A measurement study on voice instabilities during the register transition

When one of the dominant harmonics, the fundamental frequency and its harmonic components, is close to the first formant frequency, the effect of the source-filter interaction can induce voice register transition, in which the vocal-fold vibration becomes unstable and the pitch jumps drastically. In this study, we investigated the relationship between the dominant harmonics and the first formant frequency in the modal-falsetto transition to clarify the effect of source-filter interaction. While five subjects performed rising glissandi with /a/ and /i/ vowels, we simultaneously measured their vocal-fold vibration by using electroglottography and the acoustic response from the vocal tract by using the external acoustic excitation method. We analysed temporal patterns of the fundamental and the first formant frequencies in the transition section. We found that the fundamental frequency was regularly in the vicinity of the first formant frequency for the /i/ vowel. Additionally, for the /a/ vowel, it was occasionally observed that the second or third harmonic component was close to the first formant frequency in the transition. Our results indicate that the source-filter interaction is a common factor of the modal-falsetto transition for the participants.