The importance of vowel formant frequencies and proximity in vowel space to the perception of foreign accent

Abstract

This study used speech resynthesis to manipulate vowel formant frequencies in natural speech stimuli to examine the influence of magnitude and direction of spectral deviation from native norms, as well as the proximity of adjacent categories, on foreign-accentedness ratings, comprehensibility ratings and vowel identification. Experiment 1 contrasted the impact of spectral deviation on peripheral and non-peripheral vowels by shifting the first and second formant center frequencies in English vowels /ɔ/, /ʌ/, /ɛ/, /ɑ/, and /æ/ toward the acoustically closest vowel, and either the next-closest vowel, or unoccupied vowel space. To directly compare the impact of different deviation directions and proximity of adjacent categories without being confounded by F1–F2 variability within the individual vowel, formant frequencies in one English vowel, /ɑ/, were manipulated in Experiment 2 toward the four cardinal and four intercardinal directions. Across both experiments, spectral deviation that increased overlap with adjacent categories reduced comprehensibility, and increased both accentedness and misidentification. Neither the direction nor the magnitude of spectral deviations from native norms alone were sufficient to account for task performance. These findings imply that vowel space dispersion, specifically location within vowel space, plays a role in both accentedness judgments and vowel identification.