I've begun the first cuts on my latest configuration of sub bass enclosure.
This one would be more likely tagged a ninth order, because I am using a massive pressure chamber which feeds three separately tuned resonant chambers to cover the area from just above hearing to well below.
The problem I face is that I may not have adequate displacement at hand. I might have to buy, "deepthroat" long excursion drivers for this to work, lest my pressure chamber becomes starved at times, according to the best math I can muster.
There will be a supplemental driver in each chamber, but I may need a massive one in the lower tuned chamber. One typical fifteen inch driver in that chamber may be too hopeful. I will spring for a deepthroat before I give up on the design, though.