Morphological normalization: A study of vowels for Mandarin and Japanese

Reducing the morphological variances of vocal tract across different subjects would benefit articulatory data analysis and modeling. To further test such a hypothesis by a thin-plate spline warping (TPS) method, this study used articulatory data of /a, i, u/ from 3 Chinese subjects and 3 Japanese subjects, which were collected by Electromagnetic Midsagittal Articulographic (EMMA) system. The templates for the normalization of Chinese and Japanese were obtained by averaging the 3 subjects´ palates and tongue shapes in each group. The 44 landmarks in each template were then defined by a gridline system of the vocal tract. The results show that the variances among subjects were reduced in both horizontal direction and vertical direction. The similar vowel structures between pre- and post-normalization data indicate that TPS method outperforms the traditional palate-straighten method in that TPS method has reduced mid-sagittal morphological differences among speakers while keeping their individual vowel structures unchanged. The comparison results show that the articulatory differences among the three vowels are consistent with their corresponding acoustic properties.