Abstract
The purpose of the study was to assess the dependence of phonation threshold pressure (PTP) on hydration level, using a double-blind experimental approach. Twelve adults with normal voices, but without previous voice training, participated as subjects. Each subject received a 4-hour hydration treatment, a 4-hour dehydration treatment, and a 4-hour placebo (control) treatment. Following each treatment, PTPs were measured at low, medium, and high pitches. The results indicated an inverse relation between PTPs and hydration level, as predicted theoretically. However, the effect size was smaller and more variable than in a previous study that did not control for experimental biases and included trained subjects. Pending further, more direct physiological investigations, the results that were obtained are tentatively attributed to changes in vocal fold tissue viscosity associated with changes in hydration level.

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