How would I go about building a dual 12 inch Sub enclosure with deep heavy hitting Bass ?

Do you know of any programs that are browser based or work on mac OSX? winisd was recommended but it won't work for me :/ also it shouldn't be a problem to cut a port between the trunk and cabin , about how big should it be ?

We used to remove the rear deck speakers and use that as ports. In theory, there would be nothing in the way of the sub to the passenger cabin, but obviously that's not feasible. It's just one of those things you'll have to judge for yourself. I've only used WinISD, but you should be able to run it using a VirtualBox instance on OSX.

Thank you for all your help ! I don't actually have a copy of windows (virtual box needs it to create an instance ) would it be possible for you to run my speakers through winisd and give me an idea of the optimum volume/sizing ect ?

Sure. Just email my username at gmail.com with your parameters.

I'm not a car audio person, but have been involved in sub design & audio / acoustic engineering. I'd argue a (little!) bit with crisfromthelc about ported vs sealed enclosures, since which is best does depend a lot on the particular speaker parameters, box dimensions & shape, port size & location, etc. If those don't fall right, ported enclosures can be a peaky boomy mess; it it does work out, then you can gain a few dB of low-end response with a port. I'd guess, however, that ported box in cars are likely different - since you've also got fixed & largely sealed boot & cabin volumes, they'd behave more like a bandpass box design? In which case, yeah, ported boxes could be peaky as hell, so sealed would be much simpler. 2nd'ing WinISD, but the calculations from given Thiele-Small parameters are actually pretty simple for sealed boxes, so online calculators (e.g. http://www.ajdesigner.com/fl_subwoofer/subwoofer.php) are as good as anything for calculating volume. Personally I'm not as enamoured of polyfill etc as the manufacturers are, but not as dismissive as others can be. I'd use it, but not to account for a smaller-than-optimal box size. Don't overfill, but don't use too little - just a single layer, cut into a single piece shaped to line the enclosure, and stuffed loosely in place (i.e. not glued or stapled) so that it won't contact the cone. This will also allow you some leeway to tune the box if necessary. Box shape can be a bit of a dark art, but if you remember the golden ratio (0.62:1:1.62), use polyfill to minimise standing waves inside the box, and follow the usual box-building tips (e.g. http://www.diysubwoofers.org/faq.htm) you can't go far wrong. Remember to account for the volume taken by the speaker itself, as well as any damping material you may use! Don't place the driver in the centre of a panel; offset it along the long dimension to minimise the chance of end<->end standing waves. And use proper non-hardening caulk to seal the box, not a bead of glue or silicone. Oh, and for 2 drivers, add the Vas values together for your calculations.

@Pinback: You're exactly right about the the difference in open air versus a vehicle in regards to ported versus sealed. Unless you're going to great lengths to treat the entire vehicle, sealed has ALWAYS been easiest to make sound right. There's a WinISD online version here: http://www.linearteam.dk/default.aspx?pageid=newdriver I haven't really tried it, so, YMMV.