Loopback Bathtub curve

Hello,

I'm new to RoomEQ, and have a question:
When doing a loopback measurement on my prized M-audio Fast Track Pro (USB sound card), why am I getting this bathtub curve?


I realize that RoomEQ is intended mainly for a <200Hz effort, but shouldn't this work?
I've tried the with and without sound card loop back calibration, but calibration (or lack thereof) does not explain these large deviations from flat. Interestingly, when doing the calibration, I see the nearly-flat response I expect for the sound card (verified by other software).

The scope plot does not indicate the rise at low and high frequencies


And the normalized impulse looks quite clean


Wonder if I am doing something wrong with the FFT processing?

Regards
Flemming Nyboe, Copenhagen, Denmark

[JohnM]

The "C-weighted SPL meter" box is checked on the mic/meter settings, so REW is applying inverse C weighting to the measurement.

Hi JohnM,

Yes, exactly.
Thank you.

-- Flemming