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. 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 VTlength 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 VTlength, which tends to hold the phonetic identity of a vowel under large VT-length changes.
Bibliographic reference. Maeda, Shinji / Laprie, Yves (2013): "Vowel and prosodic factor dependent variations of vocal-tract length", In INTERSPEECH-2013, 3196-3200.