Vowel and prosodic factor dependent variations of vocal-tract length

We have measured total vocal-tract (VT) length, lip-tube length and glottal height during vowels on X-ray film data of short French utterances [1]. VT midpoints are determined by progressively fitting circles along the VT-length from the glottis to lip opening. The VT-length is obtained by summing up the distance between adjacent midpoints. Lip-tube length is defined as the distance between the incisors and the lip opening along the midline. Results show that the range of VT length variation is 3.2cm with the average VT-length of 16.4cm. The cause of this large range appears to be the combination of the vowel dependent VT-length and prosodic position that influences on the glottal height. For example, during a vowel at sentence final position, glottis goes down with falling intonation or up with rising intonation corresponding to, respectively, VT lengthening or shortening. The lip-tube lengths are little affected by prosodic position and exhibit a clear vowel dependency. The prosodic influences manifested on the glottal height are not compensated but rather expanded in the VT-length, yet maintaining the characteristic vowel-dependency. This suggests an underlying mechanism to maintain a uniform stretching/compression of VT-length, which tends to hold the phonetic identity of a vowel under large VT-length changes.