Resonance

Every synth filter has a cutoff frequency, and resonance sets how strongly the filter emphasizes that spot. On most analog-style filters it works by feeding part of the filter’s output back into its input, which builds a peak at the cutoff. Synths label it Resonance, Res, Q, Emphasis or Peak.

With the cutoff standing still, resonance adds an edge or a honk. Move the cutoff and the peak moves with it, picking out each overtone as it passes, which gives a filter sweep its “yow”. Rough settings, since every synth scales the knob differently:

• 0 to 20%: a slight edge, barely audible on a held note.
• 20 to 60%: the usual range for basses and sweeps.
• 60 to 90%: a whistle that can overpower the note, used for acid lines and laser zaps.
• All the way up: many filters ring on their own as a near-sine tone at the cutoff frequency, even with no note playing. This is called self-oscillation.

Acid house basslines get their squelch from the resonant filter of the Roland TB-303, with an envelope sweeping the cutoff on every note. The synth bass guide has an acid recipe, and the filter cutoff and resonance guide goes further into sweeps.

Two cautions. On many filters, more resonance also thins out the low end, so a resonant bass can lose its weight; keep it around 30 to 40% on basses that need to hit hard. And a resonant peak can jump in volume when it lands on a loud overtone, so keep your level down while you experiment, especially on headphones.

In an EQ plugin, Q sets how narrow a boost or cut is, and on many EQs a higher Q on the low-cut or high-cut band adds the same kind of bump at the cutoff. Gelato Synth puts Resonance on its Filter page, beside Cutoff and Drive.