I'm designing a sub box for two 12" Alpine Type Rs to be used in a blow-through config in my single cab Dodge, powered by a PDX 1.1000.

I want to maximise SQ but am looking to a ported enclosure for efficiency and bass extension.

I put the Type R's specs into WinISD which 'recommended' a 3.8cf enclosure tuned to 26.5Hz (with a 4" diameter x 10" length port).

26.5Hz seems low compared to others' ported enclosures, but the transfer function curve WinISD produced is very flat (see below).

Do these figures seem reasonable?

To be honest I'm unsure of how to build a dual sub ported enclosure.

Should the two drivers have isolated chambers like a sealed enclosure and does WinISD account for this?

Presumably two chambers will require two ports but when I double the number of ports in WinISD the required port length more than doubles.

Alternatively should I treat the drivers as completely separate and use WinISD to calculate the box dimensions for one driver and one port, then when I build the box use a central divider to create 2x WinISD's calculations?