Same vowels, distinct sounds

Abstract Previous research on bilingual speech production has mainly examined individuals whose first languages (L1s) differ in vowel inventories, often focusing on individual vowels and revealing distinct phonetic realizations for cross-language similar sounds. However, little is known about whether bilinguals with identical vowel inventories phonetically distinguish between their two L1s. This study examined the acoustic characteristics of 38 Basque–Spanish early bilinguals’ production of the vowels /a, e, i, o, u/ and tested whether code-switching frequency and amount of language use influence individual vowels and vowel space in each language. The results showed that /i/ was lower and /i, o, u/ were more fronted in Basque than in Spanish. In addition, bilinguals exhibited a larger vowel space in Spanish compared to Basque, with frequent code-switching diminishing this difference regardless of language use. These findings provide updated acoustic norms for Gipuzkoan Basque and Northern Castilian Spanish vowels and demonstrate that code-switching experience shapes phonetic realizations in early bilinguals whose L1s share identical vowel inventories.