Acoustic Modelling of Vowel Articulation on the Nine Thai Spreading Vowels

Abstract

The Thai vowel system comprises nine short-long vowel pairs. The nine spreading vowels occur in specific locations covering the whole articulatory area. The location of each vowel can be identified according to its acoustic characteristics. Relationships between acoustic and articulatory characteristics were found in the Thai vowel system where F1 relates to vowel height and F2 relates to vowel advancement like other vowel systems. The bark F1-F0 and F3-F2 were also found to represent vowel height and vowel advancement, respectively. Vowel height includes high, mid and low vowel groups while vowel advancement includes front, central and back vowel groups. The previous binary classification criterion is not sufficient for a spreading vowel system. Hence, three new classification strategies are proposed for the classification of complex vowel systems: (1) classification by vowel height, (2) classification by vowel advancement, and (3) classification by combined vowel height and vowel advancement. Acoustic analysis on the Thai vowel system is conducted and each vowel is acoustically modelled by its acoustic-articulatory features. The F1 and bark F1-F0 are employed in classification by vowel height. The F2 and bark F3-F2 are employed in classification by vowel advancement. Both acoustic features are integrated into a single feature vector in combined vowel height and vowel advancement classification using the Bayesian classifier. From the results, linear frequency scale provides better classification accuracy than bark in every case. The linear F1 is 88.52% correct in vowel height classification while it is 94.09% correct using linear F2 in vowel advancement classification. Both linear F1 and F2 achieve 86.33% correctness in combined classification of all the vowels. These illustrate that three classification strategies efficiently classify complex vowel systems using only static spectral cues. Therefore, the simple linear F1 and F2 values can be used to model vowel articulation and to classify each vowel in languages with a complex vowel system like Thai.