The Choirmaster's Manual/Chapter 3
CHAPTER III
ON VOICE-PRODUCTION AND THE REGISTERS
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Having obtained a fair increase in expansion, slow notes on "ah" should be taken, very softly. The student should try to imagine that breath is being inhaled, a sort of yawning feeling. This has the advantage of making the voice bright and clear by raising the soft palate, and also of saving the breath.
Exercise 1.
Work to between 20 and and 30 seconds.
Registers. On the subject of registers much has been written. The best results have been gained by "anticipating" the head-register in ascending, and bringing the head-register down in descending scales. This is the "golden rule" of singing; in many books the first half is taught. "Never allow the lower register to be forced up."
It may be taken as a safe rule never to allow the use of "chest"-tones by the boys. These notes are naturally those below E.
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Exercise 2.
The best vowel for head-tones is "oo." Take F on the fifth line, softly and clearly, then sing descending scale without altering the feeling in head or throat.

Practise diligently, taking a semitone higher each time. It is impossible to sing "oo" as an upper medium note on F or F♯ in most cases. If a clear head-tone is not obtained by all boys, it aids the effort to place the right foot slightly forward, resting the weight of the body on it, at the same time lowering the head a little, and thinking of the top of the head. With the writer, this has proved efficacious in all cases.
Having obtained clear head-notes on A♯, B, or C, or as high as you wish to train the boys, care must be taken that the "oo" vowel-tone does not become habitual with all head-notes. To avoid this use the following:
Exercise 3.

In singing this, take care that the tones on "oh" and "ah" are not louder than on "oo." These three vowel-sounds must be gradually merged into one another, "oh" being felt in the same position as "oo." This exercise gives a boy a real feeling of breath-control, especially on the lower notes, as "ah" produces a much louder tone than "oo" if left to itself.
To get rid of breathy sounds, practise Exercise 4.
Exercise 4.

(Do not sing these notes staccato, but as short as possible. Feel tones are made by a sort of "click" at the vocal cords, not by action of ribs. Do not allow the ribs to move until after the tone is made.)
Resonance. The registers of a voice are divided according to the feeling produced in the singer. The "lower medium" notes sound as if produced in the back of the mouth; the "upper medium," in the front of the mouth above the front teeth; the "head" tones are felt in the highest part of the back of the head.
The mouth is the chief resonance-chamber, and the wider it is open the greater the resounding space and the louder the voice; the pharynx and head-cavities all act as resonators, reinforcing the tones by the addition of overtones. "Ah" is the best vowel-sound for producing resonance.