Low frequency mechanical resonance of the vocal tract in vocal exercises that apply tubes

Abstract

Abstract Phonation into a tube that lowers the acoustic vocal tract resonance frequency and increases vocal tract impedance is used in voice therapy to establish effortless voice production. Additionally, keeping the distal end of the tube in the water results in the water bubbling and a consequent oscillation of oral pressure. This may feel like a massage of the vocal tract and larynx. A low frequency mechanical resonance of the vocal tract, F m , could enhance the effect of tube therapy in two ways: 1) by lowering the first acoustic resonance closer to the fundamental frequency of phonation, and 2) by introducing a coalescence of F m with the water bubbling frequency. A mathematical model of acoustic-structural interaction is introduced to clarify F m in the context of phonation into a tube with the distal end in air and in water. The numerical results from the model are compared with the resonance frequencies measured in a male subject phonating on the vowel [u:] into a glass resonance tube with the distal end in air and at 2 cm and 10 cm under water. The effects of phonation through the tube are demonstrated by registering oral air pressure and electroglottography, and by synchronous high‐speed filming of the water bubbling. The first computed acoustic resonance frequency decreased from F 1 = 200 Hz for the tube end in air down to about F 1 = 175 Hz for the tube end in water, which roughly agrees with the first formant frequency of c. 179 Hz that was experimentally found for the human vocal tract. Considering the mechanical resonance F m of the vocal tract to be c. 66 Hz, as previously estimated from measurements of a closed vocal tract, then according to the mathematical model for the vocal tract prolonged by a rigid glass tube, this frequency drops to 23 Hz. When the tube is submerged in water, F m drops further to F m = 8 Hz for the resonance tube and to about F m = 10 Hz for a longer and wider silicon Lax Vox tube. The results thus show that the mechanical resonance can be near the measured water bubbling frequency F b = 11–11.5 Hz. The results suggest that the mechanical resonance of the vocal tract tissues enhances the effects of the tube during voice therapy.