In respect to resonance, as in other respects, the human voice is far superior to all other instruments, for their resonators are fixed and unchanging, while the human resonator is flexible,-in Helmholtz's words "admits of much variety of form, so that many more qualities of tone can be thus produced than on any instrument of artificial construction."
"Resonance in Singing and Speaking"
Thomas Fillebrown
Now "voice placing" depends primarily on correct vowel placing, which in turn depends on proper adjustment of the resonators, which again depends chiefly on the positions and motions of the organs of articulation.
"Resonance in Singing and Speaking"
Thomas Fillebrown
Neglecting to cultivate the resonators.
"The Mechanism of the Human Voice"
Emil Behnke