Yesterday, a student spontaneously illustrated a concept, which, upon adequate reflection, leads this pedagogue to observe that certain auditory sensations may be a universal matter, at least for classical singers who sing with beautiful chiaroscuro tone. Why? He sees the same thing over and over again. What particular auditory sensation was illustrated?

Line.

My student was singing beautifully resonant vowels, so I asked him as a matter of inquiry: "Where do you hear that?" What came out of his mouth wasn't as interesting as the talking he did with his hands. They went up to the level of his forehead. Then he traced a line outward to the left and right with both hands. It's not the first time I've seen this. What is fascinating from this teacher's standpoint is that the student usually doesn't have any idea what  his/her hands are doing. In that sense, the words coming out of the student's mouth might be inarticulate, but the hands - indeed, the ears - know their business. Yes. I am well aware that the student can't 'send' tone anywhere, but that's not the point. What is the point?

The student has to learn to trust their auditory sensations and not logic them out.

The following three passages from Warren W. Shaw's The Lost Vocal Art and Its Restoration (1914) shows just how old this kind of thing is. 

Let the pupil look into the physiology of the throat if he so desires, but let him not think that this knowledge will enable him to sing, any more than that a runner should win a race because his knowledge of the mechanism of his legs. The latter wins because he can run faster than his competitors, and knows better how to use his legs for having trained them; and so it is with the singer.

Science is knowing. Art is doing. 

In order that the singer shall be vocally well equipped, the voice must respond to every pitch desired, as also to quality, quantity, and color, within its natural limitations. These limitations are be no means fixed, expect temporarily, for the voice is like a growing plant. Its growth and development depend upon the manner of its treatment. We shall proceed upon the assumption that the voice will gradually take on its natural habiliments in response to mental conception of effect desired and without recourse to any mechanical adjustment foreign to natural expression.

Stand erect and easily, throwing the weight of the body forward, the shoulders will back, the chest up. See that you are not in any way perturbed, and that you are not rigid. Be calm. Now, without any thought of taking breath or controlling breath in any way, sing the exercises, listening to your own voice carefully as you sing, and note the effect. Sing with the idea of producing sounds of equal volume and characteristically the same in quality. In short, sing evenly. Do not pinch or pull in any effort to adjust the position of the tones, and on no account attempt to locally control the breath. Forget it. Remember that breath control is automatic and develops without thought. Sing with the idea of expressing some idea. The mental attitude has everything to do with the result. First, sing the exercises mezzo forte, and idealize the stating of any fact. This we will call the narrative character, and narrative character is not necessarily emotional. Avoid the emotional for the present, but sing with living interest. Sing directly forward, as in earnest speech. Sing on the line. Registers change automatically, unless there is a condition of rigidity, causing interference with normal, natural vocal action. In such cases the remedy is mental energy. Use the willpower to arouse the vital force to produce the musical result and not to locally adjust the mechanism.

The idea of commencing vocal work by holding the head and jaw immovable is erroneous. It it promotes rigidity. Move them both freely, just to prove that the thing is entirely possible. You will find that true repose must be the kind that permits you to move freely without upsetting the normal vocal poise. If you don't believe in such things, try it, and you will commence to feel like a free agent, instead of a vocal slave- perhaps for the first time in your life.

Before commencing the exercises, one word more about singing on the line, that its meaning may be perfectly clear: this one idea is indeed "Multum in parvum" (Much in little). If you sing on the line you avoid the necessity of thinking a number of things at the same time, which is generally difficult and confusing.

The mental attitude is this: I want even tones, alike in quality and quantity, so that the exercise .....

Neither repress nor press. Let the mentally conceived tone pour forward. Instead of singing up and down, sing mentally on a level or line.

By this mental concept as a guide, the actual firmness of tone combined with flexibility of the parts so much desired, will commence to develop. Pay no attention to the throat. Do not hold it firmly or otherwise. Forget it. True resonance will then commence to be in evidence.

This may result in a consciousness of a gentle rising and falling of the larynx. If so, do not attempt to control it. Keep your mind on evenness of effect. The consciousness of this physical activity may be thrust upon the singer at first, but should be allowed to take care of itself.

From The Lost Vocal Art and Its Restoration by Warren W. Shaw (1914).

Want to read more? You can find Shaw's book here.