Remember how I started taking measurements saying that it was confounding? yeah...
Here's the frequency response plots for the three measurements I kept the first time, as a reminder:
And the list of predicted modes:
I went back downstairs today to pour out two more bags of sand (looks like one more will be the last one) and consider some of the things that came up in the discussion of the first set of measurements. In the process, I measured the diagonal that J_P_A mentioned in reference to the mid-30s peaks - it is about 14 feet as predicted. When I did that, I may have nudged the mic an inch or two, but I can't believe that the small movement of the mic will have changed the measurements to look like the new ones. I figured I would start with duplicating yesterday's work, just to verify set up. Well, the levels got adjusted again, so not only are these levels not absolutely right, they can't be compared directly to the first set (in terms of level).
Here's the two sets of measurements on the same plot:
I don't know what to say about that.
Next I moved the mic to the center of the rear plane of the room. The mic was hanging from the outlet where the HDMI runs through the face of the rear soffit to where the projector will go. The mic hangs down to approximately the center of the vertical height. I didn't measure when I set it up - only just now as I write this up. It's 52 inches from the riser deck, and 40 inches from the ceiling. So it's 6" above the center of the rear air space, and maybe two or three inches to the side. Without adjusting anything in the computer or soundcard - only moving and replacing the drywall "door" I generated these three sweeps.
At this point I also generated a waterfall graph for this new set:
That's when I noticed the hum in the noise floor at 120Hz. I assumed it was because I had the laptop plugged into the wall. So I unplugged it and tried again with some sweeps. This is when the impulse error I had yesterday came back. I tried three sweeps, all with the same error. I then plugged the laptop back in, and the error went away. Magic. I also noticed that the level changed when the power cord was unplugged. There's something going on here. I don't want to get into it right now if there's anything useful to say about these new measurements; but really there may not be anything worth saying, because I am now less confident in all of these based on the weird influences of the power cord that I've noticed.