Synth Filter Types Explained: Low-Pass, High-Pass, Notch, Comb

What does a filter do on a synth?

A synth filter removes part of the frequency spectrum coming from the oscillators, turning a bright, buzzy wave into something darker, thinner or more focused. Apple's Logic Pro guide puts it simply: after filtering, a brilliant-sounding sawtooth can become a smooth, warm sound without sharp treble.^[1]

Most filter sections share the same core controls: cutoff frequency, resonance, often drive and slope, plus modulation inputs from envelopes, LFOs and the keyboard.^[1] A synth filter works like the low-pass and high-pass bands of an EQ, with two differences that matter: it is built to be moved constantly, and it usually adds its own color. If you want the EQ side of the story, read what EQ is.

Cutoff and resonance: what do they actually change?

Cutoff sets the frequency where the filter starts working, and resonance boosts a narrow band right at that frequency. On a low-pass filter, lowering cutoff removes more highs and makes the sound darker; raising it lets the brightness back in.

Resonance, sometimes labeled peak, emphasizes the frequencies just below the cutoff, so the brightest remaining part of the sound stands out and filter sweeps start to sound vocal. FabFilter's synthesis guide adds two useful details: a steeper slope makes resonance more pronounced, and pushing resonance high enough makes the filter self-oscillate.^[2] A self-oscillating filter produces its own sine-like tone at the cutoff frequency, which is why cranked resonance can whistle or scream.

• Resonance 0-15% Clean tone shaping. Use on basses and pads where you want darker, not squelchier.
• Resonance 20-45% Audible peak that makes sweeps talk. The usual range for leads, plucks and wobbles.
• Resonance 50-80% Acid-style squelch and whistles. Watch the level; the peak can jump several dB as it passes a harmonic.
• Resonance near maximum Self-oscillation on many filters. Great for zaps and effects, risky on anything that has to sit still in a mix.

12 dB vs 24 dB per octave: which filter slope should you use?

Filter slope is how quickly a filter turns the volume down past the cutoff, measured in decibels of reduction per octave.^[3] A 12 dB/oct filter is gentle and keeps some buzz above the cutoff; a 24 dB/oct filter is steep and dark. Synths also describe slope in poles, where 2-pole equals 12 dB/oct and 4-pole equals 24 dB/oct.^[2]

Many modern synths offer all four. Serum's MG Low filter, for example, is a ladder low-pass offered at 6, 12, 18 and 24 dB per octave, with a topology based on classic Moog designs.^[4] If you cannot decide, start at 24 dB for anything low and 12 dB for anything that has to stay present in the mids.

Synth filter types and when to use each one

Each filter type keeps a different part of the spectrum, so pick the type by the part you want to hear. Apple lists the basic set as high-pass, low-pass, band-pass, band reject (also called notch) and allpass, which passes every frequency but shifts the phase.^[1]

Band-pass for lo-fi and focus

A band-pass filter cuts lows and highs together, leaving a narrow band that sits in a busy mix without spreading across the spectrum. FabFilter suggests a center around 1-2 kHz for an old-radio or small-speaker tone.^[2]

Comb filters: delay as a filter

A comb filter mixes a signal with a delayed copy of itself.^[5] The interference produces a row of regularly spaced peaks and notches that look like the teeth of a comb, and with delays of a few milliseconds a comb filter can model a vibrating string.^[6] Sweep the comb frequency for flanger-like jet sounds, or key-track it for pitched, metallic plucks.

What is filter key tracking?

Key tracking moves the filter cutoff with the note you play, so higher notes get a higher cutoff. Without it, a fixed cutoff sounds fine on low notes but increasingly muffled as you play up the keyboard, because more of each high note's harmonics sit above the cutoff.

In Ableton's Drift, the Key slider at 0.00 means MIDI notes have no effect on filter frequency, and at 1.00 the cutoff is lower for low notes and higher for high notes.^[7] Logic's Quick Sampler calls the same control Keyscale.^[8]

• 0% Basses and 808-style subs that should keep the same tone on every note.
• 30-60% Leads and plucks played across two octaves; keeps high notes from going dull without making them harsh.
• 100% Consistent timbre across the whole keyboard, and a self-oscillating filter that tracks pitch closely enough to play as a sine-like tone.

Filter envelope moves that shape real sounds

A filter envelope moves cutoff once per note, and its shape decides whether a sound snaps, swells or squelches. FabFilter's examples show both directions: a slow envelope attack gives an upward sweep that brightens the note, and a fast attack with a slow decay starts the filter open and closes it down.^[2] Full envelope theory lives in ADSR envelopes explained; the moves below are starting points.

1. Set the resting cutoff
   With the envelope amount at zero, lower cutoff until the sustained part of the note sounds right. That is where the envelope will return to.
2. Raise the amount
   Increase envelope amount until the start of the note opens as far as you want. Too much makes every note sound like the same zap.
3. Shape the decay
   Shorten decay at fast tempos so each note closes before the next one arrives.
4. Add velocity
   Map velocity to envelope amount so harder notes open brighter; this is the quickest way to make a patch feel played.

For repeating filter movement instead of a one-time shape, point an LFO at cutoff; see what an LFO is for rates and sync settings.

Why do ladder, SVF and diode filters sound different?

Filters with the same type and slope still sound different because each circuit design distorts, limits resonance and reacts to drive in its own way. Software filters model these designs so you can pick a character, not just a curve.

Ableton's Simpler shows the range well: its filter offers low-pass, high-pass, band-pass, notch and a Morph type at 12 or 24 dB, with circuit options including a clean design shared with EQ Eight, a ladder model with no explicit resonance limiting and a Sallen-Key model that soft-clips its resonance.^[7] Serum's list runs from the MG ladder filters to an Acid Ladder modeled on a diode ladder filter associated with acid music, plus comb, formant and notch types.^[4]

• Ladder Thick, smooth low-pass that tends to lose some low end as resonance rises. Classic for basses and warm leads.
• State-variable (SVF) Clean and flexible; one design gives LP, HP, BP and notch outputs. A good default for precise work.
• Diode ladder Rubbery and squelchy with resonance. The acid-bass character.
• Sallen-Key and similar Aggressive and gritty when pushed, with resonance that clips instead of ringing forever.

Drive feeds more level into the filter, which adds harmonics and changes how resonance behaves. Add drive before you reach for a separate distortion plugin; it often gives the grit you wanted while keeping the filter sweep intact. For more filter flavors as insert effects, see the best free filter plugins.