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"Do Subs need to be in a box to work properly? Or Can I mount them rite onto the board and have them hang from the MDF board and just have the whole inside of the trunk be the enclosure"
well, yes and no.
a quick synopsis of why you use a box: the signal being delivered to a sub is in an a/c format, meaning that theres a baseline at wich there is no electricity being passed on to the sub, but a wave is overlayed on top of that. as the wave moves above the null line the sub cone moves outward (assiming correct mechanical polarity). the wave the decends to the null line and the sub moves back to its resting point. after that, the wave makes an identical form, only below the line, causeing the sub cone to move inward towards itself. this wave is a 180 degree oppposite of the "front" wave. this applies to a sub that is hanging by a string in open air. as the sub moves, there are two physical wave fronts produced...one from the front of the subs cone and one from the back. assuming the open air setup, these waves will collide and produce what is called cancellation. because the two waves are equil in amplitude (volume and structure), but slightly off in timing, the "up" wave (made by the front of the cone) will negate the "down" wave (produced by the back of the cone). by putting a sub in a box, you change the reflexive qualities of the subs movement, as well as getting rid of cancellation caused by "down" (or more properly "rear") wave interference.
SEALED BOX: when it moves forward, it creates a partial vacuum inside the box that adds resistance to the subs movement. this is called rarification. when the sub goes on its opposite stroke (inward), it compresses the air inside the box, also creating the exact same amount of resistance. this is called compression (not to be confused w/ audio or electrical compression). this does two things: it competly isolates the front and rear waves so they dont cancell each other out. the resistance from the sealed design changes the physical properties of the subs mechanical reaction to an input signal. by putting the sub in a larger and larger box, you present the sub with less mechanical resistance, and by putting it in a smaller enclosure you increase resistance. i wont get into how that effects the sub so much, but just know that a smaller box tends to have a responce curve that is "peakier", meaning it will play at a middle-bass note louder and fall off in the low and (to a lesser extent) upper bass notes. this sounds pretty good for kick-drums and slap bass, but can sound weak w/ rap and electronically produced low bass notes. a larger box will shift the peak range down so that lower notes sound better, but it also looses some of the "punch" asociated with a smaller box. it also decreases the amount of power the sub can handle (due to the lower mechanical resistance i mentioned earlier).
VENTED (or) PORTED: by adding a tuned port (basically a tube or a cubical shaped opening that encompasses 3 dimensions) to a box, you are (in effect), re-alighning the rear wave of the sub to the front. with this setup, you are adding the energy coming from the back of the sub to the energy coming from the front of the sub in a way that doesnt cause cancellation (over a majority of the frequencies) by changing the velocity of the rear-exiting wave front. port tuning is a very complicated matter, so i wont explain much about it. just know that the the port is tuned to a certain set frequency, and the lower tuned box responds much like a large sealed box. a higher tuned box sounds much like a small sealed box. the difference is that porting adds quite a bit of volume, since you are combining (or "coupling") the rear wave to the front. the problem is that below the tuning frequency, the sub acts just like as if it was hanging from a string in open air, and this lack of mechanical resistance will cause damage to the sub quickly, plus you will get next-to no sound out of it at those frequencies.
FREE AIR (or)INFINITE BAFFLE (or) ISOBARIC: this is the situation you are asking about. In this case, you adding a very minimal of mechanical resistance to the sub because of the fact that there is no box. the sub must sit in an airtight baffle that seperates the front and back waves though, otherwise there would be tons of cancellation. the idea is that certain subs are built for this application, and thier natural mechanical characteristics lend themselves to this type of setup. most subs are designed to be put into sealed or vented type enclosure, so this isnt really a viable option for most of the product out there. thats not to say it cant sound good though, i actually prefere the natural sound of an infinite baffle setup w/ most subs, although you loose a LOT of output and the sub wont handle much power (again, because of the lack of mechanical resistance). the upside is that the install can be much easier, and (as i mentioned earlier) the responce can be a lot more natural, especially on music like jazz, where you dont want a lot of big peaks and dips in the frequency response. it sounds nearly unlistenable with more electronic stuff like rap though, because you loose a lot of impact.
so in conclusion: i would recommend you go with the manufacturers specs for a sealed box w/ those subs (ive heard them in a lot of installs in the past, and i liked that the best). as a side note, ive always had the best luck w/ building a box on the larger side if possible. ive only scratched the surface on this long ass near-diatribe about boxes, so i recommend you go to a site like www.sounddomain.com and do some research there. im sure youll find info that will takes days to sort over, lol. if you have any more questions, please feel free to ask, i love talking about this obsession of mine, lol  :shhh: i do cocaine :/shhh:
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