Erin is discussing this topic as well or is it another variant of the same idea being "Room gain"?
He is calculating the f0 for where the room gain starts from the dimensions of the room. Here in this thread here, it has been presented that the room gain is a product of pressurizing the room. I hope I have understood this right.
I have tried to measure the room gain and compare this to Erin's method with WinISD. I seem to find the gain in the measurements of my 15" 2235 speaker but it correlates with the WinISD filtered curve only if I use 50Hz as f0 (where the gain starts).
I have read the first post in this thread many times but I can not find an answer (maybe I just don't understand the text) to where from does the gain start.
Erin's video -->
Free Bass! Subwoofer Room/Cabin Gain: What it is and how to model it with WinISD