Today’s lesson is in INPUT SATURATION.  Yep, during the close mic testing I was saturating the A/D input.  This is what caused the strange differences in lower vs. upper mid-woofer 🙁

Below is a close-mic test without saturation.  The output level on the signal generator are separated by 10 dB.  The cyan line is the difference, showing that the difference is +10 db above 100 Hz.  Below 100 Hz the ambient noise on the lower volume recording interferes with the test.

Now raise the signal level by 10 dB as shown below.  No need to take a difference, the blue curve is clearly not 10 dB greater than the green line.