A sibilant is a type of fricative, made by speeding up air through a narrow channel and directing it over the sharp edge of the teeth. English sibilants are . Sibilants are louder than their non-sibilant counterparts, and most of their acoustic energy (the frication, in the case of fricatives) occurs at higher frequences than non-sibilant fricatives. [s] has the most acoustic strength at around 8,000 Hz, but can reach as high as 10,000 Hz. [ ʃ ] has the bulk of its acoustic energy at around 4,000 Hz, but can extend up to around 8,000 Hz.

The sibilant/non-sibilant distinction is especially important in English. Most people consciously know that to pluralize a regular English noun, they simply add an -s to it. But what they know only subconciously is that English plurals can take three forms (allomorphs ), those being [s], [z], and [əz]. [s] occurs after most voiceless sounds, and [z] occurs after most voiced sounds. The sequence [əz] fills in the rest, and occurs after sibilants. Thus, distinguishing between sibilants and non-sibilants is extremely important in English.