Importance of tone separation of plates...
...The best written instructions on the topic I have seen are that the plates tones should be even all over the plate and that the back plate should be somewhat higher in frequency (tap tone) than the top plate (belly).
...The acceptable range is at least 1/2 tone (one half of a full tone difference) and no more than a full tone difference. Anything more or less and the instrument will never be successful (no matter how long you wait!)
...After seven years of learning, building, testing and research... I believe that this the following is good solid proved-by-experience advice. A reasonalble goal is thus:
..."when the plates are fully mature (have stopped changing frequency) the back plate should be 3/4 of a full tone ( 1 1/2 semi-tones) higher than the top plate".
...For example, if the top plate is D4 (295HZ) then the back should be 318HZ, that is more than D#4 and less than E4.
...A reasonable survey of both the evenness of the plate tone and the separation between top and back plates can be done with taps from the fleshy tip of your finger (with a lot of practice of course).
...As I had reached this conclusion before the VSA (Violin Society of America) national convention held in Portland, Oregon in November 2004. Part of the every two year contest is a tone competetion. About 100 violins were entered by makers from all over the world. Ten instruments were awarded either silver medals (the highest tone award) or certificates of merit for tone. I examined all these insturments and they all had this evenness of the plate tap tones and about this half to a full tone plate seraration. {the same was true for the viola and cello competetion winners too.)
...Next I examined the appearance winners that did not also earn an award for tone, and they did not have this agreeable plate tuning; so I think that proper plate tuning is an important indicator for a pleasing tone.
...I have read one author who says that he has heard good violins with the top plate having a higher tone than the back plate, but it is the usual thing for the back plate to be higher.