Sibilance is the burst of energy an s, t, or sh puts into the 5 to 9 kHz region. Close microphones and bright compression push it further ahead of the rest of the voice, and lowering it with an EQ costs the whole take its top end. A de-esser only reacts while the harsh band is actually loud.
Frequency
The frequency control sets the crossover the detector listens above. The range covers 2 to 14 kHz on a log scale.
To find it, raise the range to its maximum and sweep. The setting is right when the s sounds duck hard and vowels stay put. Then bring the range back to something you would use.
Threshold and range
Threshold is the level the sibilant band has to exceed before anything happens. Range caps how far the gain is allowed to fall once it does.
A de-esser is a limiter on that band, not a compressor with a ratio: everything above the threshold comes back to it. Range is what stops that from being absolute. At 6 dB the harshest consonants are 6 dB quieter and still audible, which is usually what you want on a lead vocal. At 20 dB they are pushed most of the way out of the take.
Release
Release decides how quickly the gain returns after the consonant passes. The attack is fixed at 1.5 ms, because a slower one would let the front of every s through before the gain moved.
Short release times of 40 to 80 ms suit fast speech. Longer settings hold the reduction across a whole word, which sounds smoother on a sustained note but dulls the syllables after it.
Split band vs wideband
In split band mode the band above the crossover is reduced and everything under it is passed through untouched, so a range of 0 dB is bit-for-bit the source file.
Wideband mode applies the same gain to the whole signal. On a solo vocal that reads as the take briefly stepping back rather than as a filter opening and closing, which some engineers prefer. On a busy mix it also ducks the instruments around the voice.
Example: a bright podcast take
Frequency 6.5 kHz, threshold -30 dB, range 8 dB, release 60 ms, split band.
That is enough to stop the s sounds from stinging in headphones while leaving the consonants intelligible on a phone speaker.