Oh boy, I'll take a stab at explaining this.
Im typing on an iPhone. Some of this is going to be a bit short so don't chew my head off if there's a few smaller details missing or its not as fully explained as i could make it.
Here we go;
Theres a difference going on here that we need to talk about before things start getting weird. That is the difference between spl and sq (sound pressure level and sound quality).
First let me say that a vehicle (any vehicle) is just about the worst enviroment for both.
But people still try.
For sq (yes, spl also) a 10" sub typically needs right around 1 to 1.3 cuff of sealed box. A 12" needs somewhere around 1.4 - 1.6 cuft. Thats a lot of space to cram into a reg cab. Mount two in there and it's a challange to get it right. You can cheat and make the boxes smaller then oem recommendations, but you sacrifice sq. Subwoofers are a little forgiving here as they don't "see" the shape of the box, they only see volume. Thats why fiberglass enclosures can work and make use of evey last bit of space available in an odd shaped area.
Polyfill (ie: teddy bear stuffing) can often be used to "cheat" a sub to seeing more space in a box than there actually is, but it's a crutch at best and a poor sustitute fir proper construction. It's also not going to help you if you're so out of whack you're something like .5 or 1 cuft under the recommended volume.
Chasing spl is the land of ported boxes (and bandpass boxes). Spl is what (in street slang) "bangs" or "hits" the truck. Problem is, ported boxes require even bigger enclosures. Ported boxes trade off a wider range of response for more spl at a specific tuned point. The ports are another issue, they need to be precisely constructed or the whole thig turns to sloppy poor sounding crap which ruins both spl and sq at the same time. Ported boxes are not for amateurs....since I chase sq, I avoid ported boxes. Not willing to accept the dulling of one range to enhance another and it's just too damned easy to mess them up.
Box construction is another issue. The material has to be very rigid to not transmit sound from the negative wave to the positive side. That's why 3/4 mdf is a popular choice; its very dense and rigid compared to things like plywood (well, its also cheap and readily available). The problem with 3/4 mdf is that the thickness of the material itself takes away from box volume. Say you have a defined space and you use mdf; top, bottom, and 4 sides. Thats a lot of volume eaten up in just material. Do the math, its actually quite shocking how much you loose. Then you have to loose the volume the cone, spider and magnet use up in the box. By the time you get it all done, your outside measurent of 1 cuft for a box is now down to something like .7 cuft when its all together. Now you need more space for a bigger box to regain that missing .3 cuft and there's not too much spare room in a reg cab for that. Or, you use what you got and accept the compromise which is less spl or poorer sq.
Personally, I use 7/8 mdf and never drop lower than 3/4 if I'm fighting a space space. If 3/4 takes up too much volume, i drop my driver size down to stay within oem recommended specs for the space I have available in the vehicle.
Woodworking has to be dead nuts on; perfectly square edges, perfect joints, perfect matching angles at corners. Thats the beginning of a perfectly sealed box and solid construction. Always screwed with mdf screws and glued. I'll often cleat the corners for extra strength and sealing. I use a passthough for the wires and seal that also. If i have large flat surfaces in my construction, they either get ribs inside or some other type of stiffening structure to help prevent the possibility of unwanted resonance. Once dried, I seal it all with rtv silicone. That sits for a week (fumes are harmful to a lot of cone materials) and then the driver can go in, sealed to the box surface with double sided 3m tape. Screwed into place and then a grill or guard with a minimum of 1" more than the drivers max excursion point.
I've had guys darn near beg me to build them boxes, but I only do it for myself. Some say I'm overkill, but my sound is always crystal clear and loud. I get everything out of the hardware its capable of.
I work in fiberglass also, but that gets expensive really fast if you build a box properly. Most of my 'glass boxes are a composite glass wrapped mdf construction to get proper rigidity. Straight glass boxes often resonate too much for my liking unless an inch or so thick, incorporate a system of internsl ribbing or a honeycomb style core is incorporated. Glass can get you out of a jamb in an odd space if used right though.
You can get a 12 or 15 (or two 12's) into a reg cab, but more often than not you end up with sloppy sound or less spl that the hardware is capable of. I've never seen two 15's done properly in any cab unless they used a massive blow through style installation.
Well, maybe one guy actually got enough volume, but in an extended cab, no rear seat and floor to roof, wall to seat box. Basically, built in place in the truck and never coming out in one piece.That had lots of weird resonant sounds too, so you know the box walls were flopping around from poorly support expanses of mdf.. He thought it was cool, i didn't see the point in it. When I leaned in to get a closer look, I noticed he still had stock door speakers, a couple caps screwed to the box, lots of flashy lights and a not so great aftermarket deck. No idea where he hid the amps, might even have been inside that massive rear box. He asked me what I thought of his "bangin' system" since I was actually leaning in for a closer look. I just politely smiled at the young fellow and walked away.....sometimes its obvious there's no point in telling someone the truth...
Usually, a smaller driver in a properly sized enclosure will outperform those bigger units that have been installed in a compromised enclosure. It may be loud, but the best you know is the best youve ever heard. Big drivers and big watts dont always mean its the best system...
Wave propagation is another story. No matter what vehicle you add a sub to, you're not going to get accurate low wave reproduction. That's because the low frequency sound waves produced by a subwoofer are about the length of a city bus. The full wave needs to develop before it reflects off a surface or it ends up (for lack of a better word) clipped. Sometimes you can "cheat" the wave and find a distance that will match a part of the wave cycle and make decent reflection, but you're talking something the size of one of the competition style full size vans you see winning spl drags and such to even have a chance at decent reflection. Thats why they use vans and not a 1985 camaro or anything else small...
A bit of free air space in front of a sub is a good idea, but it really doesnt make a whole lot of difference in a vehicle. As long as the cone can reach full excursion without hitting something, its about as good as its going to get. Buttingit right up against a seat is somewhat detrimental and the material vibrates opposite of the subs output, butit's more about the crap noise the seat generates from that vibration than countering the low freq wave.
Keep in mind, we're only talking subwoofer enclosures here. I even touched on the sub itself, wiring, power, amps, sound stage or imaging issues yet.
Yes, I'm an audiophile.
Yes, I've built "one or two" very custom high end systems in my time.
No, I'm not a pro installer.
I'm just a guy who is handy around the shop, who likes good sound and is willing to learn how to get it....
