Dear Mary Beth and Vocalisters:
It is my understanding that the sustained oscillation of only the mucosal membrane occurs when the vocalis muscle is in a more relaxed state and the longitudinal tension on the vocal folds is sustained primarily by the vocal ligaments. The pitch that is produced when the vocal folds are in this mode of operation is high, that is, near or at the top of the female or male range.
In the middle or lower ranges of the voice it is typical for the vocalis muscle to supply some of the longitudinal tension on the vocal fold because the vocal ligament is not stretched at or near its maximum tension and is, therefore, not a contributor to the necessary longitudinal tension for the pitch being sung. In this vocal fold mode of oscillation, a larger portion of the vocal fold is in oscillation, including part or most of the vocalis muscle.
I am not sure without checking my resources what configuration of the vocal mechanism creates the medial tension that is often mentioned on this list. Medial tension would, by definition, be a pressing together of the vocal folds at their midline longitudinally. It would not appear to me that a complete closure of the thyroarytenoids would accomplish this by itself. Also, a closure of the space between the thyroarytenoids, which is common in full volume singing, by activating the interarytenoid muscle has a tendency to slightly open the extended portions of the thyroarytenoids which would open the vocal folds at their posterior portion. Such an opening is commonly closed by the vocal folds themselves but I am not sure what muscle action accomplishes this. It could be that this vocal fold muscle activated closure is also the cause of the medial tension but that is just a pure guess.
I know this is a lot of big word stuff but I don't how else to discuss this.
Regards -- Lloyd W. Hanson, DMA Professor of Voice, Vocal Pedagogy School of Performing Arts Northern Arizona University Flagstaff, AZ 86011
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