Sound
Sibilance
Also called sibilant, ess sounds
Sibilance is an exaggerated, hissy edge on "s", "sh" and "t" sounds in a voice. It comes from a treble peak somewhere between about 5 and 10 kHz, on the product or in the recording.
Every voice has some sibilance; the problem is when a product adds to it until vocals become uncomfortable. Pop recordings mastered bright are the usual trigger, and a slightly sibilant product can be fine on jazz and painful on a modern pop mix.
In a Frequency response measurement, the culprit is a narrow peak in the lower Treble. Equalization can tame it, and some reviewers list "sibilant" as the first thing to check with female vocals.
Sibilance is one cause of listening fatigue; harshness in the upper Mids is the other.