Using ultrasound and nasalance to separate oral and nasal contributions to formant frequencies of nasalized vowels.

The experimental method described in this manuscript offers a possible means to address a well known issue in research on the independent effects of nasalization on vowel acoustics: given that the separate transfer functions associated with the oral and nasal cavities are merged in the acoustic signal, the task of teasing apart the respective effects of the two cavities seems to be an intractable problem. The proposed method uses ultrasound and nasalance to predict the effect of lingual configuration on formant frequencies of nasalized vowels, thus accounting for acoustic variation due to changing lingual posture and excluding its contribution to the acoustic signal. The results reveal that the independent effect of nasalization on the acoustic vowel quadrilateral resembles a counter-clockwise chain shift of nasal compared to non-nasal vowels. The results from the productions of 11 vowels by six speakers of different language backgrounds are compared to predictions presented in previous modeling studies, as well as discussed in the light of sound change of nasal vowel systems.