The supraglottal differences between alveolar and postalveolar English sibilants, beside the place of articulation, involve the length of the constriction (shorter for alveolars) [1, (2), 3], the width of constriction (narrower for alveolars)[2, 4], the presence or absence of a sublingual cavity (absence for alveolars) [1, 2], the depth of tongue groove (deeper for postalveolars) [2], and also lip shape (protruded for postalveolars) [1, (2), 3].

This paper is concerned with articulatory data on lip protrusion and lip opening area (‘rounding’) of sibilants uttered by an American speaker, and the related acoustic data. Articulatory-acoustic mapping indicates that only protrusion may be playing a role in lowering the frequencies of postalveolars. An estimation based on a quarterwave length resonance predicts the difference in the tube length between alveolars and postalveolars being twice the difference measured in articulatory data. Some configurations are suggested for further acoustic modeling, in order to better examine the potential contribution of sublingual cavity, in particular, and the effect of its coupling with lips.