Thats a good question, I as well want to know the results, and what size speaker. I kinda want a 8 or 10inch sub in my rear deck more for rounding out the bottom of the midbass, and balancing the top of the low bass with it ^_^
I tried the above ^^^^ "free air" set up, 2 15 inch Elemental SP's and 2 of 4 Tsunami 1100D's (roughly 600 watts per @ 2 ohms)...it wasnt "loud" by any means but it sounded really good for the short time I had it installed this way. The low end extension was pretty intense... 30 Hz and lower would make my windshield dance quite a bit. This way would be much easier and less permanent (even though ive hacked my rear deck to hell for other reasons)
The key element in doing this is too make sure that the trunk and interior are sealed off within reason. Dont expect ground pounding bas with this method. (unless drastic mesures are taken to tune the trunk and car, in that case, some AWESOME spl's can be achived)
The basic idea is to seperate the front wave and the back wave of the speaker as best you can. Just like the rear deck speakers in ou car. The photo shown above is a good example of an infinite baffle set up. ( I can see why it had really good low frequency extension)
I haved one many types of "IB" installations, only a select few were truely "asskicken!".
Make sre the speakers you use are designed rfor this purpose, or else the displacement limited power handling will be VERY low, Ya, that 1000 watt sub willnow only take 124 watts before it reaches its mechnical limits!
I always liked the sound of an IB type setup, they were mostly well defined and offered god kick in the midbass region.
Oh ya, I remember doing a 4 10" IB in a car years ago. All run off an Orion 225 HCCA, Hit 143 dB all day every day! (but the fibregolass work! **** what a pain in the ass!)
The problem is that if you have fold down seats, its going to be tough to seal off the trunk really well. I'd prefer to use those good little JL subs, which one of these days I'll actually have the money to do.