Here's how to prevent those pesky esses from ruining your tracks!
Vocal sibilance can not only sound distracting, it can also interact unhelpfully with some of your mix processing. And it can prove a frustratingly stubborn problem. So how best can you stop over-prominent esses compromising the quality of your mix? I'll take you through some solutions in a moment but, first, let's consider what makes sibilance problematic.
The sound people make when pronouncing the letter 's' can range in character from a full-on lisp, through fairly smooth ess sounds, all the way to a whistle or a 'sh', and some of those sounds naturally draw attention more than others. It can be different in pitch, too. Female voices tend to have higher-frequency esses than male ones. All seem to have most energy in the 3-6 kHz region (where our hearing is relatively sensitive), though some have considerable energy higher up as well.
Importantly, to make any ess or 'sh' sound, we force air through our front teeth, which usually means directing a blast of air directly at the mic. That can be compounded by the mic itself: lots of vocal mics are voiced with a broad boost somewhere around 5kHz to make voices sound closer, breathier or more intimate, and that will boost any sibilance. Furthermore, cheap mics and preamps may have relatively low high-frequency headroom, which can lead to distortion, which compounds the problem.
Lots of processing and effects that you use in your production can also magnify the sibilant frequencies, and the sibilance can affect the way some processors react. Throw double-tracking and layered backing vocals into the mix, and if the esses in those parts aren't neatly aligned things can start sounding sloppy, particularly if layered parts are spread across the panorama.
The best time to deal with sibilance is before it's actually a problem, ie. when you're recording. An experienced performer should be aware if they naturally emphasise esses more than most, and may or may not have developed mic technique to address that. If not, a cheap, effective trick is to block any gap between their front teeth with dental wax (great if self-recording, though some singers will take more kindly to the suggestion than others!).