Background: High-quality voice research in the vulnerable population of patients with a voice disorder is highly needed for effective and efficient treatment of the organic and/or functional voice disorder. Semi-occluded vocal tract exercises (SOVTEs) are promising innovative therapy techniques.

Objective: To investigate the short-and long-term effects of an SOVT intervention with the use of the flow ball (FB) device  (providing biofeedback) on the objective vocal quality  of patients with a organic and/or functional voice disorder and to compare this approach with the 'treatment as usual' straw phonation (SP) and a sham condition, using a longitudinal randomized sham-controlled trial.

Methods: Subjects will be randomly assigned into a FB group, an SP group or a control (sham) group. A multidimensional voice assessment will be performed resulting  in an objective vocal quality value (Dysphonia Severity Index).

Hypothesis: Based on the use of biofeedback in the FB group an increase of the objective vocal quality value, the Dysponia Severity Index,  is hypothesized.