Dear Wim and Vocalisters:
Because the vocal folds have three basic layers (from medial outward they are mucosal, vocal ligament, vocalis muscle) the use or interplay of these layers give rise to different modes of oscillation.
In chest voice all three layers are in oscillation, In head voice the vocalis muscles relax and the longitudinal tension on the vocal folds is sustained by the vocal ligament with primary oscillation in the mucosal membrane and to some degree, the vocal ligament. What you describe as a gradual transition as the cricothyroid muscle stretches the thyroarytenoid muscles (also called vocalis muscles), must at some point make the transition from thyroarytenoid muscle activity to inactivity and vocal ligament activity. This transition is the traditional passaggio and I have never found a voice that did not have it.
-- Lloyd W. Hanson, DMA Professor of Voice, Pedagogy School of Performing Arts Northern Arizona University Flagstaff, AZ 86011
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