The lowest resonance, though tuned by overall tube length, degree of pharyngeal space, and degree of mouth/jaw opening, is primarily sensed in the pharynx and can be termed a kind of "under vowel." The second resonance, though primarily tuned by changes in tongue shape, is perceived to be located in the mouth and along the "ceiling" of the vocal tract, can be termed a kind of "over vowel." The singer's formant cluster is tuned by the epilarynx
and various microadjustments along the vocal tract, and is also perceived/felt to be higher, often in the area of the nasopharynx and in the bony structures of the front quarter of the skull (the so-called mask area).
This tissue vibration is converted to acoustic waves in the larynx canal (the epilarynx
Nature has provided an impedance coupler (transformer) by way of the epilarynx
An added benefit is that a wide pharynx, together with a relatively narrow epilarynx
tube, produces the operatic ring (singer's formant cluster).
Here we have explored vocal tract adjustments in the oral cavity only, but we suspect that enhancement of various harmonics are also facilitated by optimal adjustments in the epilarynx
tube and the pharyngeal cavity.
(1) These exercises are part of a bigger picture of training vocal fold adduction, registration, and epilarynx
tube narrowing for the best acoustic power transfer from the glottis to the lips.