That's a lot of work, and great information. Thanks for posting it here.

I wonder if your 750Hz peak, and possibly the 300Hz peak, might be caused by sympathetic vibration of the material you've used to make your phase plug. Having support on the top and bottom only, I can easily visualize it entering into a rotating vibration mode, torquing the supports and resonating at a frequency determined by its weight and the stiffness of the supports.