The impact of consonants’ functional load on intelligibility and comprehensibility of Thai-accented English in English as a lingua franca contexts

Abstract

This study investigates the intelligibility and comprehensibility of Thai-accented English within English as a Lingua Franca (ELF) contexts. The research aims to identify consonantal features contributing to intelligibility failure, assess the validity of functional load as a predictor of intelligibility outcomes, and evaluate comprehensibility across diverse listener groups. Data were collected from 30 listeners representing six first-language (L1) backgrounds: Thai, Burmese, Chinese, Japanese, Korean, and native English, through two main tasks: transcription of 33 decontextualized sentences containing target consonantal features and comprehensibility ratings of spontaneous speech samples. Speech samples were produced by six Thai speakers verified as having strong Thai-accented English. Results indicate that specific phonological modifications, particularly the substitution of initial voiced obstruents and final consonant devoicing, impair intelligibility. In addition, final position was found to be the most vulnerable site affecting intelligibility, primarily due to Thai phonotactic constraints. The findings provide initial empirical support for applying the Functional Load Principle to L2 speech intelligibility, as words containing high functional load contrasts yielded substantially lower intelligibility scores (39.09%) compared to low functional load contrasts (56.67%). Furthermore, a clear hierarchy of listener performance emerged: Thai listeners achieved the highest intelligibility and comprehensibility scores, supporting the matched Interlanguage Speech Intelligibility Benefit (ISIB). A partial ISIB was observed for Burmese and Chinese listeners, who performed better than Japanese and Korean listeners, likely due to shared typological features such as tone and syllable structure. Conversely, native English speakers provided the lowest comprehensibility ratings despite achieving moderate intelligibility, highlighting a dissociation between actual recognition and perceived processing ease. These findings challenge the primacy of native-speaker norms in assessment and suggest that pronunciation pedagogy in Thailand should prioritize high-impact features that genuinely impact mutual understanding in international contexts.