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 abruptly. We investigated the relationship between the dominant harmonics, the first formant frequency, and the pitch jump width in the modal-falsetto transition to examine the effect of sourcefilter interaction. We measured temporal patterns of the fundamental frequency and the first formant when subjects performed rising glissandi with /a/ and /i/ vowels. For the /a/ vowel, there were weak proximity relationships between the dominant harmonics and first formant during the transition, indicating that source-induced transition occurred. For the /i/ vowel, in contrast, the fundamental frequency was regularly close to the first formant in the transition, indicating that the acoustically induced transition was caused by the source-filter interaction. Additionally, it was found that the difference between these two mechanisms had little influence on the pitch jump width. Finally, we concluded that the source-filter interaction is a contributory factor of the modal-falsetto transition, in agreement with foregoing studies.
We simulated acoustic interaction between the voice source system in the larynx and the acoustic filter of the vocal tract. The vocal tract of a soprano was first scanned in three dimensions using magnetic resonance imaging while she produced four musical notes (A3, E4, D5, and A5) with /a/ and /i/. These images were used to simulate voice production, including the vibratory motion of the vocal folds and the behavior of glottal airflow. Images for the /i/ vowel were used in the simulation, because a good proximity relationship was found between the fundamental frequency and the first impedance peak of the vocal tract. The simulation results revealed that the fundamental frequency (vibration frequency of the vocal folds) was decreased to a large extent by the interaction especially when their natural frequency was in the proximity of the impedance peak. In a specific case, the acoustic load of the vocal tract exerted on the vocal folds changed as a result of the interaction, so the vibratory motion was effectively assisted. These interaction effects were also examined in terms of the phase relation among the temporal waveforms of the glottal variables.
The human whistle is a typical aeroacoustic sound. Downstream of a small orifice made by the lips, a jet is formed by airflow with a high Reynolds number. A sequence of vortex rings is then produced, and periodic air pressure changes result in a characteristic whistling sound. Although the vocal tract has been reported to act as an acoustic resonator determining the blowing pitch, the precise shape of the vocal tract and its resonance properties during whistling remain unclear. In the current study, the morphological and acoustic properties of the vocal tract were examined during the act of whistling in a single participant. The vocal tract was scanned in three dimensions using magnetic resonance imaging while four musical notes were produced. The data revealed that the tongue constricted the vocal tract in different ways depending on the note, and the location of the constriction moved forward when the blowing pitch increased. Acoustic analysis of the vocal tract showed that the second peak of the lip input impedance was largely in accord with the whistling pitch. In addition, specific regions in the vocal tract were highly acoustically sensitive to small deformations.
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