In the last post, I dealt with neither the Aryepiglottic Fold (epilarynx) nor inertial loading that causes the pulse skewing that Martin Berggren referred to in his excellent comment.
For simplification, the ratio of the diameter of the aryepiglottic fold (sometimes called the collar of the larynx) and the vocal tract must be approximately 1:6 in order for the Singer’s Formant to provide enough of an energy boost in the 3kHz region. The scientists agree that this is an independent frequency that is not based on the the vowel formants, although it may cluster with the upper vowel formants (cluster because they occupy the same acoustic space). It also says that the 1:6 ratio occurs when the aryepiglottic fold narrows and the vocal tract widens by lowering the larynx.
This is one of the instances where I think the scientists make a crucial error. The quality of a vowel and the frequency of its formants is strongly affected by laryngeal depth. So to say that the resonance of the epilarynx is totally independent of vowels is at best inaccurate. Scientists must define their terms very narrowly. So I suppose that vowel alteration constitutes a perceivable difference on how the vowel is recognized rather than the alteration of the vocal tract. But in singing when we talk about vowel modification, we are indeed talking about minor alterations that may not affect vowel recognition at all but simply renders the sound production more efficient.
Efficiency also touches upon pulse skewing as related to supra-glottal inertia. The way I look at all of this is that it is all interdependent. The lower larynx that creates the conditions for inertial loading and the resonance of the epylarynx that yield the Singer’s Formant cannot be separated from fold posture. The proper mechanics of phonation lead to the kind of laryngeal depth that produces the 1:6 ratio that produces the SF. These mechanics also influence inertial loading.
The central issue I believe is whether the laryngeal musculature can maintain the configuration necessary for the 1:6 ratio. I am certain that adequate fold depth sets up a balanced fold posture that has a direct influence on laryngeal depth. If the larynx is depressed without balanced fold posture, I believe the results will not yield the beneficial acoustic results we speak of here.
Vocal tract adjustments, including vowel modification will have an effect on the laryngeal oscillation and consequently on laryngeal depth. From the pedagogical point of view, the question is “how do we achieve the 1:6 ratio?” I have been able to ascertain that vowel modification alone does not alter the formants significantly enough to alter resonance strategy unless fold posture is adequate relative to the specific voice.