A Preliminary Analysis on the Correlates of Stress and Tones in Mizo

Stress is the property of a language to exhibit prominence or distinction in one or more syllables in a given domain. The existence of word stress has not been suitably explored in previous acoustic studies of the Mizo language, which is a tonal language of the Kuki-Chin sub-category in Tibeto-Burman language families. In this study, we attempt to analyze word stress on disyllabic target words, specifically in three lexical categories – adjectives, nouns, and verbs. Utterances of the target words are recorded in isolated setting (out of focus) and in sentence frames (in focus). First, averages of features, namely – duration, intensity, F0, formants and spectral tilt, are extracted and investigated for identification of stressed and unstressed syllables on a total of 2880 samples. Next, the interaction of word stress on the four tones of Mizo is investigated. While it is found that H-tone is generally stressed, inferences are made that stressed syllables are not unique to a specific tone. Thirdly, significance of the selected features are validated using a two-tailed paired sample t-test. Our analysis indicate that the mean differences in duration, intensity, and F0 of the stressed and unstressed syllables are significant across the lexical categories at p < 0.05. Next, validations on the significance of the mean differences are carried out using Cohen’s d effect size and Pearson’s Correlation Coefficient (r). Finally, three machine learning models - Support Vector Machines (SVM), Naive Baye’s, and Ensemble learning methods (AdaBoost and Boosted Aggregation), are used to identify stressed and unstressed syllables associated with tones in Mizo. Discriminating differences, especially in disyllabic verbs, are observed between stressed vs. unstressed syllables. Conclusions are drawn that duration is a strong and robust cue for acoustic correlates of stress, while intensity is a medium cue for stress and F0 a weak cue for stress.