Vocal mechanics are required in mid-range. No really good vocalist can do without some kind of knowledge of what happens in the mid-range pitches where the voice must change gears, with two exceptions: a counter tenor and a true low bass. Even a lyric coloratura soprano has to descend into lower pitches these days. It really isn’t possible any longer to have a major classical career singing only in head voice as a female.
Those who do not understand or deal with registration will always be at a loss in explaining or teaching vocal technique in mid-range, relying primarily on “resonance strategies” which are ineffective as a substitute for register balance across the “primo” or first passaggio at approximately E or F above middle C. This break, which is at more or less the same place for every voice category, male and female, young and old, can be avoided. It can also be pushed out of the way (but at a cost) and it can be uncultivated or invisible in a weak, small voice. What it cannot be is eliminated.
If you assume that most people speak in “modal” or chest register, not in “head” or loft, then in most people the weak register is head. The crico-thyroid muscle, which stretches, thins and tightens the vocal folds to raise pitch, produces a “lighter sound” as it activates the folds to vibrate only on their upper edges. It takes a good deal of strength in the edges to resist air pressure coming hard on from below, so head register is typically breathy in untrained voices, but the capacity to develop that strength is an absolute requisite if the voice is to function optimally. That is one of the main points of functional training.
I teach all my students to balance their voice across the break in order to maximize vocal freedom and have the most amount of artistic choice about where to go and how to get there. This approach has kept my own voice, now at 61, able to vocalize through four octaves (F below middle C to F above high C) and to sing a belt sound to D, a mix to high C, and head above that. The choice, when I am practicing regularly, is up to me, insofar as how I sound when I sing and I do not mix my sounds unknowingly. Pop music does not accidently sound like jazz, and Broadway belt songs don’t sound like Mozart, particularly in my mid-range pitches. They are the hardest to deal with and keeping them in balance, so that the middle “pivots” as needed is very hard, continuous work, but it is not so hard as to be unavailable or unlearnable. If you study with me long enough, you will do it, too.
In order to have strength in the middle voice, you must develop head register first, crossing it down past the break until it “settles” and develops the ability to withstand breath pressure. Then you can work on strengthening chest register, without pressuring the tongue in the back, and carrying that up to about E/F/G above middle C. If you do this enough, mix emerges and you don’t have to work on it. It shows up because the instrument has to make an adjustment in order for the larynx to remain freely moveable. Of course, most people need assistance to get this to happen because there are myriad ways for it to go off on a not-too-wonderful tangent that causes, rather than eliminates, vocal problems. Breathing here is an ingredient, but THIS CANNOT BE DONE THROUGH BREATH SUPPORT. No. Nor can it be accomplished through manipulation of “the resonators” since the only factors involved are the tongue itself (it’s position and shape), the soft palate, the lips and the jaw. You cannot vibrate your sinus cavities. They do not add anything to the sound (FACT — go read Johan Sundberg’s book if you do not know this). After that, if you want to learn to belt, you can, but you have to gradually take “chest” up without pressure or weight and without changing the vowel sounds too much. There is more than one way to belt, of course, just like there is more than one way to sing classical or Broadway music, but you only have the vocalis and the crico-thyroid to adjust the source (the vocal folds) so you are in chest if you aren’t in head (as the primary default) or vice versa. The people who say that belting isn’t chest register do not understand register function…..and usually do not themselves, belt. Read the science, not the pedagogy.