1) Does the singer desire for the entire usable range to be driven by a mucosal (cover) vibration or does the deep part of the folds (body) participate in some parts of the range (e.g. the lower range)
2) Does what scientists observe and publish as norms agree with the vocal function of the very best lyric singers?
The two questions need to be dealt together. First of all, what do scientists observe and publish as norms?
Here is an excerpt from the website of The National Center for Voice and Speech, arguably the most respected source, providing information on vocal function from the scientist's point of view:
Chest register is perceived when the timbre is richer or heavier; this quality is produced when the singer contracts both the CT and TA muscles at the same time, but the TA is more active, thus tending to shorten the folds and produce a lower pitch range. The Fo and lower overtones are stronger than higher overtones in chest voice, and a large amount of the vocal fold tissue is in vibration. In addition, the vocal folds are usually closed through more than half of each cycle of vibration.
Head register is perceived when the timbre is lighter or thinner. Both the CT and TA muscles are contracted, but the CT muscle predominates, and so the range of pitch for head voice is higher, since the folds are lengthened, thinned and stretched. A smaller portion of the folds is in vibration in head voice; only the outer layers of the cover vibrate. The Fo and all overtones are weaker than in chest voice, and the folds are open for a larger portion (more than half) of each vibrational cycle than in chest voice.
Are the color-coded statements accurate? Is this article addressing professionally viable lyric singers?
Relative to the statements in blue, Donald G. Miller, of Voce Vista fame, has observed in the male voice that operatic head voice production can yield up to 80% close quotient (the percentage of the vibration cycle that the folds remain closed) particularly among more robust voices. This is documented in his extremely informative book, Resonance in Singing, as well as in this online presentation.
The discussion of the two tenors singing a crescendo-diminuendo is fascinating when one listens to the two sound clips provided. We will return to the sound clips! For now, simply looking at Miller's findings (i.e. close quotients of 75% in both subjects singing in the traditional head voice range), it would seem that there is a disagreement as to the definition of head voice. Must head voice production yield a high open quotient (i.e. a low close quotient) as suggested on the website of the NCVS? Are the tenors in the Voce Vista website singing in chest voice, if the CQ levels are as high as 75%?
Let us consider the statements in red in the above quotes from the NCVS! The statements suggest that head register is weaker than chest register and that the deep layers do not participate in the vibration, in head voice. At face value, at least the first part of that statement is wrong when we look at Miller's spectrograms. The examples of the two tenors show very strong energy in all harmonics, particularly when both singers sing ff. Miller's presentation cannot tell us how the folds are producing the high CQ levels in the two tenors singing in head register. Why is tenor1 produce a very bright sound and also a falsetto when he reduces volume to pp and tenor2 is able to maintain a high CQ as he reduces the sound (granted that tenor2 does not reduce the sound as much as tenor1)?
I believe the answer lies in the difference between the way the two tenors achieve their high CQ. Let us look at the picture of the fold layers again:
The simulation does show an increase in the mucosal contact area, but it illustrates more obviously that the medial layer (ligament) participates in the vibration. I do not believe this is necessarily the case when it comes to the upper modal range--what we call full the lyric full voice.
At this point we must understand what creates pitch. A pitch is to put it simply a number of vibrations per second. A4, right above middle C, is also called A440 because 440 vibrations (i.e. open-close cycles) are necessary for us to hear the pitch 440 Hertz or 440 puffs of air released in one second. The speed of the vibration cycle is what controls pitch. It is important to know that there are two components to the speed of the cycle: How much of the fold edge is participating in the vibration (i.e. how deep is the approximating edge) and how tightly the folds come together during vibration.
The simulation above shows the vibration occurring at the mucosal level--that is the outside layers barely touch. If the folds are touching more tightly, two parameters must change: 1) the time of the close quotient will increase, which means normally the fundamental frequency (pitch) would lower. In such a case, for a given frequency if the singer desires to maintain pitch with this pressed mode of vibration, the amount of vibrating edge must lessen 2) the outer layers (mucosa) would be pressed against the other layers, which are less flexible. Greater sub-glottal pressure would be necessary to maintain the vibration. In such a scenario, the voice will sound more intense but less air would be released per vibration cycle. The sound pressure would be less than a tone that is produced with full closure at the mucosal level.
Compared to pressed phonation, to produce a long enough vibration cycle when the folds are closed completely only at the mucosal edge, the amount of contact area would have to increase. The mucosal edge would have to be deeper vertically, on the y-axis of the animation, as opposed to the pressed sound which would require deeper contact on the x-axis.
