Cutoff frequency

On most synths the cutoff knob is labeled Cutoff, Freq or just Filter. Some show the setting in hertz, others as 0 to 100% or 0 to 127.

Play A2 on a sawtooth wave and you get the note at 110 Hz plus overtones at 220, 330 and 440 Hz and every multiple above. Through a low-pass filter, frequencies below the cutoff pass and those above it fade. Fully open, the saw buzzes. With the cutoff around 1 kHz it turns warm and round. Near 300 Hz only the note and its first overtone come through cleanly and it sounds dark and muffled, and below 110 Hz even the note itself starts to fade. On a high-pass filter the knob works the other way: raising it removes more of the low end.

The cutoff is not a hard edge. On a filter with no resonance, the level at the cutoff is already 3 dB down, and past it the level keeps falling at the filter’s slope, most often 12 or 24 dB per octave. Resonance boosts a narrow band right at the cutoff, so turning the knob with resonance up makes the sweep talk, like a wah pedal.

Synths also move the cutoff for you. An envelope can open it at the start of each note and close it again, which is how synth basses and plucks get their bright-then-dark shape; the ADSR guide covers that envelope. An LFO can rock it back and forth for a wobble. Key tracking raises the cutoff as you play higher, so notes keep about the same brightness from the bottom of the keyboard to the top.

In Gelato Synth, Cutoff is on the Filter page next to Resonance and Envelope to filter, and the LFO can target the filter. The filter cutoff and resonance guide has settings to try on any synth.