Equalizers are audio processing tools that allow the boosting or cutting of sound frequencies across the spectrum. This allows a producer or engineer to highlight the desirable parts of a sound, or reduce the prominence of unwanted ones. For instance, boosting the low end on a kick drum would give it more weight. Cutting the mid range of an electric guitar would allow the bass and treble frequencies to ring out without being muddied. Removing sibilance, also known as "De-essing," is a common application for equalizers on vocal tracks.
Solo the vocal track and play it through. Look for a section that is particularly full of sibilance. The high-frequency "s" and "t" sounds are usually the worst culprits. Sibilance can be heard as a hissing or clicking sound that is usually disproportionately loud compared to the rest of the vocals. If your recording system allows it, set up a loop that constantly repeats the problem section.
Engage the equalizer as an insert on the vocal channel in the mixer. It's important that you don't set the equalizer up as a remote send, as you are looking to remove the sibilant frequencies. Setting up your EQ as a send will blend the dry and processed signals, which won't have as clean an effect. This will require you to remove more of the high end, which will give your vocals a dull sound.
Enable a high band pass filter on the equalizer; if your unit has four bands, you should select the third one. Roll on about three to four decibels of additional gain and reduce the "Q" setting to make a narrow spike.
Use the gain control to roll the spike left and right through the frequency band. Focus on the range of 9,000 to 10,000 Hertz; this is where the "ess" sounds reside.
When you hear a dramatic increase in the prominence of the sibilance, you've found the correct spot. Roll off the gain of the spike and reduce it until the sibilance sounds less pronounced, but the rest of the vocal's high frequencies still remain.
Turn off the track solo function, so the rest of the parts return and you are able to hear the vocal part in context. Continue to make fine adjustments using the gain and Q settings until the sibilance is reduced, but the high-frequency detail of the vocals still remains.
There are dedicated "De-esser" plug-ins that specialize in removing sibilance. They are essentially equalizer units that focus on the standard problem frequencies.