Changes to Mandarin vowel and lexical tone acoustics in clear speech production

This study examines the effects of clear speech on the acoustics of vowels and lexical tones in Mandarin. Thirty female speakers produced both casual and clear speech, with target words embedded in carrier sentences. Clear speech was associated with an expansion of vowel space area and a shift in vowel space distribution driven by higher F1 for /a/ and lower F2 for /u/. Tone space area increased in clear speech but showed no clear interaction with vowel type. Clear speech was associated with changes in pitch and timing parameters but not equally so across all tones. In general, clear speech tones exhibited greater mean, slope and range of fundamental frequency, longer durations, and reduced harmonic-to-noise ratio. A principal component analysis identified pitch change as the primary psychoacoustic parameter, while pitch height and duration were secondary. A logistic regression of principal components revealed an apparent coupling between duration and pitch change in casual speech that was not apparent in clear speech. Heightened contrasts of lexical tones in clear speech may therefore be associated with an increase in degrees of freedom for more precise control over individual tone parameters, independent of vowel characteristics.