Re: Bandpass, ported, or sealed?
Don't forget about transmission lines. ;-)
As was said, there isn't a black & white, one box fits all answer but that's mainly because real estate in vehicles is so hard to come by and of course, because different strokes for different folks.
My personal preference is that I'd almost rather not have any system in my vehicle than to have subs imprisoned in a crummy sounding sealed enclosure. Sealed boxes are, to me, a waste of lumber and a waste of potential. The gains you get from them with respect to group delay are far outweighed by the poor low end response they produce and there is virtually no possibility of ever achieving any acoustical gain from them.
Ported boxes are the best all around compromise between space and performance. A properly designed an built ported enclosure will be accurate and loud and will have a lot more low end extension than sealed, all other things being equal.
Fourth and Sixth order bandpass boxes work very well for getting stupid loud in a narrow bandwidth and they're pretty convenient in instances where you need the subs to be safe from damage or in blow throughs, as you only have to have the port(s) firing through the back of the truck's cab.
The best performing box you can build is a ¼ wave transmission line but they have two down sides that makes them impractical in car audio. First, they take up a lot of space and second, they present a huge amount of design difficulty, owing to the fact that they're basically a 7 to 9 foot long port that has to be snaked into a small area. That said, you get the widest bandwidth, flattest response and most accurate group delay that you can get with them so in my opinion, the design difficulty is well worth the trouble.
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