In speech production research, a good knowledge of the articulatory-acoustic relationship is a key factor.As lips are one of the major vocal tract acoustic control points, it is of utmost importance to gain insights into the relationship between their geometric and acoustic characteristics.Therefore, video recordings and plaster replicas of the lips of a subject were made for a set of sustained vowel and fricative articulations.Video images of the replicas were also recorded.From these recordings, geometric labial horn parameters, such as labial horn depth and intralabial area, were computed.The acoustic radiation impedance of the replicas, at the coronal plane located at the lip corner, was measured by a two-microphone method, and the length and area of a single uniform cylindrical tube of identical impedance were determined through an optimization algorithm, leading to an acoustic equivalence of the lip horn.Various correlations between the geometric parameters have been confirmed.It has also been shown that the acoustic equivalent intralabial area linearly depends on the geometric area, whereas the equivalent length is roughly independent of the geometric length.It was concluded that lip corner position is the main determinant of the vocal tract length on the lip side.