As auditory sensations go, listening to the head ring with tone is perhaps the oldest one in the book. In point of fact, the idea comprises the very heart of the Lamperti School as presented by William Earl Brown, who recorded the teaching of Giovanni Battista Lamperti in Vocal Wisdom (1931). Here are the relevant excerpts.
After the voice is developed, stop thinking of the growing years, and sing from the head down. The presence of resonance in head, mouth and chest (overtones) is proof that your voice is full-grown, full-fleshed. Thereafter you get ready to sing from your head downward, because your head is the instrument.
No matter what the character of the voice be (bass, tenor, contralto, soprano) it should feel high placed and sound high focused.
The slightest escapement of compressed breath must arouse the pitch of the desired tone, vibrating intensely enough to be felt in the skull.
Your voice is focussed only when in its entire range it is intense enough to feel started and stopped in the same spot- the center of the skull.
Your voice will continue focused in your skull so long as you sing with regular vibration in the throat, fed by compressed breath controlled in the pelvis.
Vibration of the voice is like the string. Breath is the bow. What is this added help? Regular vibration. Like resin, it prevents slipping. Energy can play on it, but not by push or pull. It is not heard by the audience. But it is felt by the singer, in his head. It is sometimes mistaken for hoarseness of phlegm. When it appears in the voice it re-educates the entire process of singing because it becomes master. It is this that makes the voice feel like one register, one mechanism from top to bottom.
The feeling of perpendicular vibration from the focus in the middle of the skull to the pelvic control is a sign that the right and left vocal-cords and sides of the body are concertedly functioning, "synchronizing."
Though the start of the tone seems hum-like, and felt at a certain spot in the bony structure of the head, it is useless to insist on the sensation of a focus, until the body instinctively compresses and pelvicly controls breath. The spot where tone seems to start, is the place where the vibration of "ng" (as pronounced in the word "England") is located. Because all tones high or low seem to start in the same place, the voice is said to have one register but three resonances.
When this composite sound is focused in the middle of the skull, it can be moulded by the lips into any form or shaded to any color, changed from one vowel to another and made to open or close at will. It is the "dark-light" tone, which unites all registers, that can be sung with mouth open or shut. It demands control of all muscles from top of head to middle of waist. Keeping such a tone focused in the head depends on the connected energies from waist to pelvis.
The focused start of all tones must be so powerfully charged with energy that it needs a guiding, restraining "hand" (diaphragm). Only when this self acting "attack" is full of controlled energy can it be felt in the skull.
Focus of tone consciously starts and remains in the bony structure of the skull. The sense of touch realizes it.
A "focus" is the spot in the skull where the concentrated rays of vibration (made in the throat) impinge.
The sympathetic reverberation of the middle sinus in the skull- an enclosed cavity in the head directly above the pharynx. In fact, the bony structure of the skull reports all that happens in the throat.
When you realize that nothing leaves the throat (which only sets up vibrations) you will stop pushing and pulling to make your voice "carry." The "carrying power" depends on the regularity and intensity of the vibrations, and not on your efforts.
Lamperti is right. You can't find 'regular vibration' by pushing air through your throat. Nor will it appear via visualization. When brings it forth? Calling. Calling with the strong desire to communicate. Closed vowels like /i/ and /e/ aid it.
"/eeeeeeeeeeeee/ Giovanni!" The Italian street vendor cries across the piazza. The desire to be heard allows the voice to come forth clear as a bell, not some complicated pushing and pulling of muscles.
The regular vibration that Lamperti is talking about? /e/ and /i/ have it when called clearly, /e/ often being more successful in the beginning. Transfer this 'vibration' to /a/ and you are half way there. Obtain it on /o/ and /u/ and you have really learned something, since they are least likely to have it. The next step is exercising your voice on all five vowels within a two octave range. This is what is needed for an operatic role or a pop song with range.
The singer who experiences regular vibration feels all the muscles between pelvis and crown of head lift. In fact, they lift on the inhalation before any tone comes forth. Again, the strong desire to communicate makes this happen.
What is regular vibration? Heightened bone conduction. It leads the voice. He who masters it has a voice.