I should have said "bass leaking in to the room measurement."
I'm setting an impulse response window on my measurement at the listening position to make it more like an anechoic measurement. 7 cycles means the window of time that's allowed into the measurement is 7 wave cycles, so at 1000 Hz that's 7 milliseconds, so I only get in the measurement what's reached the microphone in the first 7 milliseconds at that frequency. By 500 Hz it's 14 milliseconds, which means more reflections are getting included because my room is small. By 100 Hz it's 70 ms, which means a lot more reflections are getting included. So, more sound power reaches the microphone as the frequency goes down, and the measurement becomes less anechoic. The goal is to get an anechoic response that's flat on axis. But since I don't have an anechoic environment to test in, reducing the timing window to 7 cycles gives me a closer idea to the anechoic response, but becomes less accurate as the frequency goes down. If I turn the timing window down to below 7 cycles the analysis becomes increasingly less accurate, so without some more sophisticated trickery of multiple measuring distances like a Klippel machine can do it's not easy to get an anechoic response in the bass. I know more energy from reflections is making it into the measurement in the bass, so I have to shoot for a rising response of some kind as measured to get close to what a true anechoic flat response would be.
One way to attempt it is to measure right up close to the woofer. If you just have one fairly small woofer this can be of some use. In my case, I've got stacks of woofer horns in each corner, which produces surprisingly different listening position response than when measuring up close. If I EQ the woofer horns flat up close, the response at the listening position has a surprising upward tilt towards their upper cutoff frequency, I think due to the increasing directivity of the signal as the frequency goes up with such a large radiating surface that's 2' wide and floor to ceiling height. at 500Hz that's nearly a full wavelength width. I run them up to about 300 Hz or 262 Hz more precisely seems to be where the phase and amplitude integrates most neatly with the midrange horns.
Honestly, I'm not sure how to make sense of it, because the increasing directivity should cause the total sound power in the room to go down, not up. I'll have to think more about that. It's probably more complex than my explanation. It could be that my room is leaking lower bass faster than it is upper bass. I've looked at decay times and that doesn't seem to be the case. The upper bass is actually decaying faster, around 300ms RT60, while the lower bass is much longer. Also clarity is higher in the upper bass, which means the ratio of early to late energy is higher.