There's a term for a bass trap design which has a solid surface on both front and back with absorbent material between, but I can't remember it. Ahh, here we go -- a membrane absorber or panel trap. Anyways, I guess the foil facing acts as the front part of that mechanism...

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8