Low-pass filter

On a synth, the low-pass filter sits after the oscillators and trims their upper overtones. A sawtooth wave on A2 has its note at 110 Hz plus overtones at 220, 330 and 440 Hz and every multiple above that, and those high overtones are what make it buzz. As you turn the cutoff down, the overtones fade from the top and the saw gets steadily darker, until it sounds like music through a closed door. Turn it back up and the buzz returns.

On most synths the low-pass is the default filter, so the Cutoff knob controls it, and a filter type switch marks it LP or LPF. Mixing EQs often label the same thing High Cut.

How steeply it cuts is the slope, measured in decibels per octave. A 12 dB (two-pole) low-pass is gentle and leaves some sparkle. A 24 dB (four-pole) low-pass, the slope of Moog’s ladder filter, cuts harder and sounds darker and heavier. Add resonance and the filter boosts a narrow band right at the cutoff, which gives filter sweeps their squelch.

Low-pass filtering sounds natural because acoustic instruments behave the same way: a plucked string loses its high overtones faster than its low ones as it fades. A filter that opens at the start of each note and then closes copies that, and many synth basses and plucks are built on it. The filter cutoff and resonance guide has settings to try, and how to make a pluck synth builds a whole sound around it.

Its mirror image is the high-pass filter, and a band-pass filter keeps only a slice around the cutoff. Gelato Synth has all three on its Filter page (Low, Band or High pass), next to Cutoff and Resonance.