its a folded horn.
have you ever seen a trumpet?
well instead of blowing into it with your mouth, imagine a speaker there. hey, kinda like a bullhorn.
as far as car audio suboofers go, the USUAL folded horn looks something like this inside.
but for suboofers. so they are big. and i have yet to learn how they work or are made. so dont ask.
or maybe a transmission line. wich is essentially a ported box, but with no "chamber" just a real long tube.
"A transmission line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic waves or acoustic waves"
basically its a waveguide for real low frequencies (in this case for suboofers) - generally speaking, they are "tuned" by having the "tube" 1\4 the wavelength you are wanting the box to peak at- from what i recall correctly that is.
think of all "ported" enclosures as a physical amplifiers //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif