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This is a CSD plot of a dynamic driver (from the early '80s) with a convolution filter applied to the test signal before being fed into the phones. The decay measures as being instant over much of the frequency range, although the most stubborn spots of driver ringing persist. Not seen a single electrostat measure similarly by natural means.
For the sake of clarity, would you be able to verify that it was only the driver being measured (and not other possible sources of resonances)? Also, I assume that the driver was measured "free-field". Is this correct?
I am testing a theory based on your data and some of my own, but am waiting to post any results until I can confirm their relevance.