Longitudinal Investigation of Early Vowel Development in Korean Infants

Objectives: This study investigated early vowel development in Korean-learning infants by analyzing longitudinal vocalizations at 6-8 and 12-14 months. The study aimed to identify developmental changes in the frequency and distribution of vowels and diphthongs, providing empirical data on the early organization of the Korean vowel system. Methods: Fourteen typically developing infants (7 boys, 7 girls) participated. Naturalistic daylong recordings were obtained in the home environment using the Language Environment Analysis (LENA) system. From each session, 100 minutes with the highest infant vocal activity were extracted and analyzed with the AACT software. All utterances were segmented into breath groups and phonetically transcribed by trained coders in blind conditions. Vowel productions were analyzed based on the frequencies of isolated and consonant vowel (CV)-bound vowels and diphthongs within canonical babbling. Longitudinal changes were examined between 6-8 and 12-14 months. Results: The total number of utterances and syllables increased significantly across the two age periods. Across both ages, /a/, /ɯ/, and /ε/ were most frequently produced, with /a/ consistently dominant. High back vowels (/o/, /u/) remained rare, while diphthongs such as /ja/ and /wa/ were produced with relatively high frequency at 12-14 months, suggesting increasing articulatory coordination. Conclusion: By longitudinally analyzing vowel productions in the babbling of Korean infants aged 6 to 14 months, this study delineates the developmental trajectory of the Korean vowel system and provides baseline data for understanding early phonological development.