Extending the category precision hypothesis to articulation: Front vowels in Korean and English

This study examines individual differences in L1 and L2 vowel production, extending the category precision hypothesis from the revised Speech Learning Model J. E. Flege and O. S. Bohn, in Second Language Speech Learning: Theoretical and Empirical Progress, CUP, 2021 to the articulatory domain. The hypothesis posits that variability in L1 phonetic categories influences L2 speech learning. While prior studies have tested this claim acoustically, its articulatory basis remains unexplored. We examine whether acoustic and articulatory precision are correlated within speakers, and whether this relationship differs between L1 and L2. Ultrasound data from the SNU-MAC Corpus https://snumac.wordpress.com are used to analyze front vowels in L1 Korean (/i/, /e/) and L2 English (/i/, /ɪ/, /ɛ/, /æ/). Acoustic precision is measured via F1–F2 variability, and articulatory precision via tongue height derived from ultrasound contours. Results show that in L1, speakers with lower acoustic variability exhibit more consistent tongue positions, indicating a tight coupling between acoustic and articulatory precision. This pattern is not observed in L2, suggesting that nonnative speech may involve less integrated control across the two domains. Extending the category precision hypothesis to articulation, these findings highlight the value of multimodal approaches in speech learning research.