What Do Filters Do?

Filters shape sound by allowing certain frequencies to pass through while cutting others. They’re a core part of subtractive synthesis — the "subtractive" part — because you start with a harmonically rich waveform and use filters to carve it into something new.

The most common filters are low-pass, high-pass, and band-pass, but there’s plenty of variation depending on the filter’s design (ladder, state-variable, diode, etc.). Filters can be smooth or aggressive, clean or gritty, and can completely change the vibe of a patch.

Types of Filters

• Low-Pass (LPF): Cuts high frequencies, lets the lows through. Essential for mellowing out bright waveforms.
• High-Pass (HPF): Cuts low frequencies, lets the highs through. Useful for thinning out bass or creating sharp edges.
• Band-Pass (BPF): Passes a narrow frequency range, cutting both lows and highs. Great for vocal-like tones and sweeps.
• Notch (Band-Reject): Cuts a narrow range, lets the rest through. Used for hollow or phasey sounds.
• Formant / Comb / Resonator: More complex or specialized filter types, often for vowel-y or metallic effects.

Key Features

• Cutoff: The frequency where the filter starts cutting. Usually modulated by envelopes, LFOs, or hands.
• Resonance (Q): Boosts the frequencies near the cutoff point. Can create whistly peaks or self-oscillation.
• Drive / Saturation: Some filters add warmth or distortion when pushed — depends on the circuit.

Filters are one of the most expressive tools in a synth. A static waveform can come alive with just a resonant sweep or some cutoff modulation. And no two filters sound quite the same — especially in analog gear.