The Role of the Vocal Tract's Acousto-Mechanical Resonance on the Mechanism of Bubble Release From Tubes Used in SOVTE.

AND OBJECTIVES: Semi-occluded vocal tract exercises involving a tube with a distal end immersed in water have been used as a voice rehabilitation technique for nearly 60 years. Advantages of this technique include a constant flow resistance, which is mainly dependent on the tube's immersion depth, and an oscillatory component of the back pressure due to bubble release, which is assumed to provide relief of muscle hypertension around the laryngeal and pharyngeal regions. The goal of this study is twofold. First, to shed more light on the contribution of geometrical parameters, such as tube inner diameter and inclination, on the amplitude of the back pressure oscillation and on the bubble release frequency (BRF). The second goal aims to understand the role of vocal tract compliance on the mechanism of bubble release. We hypothesize that the low-frequency acousto-mechanical resonance of a real compliant vocal tract becomes strongly coupled with the mechanism of bubble release, thereby augmenting the back pressure oscillation and dictating the BRF.

METHODS: The BRF and the back pressure are assessed experimentally as a function of flow rate for tubes with a non-compliant upstream condition, considering different diameters and inclinations. These results are compared with theoretical predictions and the experimental data obtained with six adult subjects, from which the vocal tract acousto-mechanical resonance frequencies are also assessed.

RESULTS: The results obtained with tubes involving a non-compliant upstream condition agreed well with the theoretical results and did not indicate a significant influence of the inclination nor the inner diameter on the BRF and on the oscillatory component of the back pressure. Despite the good agreement with the theory, the results obtained from the tube with a non-compliant upstream termination diverged significantly from those obtained with human subjects. In the latter case, the back pressure fluctuation was considerably higher and the BRF measured for each subject coincided with the frequency associated their respective vocal tract acousto-mechanical resonance.

CONCLUSIONS: Results indicate that the BRF is essentially driven by the acousto-mechanical resonance of the vocal tract. Furthermore, the substantial increase on the amplitude of the oscillatory component of the back pressure suggests a strong feedback loop between the source of pressure (bubble release mechanism) and the resonator (vocal tract). The high-pressure fluctuations within the oral cavity implies that subjects need a considerable extra effort to adjust their vocal tract in order to achieve an adequate impedance match between vocal folds and vocal tractsd to produce voice.