The aim of this study is to analytically describe the frication noise spectrum in terms of formant affiliation to vocal tract cavities. The high-resolution, teeth-inserted MRI data of sibilants in 7 languages [30 subjects in total] as well as 1D and 3D acoustic modeling are involved. The results are summarized as follows: 1. Sibilants’ dispersion within the articulatory space depends on the language’s phonemic inventory [with contrasts involving [+/- anterior], [+/- distributed], or both features]. A large amount of inter-speaker variation [7 subjects] is observed in French /s/ and /ʃ/ contrasted by [+/- anterior]]. 2. This variation is due to two articulatory variants of the French /ʃ/ : [a] apical, with a sublingual cavity, and protruded lips, like Polish /ʂ/ ; [b] palatalized, with a domed tongue dorsum, like Polish /ɕ/. Systematic acoustic modeling provides evidences about their acoustic equivalence. 3. In Polish, where /ʂ/ and /ɕ/ are contrastive, a super-low peak [1.5 – 1.8 kHz] characterizes the former. The acoustic modeling results using realistic area functions of two subjects show that this peak is affiliated to the front oral cavity. 4. The articulatory data show a systematic narrow constriction at the teeth in all of the examined sibilants. Acoustic modeling shows that lip protrusion results in lowering the formant affiliated specifically to the lip cavity. To conclude, the narrow tongue and teeth constrictions in sibilants allow the speaker to control quasi independently
