Effects of oral-nasal coupling on whispered vowel spectra

This study was designed to investigate some acoustic (formant) effects of systematic changes in oral-nasal coupling area during whispered vowel productions. One adult female cleft palate subject was fitted with a specially-designed prosthetic speech appliance. The appliance was drilled to provide seven controlled diameters of oral nasal coupling, ranging from no coupling to a maximum of 14 mm. At each coupling condition, the subject's whispered productions of each of two test vowels (/i/,/u/) were magnetically recorded. From narrowband (3 Hz) acoustic spectra of the recorded vowels, measurements of formant frequency, intensity, and bandwidth were obtained for the formants visualized below 4 K HZ. The most reliable acoustic indicators of increased coupling were changes in the measurements for formant two of /i/ and for formant three of /u/ and, for both test vowels, the appearance of "extra formants." In general, however, the findings appeared consistent with the view implicit in acoustic transmission line theory that the spectrographically-delineated formant effects of oral-nasal coupling may be inherently inconsistent and difficult to predict.