The issue here is flexibility and sound pressure. Greater contact area along the mucosal edge provides enough time for the vibration cycle such that pressed phonation is not necessary. Given that the folds meet softly (but completely), less sub-glottal pressure would be necessary to start and maintain the vibration.
Dr. Zhang, Zhaoyang at UCLA, in a 2008 article shows that a stiffer fold body will isolate the vibration of the vocal folds along the mucosal edge. These two images from his article illustrate the different mode of vibration:
This first picture represents a model of loose fold body and loose fold cover. When there is not enough antagonism between vocalis and crico-thyroid the body (deeper layers) vibrate with the cover. The vibration will tend to be more difficult in such a case. More sub-glottal pressure will be necessary to start and maintain the vibration.
This simulation, on the other hand, represents a stiff fold body rendered by contractions of both main muscle groups. This antagonism makes the fold body less mobile and isolates the vibration of the folds along the mucosal edge. The antagonism between vocalis and crico-thyroid also increase the contact area along the mucosal edge.
The simulations obviously do not illustrate the difference in contact area that I believe should be reflected here. The necessary contraction between the vocalis muscle and the cryco-thyroid muscle that causes the stiffness in the body of the folds would also change the fold depth, such would make up for the faster opening of the fold from reduced medial pressure (pressed phonation).
I believe this is the difference we hear between the two tenors compared on the Voce Vista page.
Tenor1, to my ears sounds exciting and pressed. As he reduced sub-glottal pressure to reduce volume, he no longer had enough pressure to sustain the vibration of the pressed folds. He had to give up the medial pressure and did so gradually and rather effectively. But I believe he had to relax the arytenoids (back of the folds) and let air out to reduce the volume even while the folds stayed together--In essence a falsetto production as reflected in both the acoustic signal and the glottal signal (CQ).
Tenor2 on the other end seemed to exhibit a mellower sound that by the looks of both the acoustic signal and the CQ kept enough glottal resistance without excessive medial pressure. From what I hear of his sample, he could have continued to diminuendo to a softer sound, but he stopped.
The change in phonation mode that we hear in Tenor1 is very symptomatic of pressed phonation. When a pressed sound reaches an excessive sub-glottal pressure threshold, it will tend to un-dynamically shift to a new mode. We often hear this in voices that crack from excessive sub-glottal pressure.
A dynamic exchange in dominance must occur between the vocalis and crico-thyroid muscles as pitch rises. Sometimes a singer may have a balanced phonation on one note and a poor phonation on the next. This mezzo example in Miller's website is a perfect case. Her lowest note on the A3 to A4 arpeggio is to my ears perfectly balanced, with enough antagonism between the vocalis and crico-thyroid groups to create a stiff fold body that encourages a vibration dominated by the cover (It is possible that the ligament, the medial layer is also active, but to my ears, not the muscular layer). Yet, the excessive thinning of the folds on C4# would have caused pressed voice and a change in quality. Instead, I believe the singer allowed the arytenoids to loosen, allowing free air to escape at the back end of the folds while maintaining the pressed vibration along the edges. An effective trick, but in the end the voice seems to lack substance all the way up to the A4--A consequence of the imbalance on the second note.
To summarize, it would seem that the scientists make a distinction between chest voice and head voice based on the amount of medial pressure. In essence, what is often referred to as chest voice is a pressed phonation that necessitates the participation of deeper, less flexible layers in the vibration of the folds. Head voice is defined by a vibration mode that confines the oscillation to the mucosa but perhaps lacking in fold contact.
What I suggest is a middle ground found in the two approaches that is possible when there is enough contact area along the mucosal edge. In such a case, complete contact is possible between the folds without necessitating pressed phonation, such that would impede the easy oscillation of the mucosal edge. Parenthetically, I believe this is the quality that is suggested by the phrase, voce piena in testa (full voice in head mode), coined by the late Richard Miller. Indeed, the increased superficial contact area produces a richer spectrum while allowing easier airflow, consistent with mucosa-based vibration of head-voice mode.
To answer the original first question, I think it is of benefit to the lyric singer to develop such a balanced mode throughout the range, consisting of increased contact area along the superficial layers. The medial layer, the ligament, possibly participates in the vibration of the lowest pitches, but I do not think the muscular layer needs to participate, unless the singer should attempt a raw chest tone. For a temporary effect it can work, but habitual use of the raw chest voice will only create habitual non-dynamic adjustments over time.