The belief that singers should sing the exact vowel written by the composer is entirely logical. However, to do is not natural to the vocal instrument. A singer whose vocal resonance is even and consistently good from note to note—high or low, soft or loud—is changing the vowels semitone by semitone (whether or not the listener can sense it), and the vocal tract is constantly changing form (whether or not the singer takes note of it.)
This cannot be avoided. This is the way the voice works. As Oren Brown, the noted vocal pedagogue reminds us, “Good singers, whether consciously or not, depend on finding an easy adjustment for the pitch. This will be a modification [my emphasis].” Moreover, when voice teachers or choral directors ask their singers to, for example, sing [i] (ee) but drop their jaws while doing it, they, too, are modifying the vowel, for an [i] (ee) sung with a very large mouth will be an [ɛ] (eh).
With the aid of vowel modification singers will have fewer intonation problems, better resonance across their ranges, more carrying power, easier production of forte and piano, clearer diction, and, if choral directors could persuade themselves to use the modification suitable to each section in place of that common vowel indicated for all the voices, a much better blend.
Perhaps this sounds too optimistic to be true. Your doubts will be alleviated by understanding that the described results are governed by the extent to which the tongue controls events of the resonator tube (the vocal tract), and the tongue’s effect on laryngeal efficiency. For optimal results, the tongue tip should rest at the top of the bottom teeth. This position can be taught by saying, “Hmm!” which maneuver will place the tongue tip correctly. Trying to use other tongue postures in an attempt to achieve more resonance does not allow the proper shapes for the vocal tract and creates tongue tension.
Putting the tongue tip at the root of the bottom teeth produces a dull sound.
Pulling the tongue tip up and back distorts all the vowels.
Pulling the tongue back into the mouth forces the larynx into a very low position, delivering
unclear diction and a muddy sound, if a darker one.
Pushing the tongue tip against the back teeth makes for harsh and tinny timbre.
Each inhalation is best executed with the tongue tip on the top of the bottom teeth. Using this position is not difficult and the rewards are great.