Thank you so much for your comments, Sean.
I am used to trying to figure out how things work through black-box analysis, rarely getting any help from the developers who actually know how they work. Your transparency - through blog and forum posts - should be an inspiration to others, many of whom would rather propagate mojo-myths than explain how something works.
Phase-coherency is one aspect I've never dwelt upon, because I figured it was way too complex to understand.
Obviously, in acoustical reverberation phase is all over the place as each reflection is delayed by different amounts. It stands to reason that this would be a huge factor in determining how the reverb wash sounds. But gosh, how does one go about analyzing it? You'd need a supercomputer to ray-trace every possible path, calculate their phase shifts, and sum them moment-by-moment. (Although analyzing a plate has got to be far, far simpler than a cathedral.)
And now you tell me that the speed of sound varies with
frequency in a plate. This is a mind-blowing revelation. Why is that the case in steel but not in air? In all my reading on acoustics, I have never come across any reference suggesting that the SoS is not constant in homogeneous materials. You see plenty of tables that say "here's the speed in water, copper, steel" but none have a footnote saying "this is at one kilohertz only".
Not that I doubt it - you've ascertained it by measurement. I now see that I should be testing using different frequencies of sine waves rather than the single square wave impulse (1000 Hz) that I used.
Thanks again, Sean, for the information, for another great product, and (best of all for us po' folk) keeping the price down to a week's worth of peanut-butter-and-jelly sandwiches.