Paralinguistic variation and invariance in the characteristic frequencies of vowels.

It is shown that within-speaker variations in vocal effort and phonation affect fundamental frequency (F0) and the formant frequencies of vowels in the sense of a linear compression/expansion of the spectral separations between them, given an adequate scaling of pitch. Between-speaker variations in size correspond to a translation of the spectral peaks shaped by F0 and the formants if pitch is scaled tonotopically (in Bark). On the basis of these observations, invariant cues to vowel quality are suggested. It is further shown that vowels produced by adult women tend to be phonetically more explicit and, hence, more peripheral in 'vowel space' than those of men and children. It is also shown that the formant frequencies of vowels subjected to paralinguistic variation are related by power functions of frequency.