Acoustic Analysis of the Open End Effect Using Solid Vocal Tract Models Constructed from MRI Data during Vowel Production

Abstract

This paper investigates acoustic effect of the vocal-tract open end to examine whether the open-end correction coefficient known to date is adequate for modeling the vocal tracts with a wide-open mouth. To do so, MRI data were examined for two females (LH and SC). 3D vocal tract shapes were extracted, and solid vocal-tract models were built from volumetric MRI data during production of Chinese vowel /a/. The measured formants from the solid models and calculated formants from the area functions were compared to estimate the wide open end boundary effect. It was found that the calculated formants with the excess lengths of 0.44 and 0.28 open end diameter for subjects LH and SC are closer to the measured formants than the calculated formants with the common open-end correction coefficients (in the range of 0.3 to 0.4 diameters of the open end). The results suggest that the coefficient for correction needs to be adjusted in models to reflect the geometry of the lip end of the vocal tract.