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This experimental study investigated the back pressure (p back ) versus flow (U) relationship for 10 different tubes commonly used for semi-occluded vocal tract exercises (SOVTE), i.e., 8 straws of different lengths and diameters, a resonance tube and a silicone tube similar to a Lax Vox tube. All tubes were assessed with the free end in air. The resonance tube and silicone tube were further assessed with the free end under water at the depths from 1 to 7 cm in steps of 1 cm. The results showed that relative changes in the diameter of straws affect p back considerably more compared to the same amount of relative change in length. Additionally, once tubes are submerged into water, p back needs to overcome the pressure generated by the water depth before flow can start. Under this condition, only a small increase in p back was observed as the flow was increased. Therefore, the wider tubes submerged into water produced an almost constant p back determined by the water depth, while the thinner straws in air produced relatively large changes to p back as flow was changed. These differences may be taken advantage of when customizing exercises for different users and diagnoses and optimizing the therapy outcome.

This study provides data on the physical properties of resonance tube phonation with the tube end in water. This information will be useful in future research when looking into the possible effects of this type of voice training.