Since I have not been part of the Higher Education environment for the last fourteen years, I miss out on a lot of interesting publications. What is interesting is that an article by Ingo Titze in The Journal of the Acoustical Society of America dating back to 2014 corroborates the theories I have shared on this blog relative to the necessity of deeper fold adduction. Titze’s conclusions (a culmination of more than a quarter century of research and shared research with colleagues) are so accepted in the field that they inspired research in surgical procedures to achieve the so-called rectangular glottis in phonation. I will attempt to clarify the basics as Titze explains in his article:
Titze concludes there are three muscle groups responsible for different aspects of glottal adduction: 1) The Lateral Crico-Arytenoid (LCA) pair that bring the folds to midline at the superior aspect (upper portion) of the folds. 2) Thyro-Arytenoid (TA), which includes two muscles, vocalis and muscularis, thicken the folds vertically and brings the folds in contact at the inferior aspect (lower portion) of the folds. Finally 3) Inter-Arytenoid (IA) pairs seal the posterior gap to prevent air leakage (IA is less relevant to this discussion since it only prevents leakage beyond the vibratory edge of the folds)
The LCA adducts the vocal processes of the arytenoid cartilages, which bring the superior edges of the vocal folds together…The bottom of the vocal folds is adducted by the TA.
But Crico-Thyroid (CT), the primary function of which is the lengthening of the folds (also called the pitch muscle) has an adductory function as well.
Titze explains further:
What role does CT activity play in registration? Its main function is anterior–posterior fiber stiffness regulation of the vocal fold tissue layers by vocal fold lengthening, but CT also plays a role in adduction. LCA and CT activity are often highly correlated in speech (Atkinson, 1978). One reason is that, when fundamental frequency is high and governed by tension in the vocal ligament (Van den Berg, 1960; Titze et al., 1988), amplitude of vibration is small. Smaller amplitude requires more adduction at the vocal processes to allow the vocal folds to reach contact in vibration.
Titze confirms what I have attempted to explain here–That shallower glottal posture (i.e. convergent glottis, as Titze refer to it) requires “more adduction.”
Thinner tends to be pressed (my words).
In addition, elongated vocal folds are retracted from the glottal midline because their cross sectional area is reduced. This retraction requires further LCA adduction, which adjusts the glottis toward a convergent shape if TA activity is not simultaneously increased (Hirano, 1975). In other words, bottom adduction may not follow top adduction of the vocal folds when CT is much more activated than TA (Berke et al., 1989). At the opposite extreme, if TA activation is strong and the ligament is lax due to little CT activation, a divergent pre-phonatory configuration can be the outcome. Adduction at the top is then weaker than adduction at the bottom. For mixed registration, it is hypothesized that the two extremes are avoided with appropriate muscle balance so that a near rectangular glottis is achieved and stiffness is balanced in the tissue layers. A small convergence angle is probably not detrimental, but large divergent or convergent angles are not conducive to low-threshold self-sustained oscillation.
Titze concludes (perhaps correctly) that LCA (the primary adductory muscle) approximates only the upper portion of the folds and that TA is needed to adduct the lower aspect. He then correlates LCA and CT. He refers to convergent glottis as related to LCA dominance and divergent glottis to be TA dominant. That is one way of looking at it. Far be it from me to question the research of a top scientist far more fluent in his understanding of such matters than I am. Nevertheless, I have a suggestion purely from the singer’s point of view:
I believe that pre-phonatory postures are a result of the singer’s expectation of vocal timbre. If I imagine myself to be a bass, I will tend to expect a deep sound, which might lead to exaggerated TA activity resulting in a divergent glottis. The sound feels like it is rooted in the chest. This posture prevents the folds from closing at the upper portion, which is related to a lack of CT activity (lengthening). That imbalance results in lax vocal folds and result in a rather dull timbre. This, of course begins with the singer’s desire to sound bassy (this would work for a mezzo as well). The opposite can be said for a tenor (or soprano)–that is a tendency to desire brightness over darkness in the voice, resulting in an excessively convergent glottal posture. Naturally a tenor or soprano can desire to be excessively dark just as a bass or mezzo may seek to be excessively bright.
If we take as given that fold posture is primarily influenced by the singer’s imagined sound, then I would explain more simply that LCA is the primary adductory muscle and that the shape of the the glottis will depend on the balance between CT and TA.
Titze concludes that a balanced (or mixed) coordination in the antagonistic relationship between CT and TA will produce a desired rectangular posture that requires less adduction than the convergent (relatively pressed) glottal posture. Ideally, the folds approximate gently along the entire vertical cross-section of the folds.
The rectangular posture gives both richness and brilliance to the tone–In bel canto terms, chiaroscuro.
Titze also mentions that the extreme postures, convergent and divergent, are more stable, which is why more singers tend to find comfort in a one-sided posture (convergent or divergent). The most admired singers achieve a successful tightrope act of balancing CT and TA and regulating breath pressure as to not blow apart the gently adducting rectangular glottis and adjusting vowels in order to achieve acoustic configurations that encourage the dynamic coordination that produces the rectangular glottis.
The singer’s challenge is to figure out how it feels to access the two sides of adduction. How does appropriate TA activity feels such that it produces ideal posture on the lower portion of the folds? And how does one combine that sensation with the sensation of appropriate CT activity to close the upper portion of the folds? That is what we must achieve in voice lessons. The benefits of a mixed (rectangular) posture include an easier, more flexible tone; more dynamic transitions between registers; greater volume control; greater stamina and less wear on the folds. And more!
Lastly, health matters! Our folds’ surface is not always flat and smooth. It is usually somewhat uneven from inadequate sleep or viscosity from inadequate hydration or reflux or allergies or colds, etc. When the surface is uneven, we often compensate by adducting a little more firmly to avoid gaps in the closure (which sounds raspy and produce subharmonics). In so doing, we are creating a chain reaction that results in a more convergent glottis, lacking in depth and causing abrupt register changes. Achieving this balance is the singer’s daily work for a lifetime. Additionally, achieving balance in speaking has a great influence on the ability to achieve a mixed (Bi-stable fold adduction, per Titze’s article) posture in singing.
Copyright 4 December 2020