Effects of English exposure on Korean vowel production in Korean–English code-switched speech*

This study examines how immersion experience in an English-speaking environment shapes bilingual phonetic behavior in 54 Korean–English bilingual speakers, with a focus on vowel production in code-switched speech. Using linear mixed-effects modeling, F1 and F2 values of Korean vowels were analyzed in Korean-only and code-switched Korean conditions for speakers with and without immersion experience. Vowel height (F1) exhibited robust phonemic contrasts with minimal influence of immersion or code-switching condition, whereas vowel frontness/backness (F2) showed significant sensitivity to the code-switched context. A significant interaction among condition (code-switching), vowel, and immersion emerged for F2, driven primarily by speakers without immersion experience, who exhibited greater backing of /u/ in code-switched speech than in Korean-only speech. In contrast, speakers with immersion experience showed more stable vowel realizations across speech conditions. These findings suggest that the absence of immersion is associated with greater sensitivity to the surrounding linguistic context during code-switching, particularly for vowels occupying regions of the vowel space where Korean and English differ in frontness/backness, such as the high back vowel /u/. By contrast, immersion experience appears to promote greater stability and stronger language-specific phonetic control.