This faculty of the ear to tell from the sounds of a voice how its tones are produced is capable of being greatly developed and refined. For people generally it would probably be of little value to be able to judge voices critically in respect to the manner of their production. But for those who are [...]
A wonderful example of mezza voce singing, especially in the last note on "in vain." http://www.youtube.com/watch?v=3BXLMIZ2sqM
Without troubling themselves to reason the matter out, they [the Old Masters] always obeyed and applied the laws which Nature has imposed on the voice. The first of these laws is, as we have seen, that the voice is guided by the ear. Yet it may be doubted whether they could have stated in plain [...]