… The possession of a fine “natural voice” is not an anatomical phenomenon but a technical one, inasmuch as any healthy voice can, theoretically, become a great “natural” voice if it is really well produced – i.e. properly trained. Again, since the correct use of the voice depends upon the proper development and coordination of all the muscles which should be used in the act of phonation, and non-interference by the muscles which should not be so used and, since such coordination depends upon mental concepts, a great “natural voice” is, in the final analysis, primarily a psychological and physiological phenomenon.
Stanley, Douglas, Joseph Pease Maxfield, and Alma Stanley. The voice, its production and reproduction. Pitman Publishing Corporation, 1933.