Oh really?
OP, how much space do u have? 2 18s will take up around 2x the space of 4 12s in a properly designed enclosure.
Not exactly. You can do 2.5 cubes per 12. I'd do 6.5 cubes Per 18, so 10 cubes - 13 cubes.
Imo 2.5 cubes for ported is pretty big for most of the underground subs, unless if theyre being underpowered...but thats beside the point.
Maybe I was off a little when i said "around 2x," but ill explain my reasoning. I bigger box needs a longer port to achieve the same tuning as a smaller box. Also, a bigger box would require more bracing than a smaller one in order to hold the same weight without flex (theoretically at least)
Thats means that the box for 2 18s would need to have a bigger NET volume while adding more displacements (port and bracing). This would result in much more (for the most part) GROSS volume.