Thank you all for the kind words, and thank you Al for correcting my sloppy math . . .
On the issue of acoustic feedback, I'd just like to add that it rarely occurs at the tonearm/cartridge main resonance point, simply because it takes a speaker with extremely powerful subsonic response to produce enough energy to "close the loop" at these frequencies. But with a big subwoofer, it can definitely happen. Usually it seems to occur at a frequency that's modally related to room dimensions, or in the mid-bass region where the loudspeakers are efficient, but the turntable's suspension isn't.
I have (and had) combinations which have been between 9.7 -10.0- 10.4
Nearly super (from that Theory) but I was amazed that some carts performed MUCH better in Arms where the "calculation" showed different datas.
If you're not actually measuring the resonance point, there's no way to know whether or not you're getting the best results within a given range. Keep in mind that manufacturer compliance and effective-mass data varies wildly in its accuracy - and poor data isn't an indicator that the theory of operation is incorrect. Similarly, many speaker drivers have inaccurate specifications for their Thiele-Small parameters . . . but this doesn't mean that the matching of the driver and the cabinet is any less important.
Of course, the tonearm/cartridge resonance envelope is just one of myriad factors that determine sound quality in LP playback, but it is one of the fundamentals. It's also something that is particular to playback . . . that is, it's one of the ways in which a turntable is fundamentally different from a cutting lathe.
