Thanks to Kenneth Bozeman, Ian Howell, and Chadley Ballentyne for a stimulating two days on acoustics. I connected dots back to Witherspoon's ideas on the subject from 1925 and found many striking parallels.
I am of the opinion that vocal masters of the past were listening to the play of overtones in the voices which they were training - it is they who developed the term, "registers." And, I know that better and faster results are obtained when a somnambulistic approach is taken in this acoustical type of [...]
We frequently hear agreeable tones, as far as musical quality is concerned, but we have, in nine such cases out of ten, to dispense with a clearly defined vowel, as, in order to obtain the musical quality, the singer generally sacrifices, or distorts, the vowel, so that it might be anything but that which it [...]