Resonance is a setting that emphasizes the area around a filter’s cutoff. It is sometimes labeled Res. When you move a low-pass or high-pass filter, a peak forms at the boundary frequency and that part of the sound is emphasized. In synth sound design it gives character, but in a mix it can lead to harsh peaks and jumps in level.
What gets emphasized
Raising resonance makes the area near the cutoff stand out. With a low-pass, a specific frequency rings out as the sound gets darker; with a high-pass, the boundary where the low end is removed is emphasized. At some settings it becomes a sharp, whistling tone that changes the character of the source dramatically.
Using it for sound design
Synth filter sweeps, sound effects, and build-ups can all use resonance to create movement and character. Combining it with cutoff automation is especially effective. Because peaks can suddenly get much louder when you push it, keep an eye on your level meters too.
Things to watch in a mix
If you are only cleaning up unwanted lows or highs, a low resonance setting is usually enough. Push it too far and the band you meant to reduce with EQ stands out at the boundary instead. Check across the whole song that the same frequency is not being emphasized on several tracks at once.
How it differs from Q
Q is a setting related to the width over which a peak EQ or a filter acts. Resonance relates to how much the area around the cutoff is boosted. The relationship between the two differs by unit, but both share the trait that narrower and stronger processing changes tone and level more easily.
Adjusting it safely
After raising resonance, lower the output to compare at a matched level, and check in both stereo and mono. Keep it clear whether you are going for an effect or for cleanup. You can review the basics of EQ in the article on EQ.
