Hope it works out for you.
If you're looking for accurate measurements above ~4KHz
with the ECM8000, I've compared a few with an Earthworks
cal mic and they all had a few dB bump above 4KHz then back
down (and falloff) at 20KHz. The manufacturer typical curve
published shows the general shape, the peak amplitude just
seems to vary a bit, some more than others.
The ECM's can still be quite useful, just understand that it will
read a bit higher than "truth" in the highs.