Purpose: Voice therapy with a semi-occluded vocal tract has a long history. The use of lip trills, tongue trills, bilabial fricatives, humming, and phonation into tubes or straws has been hailed by clinicians, singing teachers, and voice coaches as efficacious for training and rehabilitation. Little has been done, however, to provide the scientific underpinnings. The purpose of the study was to investigate the underlying physical principles behind the training and therapy approaches that use semi-occluded vocal tract shapes.
Method: Computer simulation, with a self-oscillating vocal fold model and a 44 section vocal tract, was used to elucidate source-filter interactions for lip and epilarynx tube semi-occlusions.
Results: A semi-occlusion in the front of the vocal tract (at the lips) heightens source-tract interaction by raising the mean supraglottal and intraglottal pressures. Impedance matching by vocal fold adduction and epilarynx tube narrowing can then make the voice more efficient and more economic (in terms of tissue collision).
Conclusion: The efficacious effects of a lip semi-occlusion can also be realized for nonoccluded vocal tracts by a combination of vocal fold adduction and epilarynx tube adjustments. It is reasoned that therapy approaches are designed to match the glottal impedance to the input impedance of the vocal tract.