Speech Prosody 2004
High vowels sound longer than low vowels when acoustic durations are equal. Also, the perceived vowel duration of diphthongs is longer than that of phonetically similar vowel-glide combinations. These experimental findings lie at the basis of two vowel splits that took place in tonal dialects in the Netherlands and Belgium in which duration is used as enhancement of a tone contrast.
Bibliographic reference. Gussenhoven, Carlos / Driessen, Wilske (2004): "Explaining two correlations between vowel quality and tone: the duration connection", In SP-2004, 179-182.