I am now of the opinion that the bass cabs with a back panel of foam may be some kind of crude passive cardioid. There is tuning to do of course if no dsp is used with it. Amount of foam, distance between front and back of the driver, size of opening at the back of the cabinet.
Anyway, what use are words to describe what it is. Why should it have to be compartmentalised or pigeon holed. Cardioid is a bullshitters buzz word on forums at the moment with lots of peeps raving on about it. If i manage to perfect the rear panel of foam approach and like it more than true sealed box with no stuffing then i will continue with it. If not then back to ffing about with aperiodic.
With the fear of disappearing up my own arse i am going to stop using words like cardioid, aperiodic etc. What it is makes no difference to anybody, what it sounds like is a different matter
. I also am aware that i have been on a bit of an enclosure reduction crusade
Nothing wrong with that i guess as long as the performance is not audibly compromised .
Ways to reduce box volume;
Use different drivers with lower Vas - not gonna happen
Put up with overblown bass - not going to happen
Use two drivers in isobaric loading - yes, yes, yes
Design a hole in the back of the cab with foam in it or other resistive material - yes worthy of investigation
Enclosure with a foam back panel - yes, worthy of investigation
Complete open baffle or no baffle - yes, worthy of investigation but probably impractical for true dipole due to distance to back wall requirement
Hartley Boffle - not sure about this. Its basically a resistive back panel with internal curtains