The Free Voice is being republished by Oxford University Press. It should be out Summer of 2018.
Here is an excerpt from Chapter Five, Aesthetic Judgment:
There are many influences at work in determining quality. But beyond such factors as technical condition, aesthetic goals, and musicality, there is a quality which is intrinsic to the personality and the mechanism itself. This “natural” quality is determined by anatomical structure, combined with peculiarities of temperament. The thickness of the vocal cords, the size and shape of the cavities of the mouth, throat, and head, all contribute to a state of being that fixes the boundaries of ultimate potential. The way the instrument is used, however, alters the character of the tonal emission and to a considerable extent leaves potential unrealized. According to the efficiency of the vocal response, quality will be either a true or false likeness of the intrinsic timbre. As the natural quality of the voice is almost always unknown because of mechanical imperfections, no attempt should ever be made during training to cultivate what is thought to be the natural quality. Genuine tone quality can only be revealed by purifying the vowel quality and correcting the imbalances within a maladjusted registration.