decides how many of them get through.

The cutoff frequency is the dividing line. With a low-pass filter, overtones below the cutoff pass at full level and those above it get quieter the further past the line they sit. How fast they fade is the slope, measured in decibels per octave. A 12 dB filter (two-pole) is gentle and leaves some sparkle. A 24 dB filter (four-pole, the slope of Moog’s ladder filter) is steep: each octave further above the cutoff comes out about 24 dB quieter, twice the drop of the 12 dB kind.

Some synths mark cutoff in hertz and others as 0 to 100%. With a sawtooth on A2, these points are easy to hear:

Cutoff also interacts with pitch. At a fixed cutoff of 800 Hz, A1 (55 Hz) keeps its first fourteen harmonics under the line and sounds full, while A5 (880 Hz) starts above the cutoff, so it comes out quieter and close to a plain sine. Key tracking fixes this: at 100% the cutoff rises one octave for every octave you play, so each note keeps the same brightness. If your high notes sound dull and your low notes sound buzzy, check key tracking before you touch anything else.

What does resonance do on a synth?

Resonance, also labeled Q, Emphasis or Peak, feeds part of the filter’s output back into its input. That creates a boost in a narrow band right at the cutoff frequency. With the filter still, you hear it as an extra edge or honk. Move the cutoff and the peak rides along with it, picking out each overtone as it passes, which is the vowel-like “yow” people mean when they call a filter squelchy.

Rough settings, since every synth scales the knob a little differently:

• 0 to 20%: a slight edge at the cutoff, barely noticeable on a sound that is not moving.
• 30 to 60%: the usual range for sweeps and basses, clearly vocal when the cutoff moves.
• 70 to 90%: a whistle that can overpower the note, good for acid lines and laser zaps.
• All the way up: on many filters the peak rings by itself as a sine wave at the cutoff frequency, even with no input. This is self-oscillation, and some players tune it and play it as an extra oscillator.

Two things to watch. On a classic four-pole ladder filter, more resonance also turns down everything below the peak (at full feedback the low end drops to about a fifth of its level), so a resonant bass can lose its weight. Keep resonance near 30 to 40% on basses that need to hit hard. And a resonant peak that lands on a loud overtone, or a filter ringing on its own, can jump in volume without warning. Keep your level down while you experiment, especially on headphones.

Low-pass vs high-pass vs band-pass filters

The filter type decides which side of the cutoff survives. Synths that offer more than one type often label them LP, BP and HP.

A low-pass filter is the default on most synths because acoustic sounds behave that way: a plucked string loses its high overtones before its low ones as it fades, so a closing low-pass sounds natural to the ear. Reach for it on basses and pads, and any time a raw saw or square is too harsh.

A high-pass filter is the mirror image. It keeps the top and thins out the bottom, so choosing high-pass vs low-pass comes down to which end of the sound you want to lose. In a song, high-pass does most of its work in the mix. Set it around 150 Hz on a pad or a strummed chord part and the low end stays clear for the bass and kick drum. Sweeping a high-pass slowly upward over the eight bars before a drop thins the track out and builds tension, a standard move in dance music.

A band-pass filter keeps only a slice around the cutoff and cuts both sides. Telephone lines pass roughly 300 Hz to 3,400 Hz, so a band-pass centered near 1 kHz is the quickest route to a phone-call or pocket-radio sound. On most synths the band-pass has a single cutoff knob that sets the center of the slice, and resonance makes the slice narrower. Notch, the rarer fourth type, cuts that same slice out. A phaser effect works by sweeping several notches at once, which is why a moving notch filter sounds like one.

What is drive on a synth?

Drive turns up the signal going into the filter, or into a saturation stage next to it, until it starts to clip softly. Clipping flattens the peaks of the waveform, and a flattened wave is richer in upper overtones, so the sound gets denser and brighter. A little drive, 10 to 30%, makes a bass or an electric piano sound thicker and warmer. Past about 60% it turns gritty and aggressive, the range for distorted leads and acid lines.

Drive changes how resonance behaves too. With resonance turned well up, the peak is the loudest part of the signal, so it clips first, and on a heavily driven filter the whistle sits back while the low end sounds fuller. On Moog’s ladder filter, pushing the input hard produces a distortion that became part of what people call the Moog sound.

Where the drive sits changes the result, and manuals do not always say. A quick test: turn drive up, then close the cutoff. If the grit fades as the filter closes, the drive comes before the filter and the filter is smoothing away the new fizz. If the grit stays, the drive comes after it. Drive also makes the patch louder, so lower the volume as you raise it, or you will judge the louder version as the better one every time.

Filter moves to try: sweeps, wah and acid

A filter left in one place works like a tone control. Filters get their character when the cutoff moves while notes play, and three things can move it: your hand, an envelope or an LFO.

On iPhone, our own Gelato Synth has a Filter page with a Low, Band or High pass switch, Cutoff, Resonance, Drive and Envelope to filter, and its LFO can target the filter for the wobble. Its Hold button latches the notes you play, which leaves both hands free for the sweep.

Start each recipe from a single sawtooth through a low-pass filter, 24 dB if your synth gives you the choice.

1. The slow sweep. Hold a C major chord (C3, E3 and G3), set resonance to about 40% and turn the cutoff from nearly closed to fully open over eight bars. Run it backward to fade a section out. It is the easiest filter move to hear, and the fastest way to learn what your resonance setting does.
2. The filter envelope. A filter envelope is an ADSR envelope aimed at the cutoff, so every note opens and closes the filter on its own. Set cutoff to about 20%, envelope amount to 60%, attack 0 ms, decay 250 ms and sustain 0, then play A1. Each note opens bright and snaps shut, the “bwow” of a classic synth bass. Stretch the attack to 150 ms and raise sustain to 50% and the same patch swells like brass.
3. The wah. Switch to band-pass, or stay on low-pass with resonance near 65%, and rock the cutoff between about 400 Hz and 2 kHz in time with the music. A guitarist’s wah pedal sweeps a band-pass filter with the foot, so this is the same move by hand. It suits choppy, funky rhythm parts.
4. The wobble. Point an LFO at the filter cutoff, choose a sine or triangle shape and sync its rate to the tempo. Quarter notes give a slow pump, eighth notes a busier wobble. If your LFO has no sync, set the rate by number: at 140 BPM an eighth-note wobble is about 4.7 cycles a second.
5. Acid. Sawtooth or square, resonance at 75%, cutoff low, envelope amount high and a short 150 ms decay. Play a sixteenth-note line that jumps between A1 and A2, then turn cutoff and decay slowly while it loops. This is the sound of the Roland TB-303 Bass Line from 1981, a small box with knobs for cutoff, resonance, env mod, decay and accent but no drive, so leave drive at zero for the clean squelch and push it past 60% for a harder, distorted take. Phuture’s “Acid Tracks” from 1987 is often credited as the first acid house record.