i am about to get 2 12 inch alpine type x subs. i probably won't get it til next week. but i am already think about enclosure. sealed or slot port? i'll be working with about 1.7 - 2.0 cu ft with polyfill inside.
usualy ported is louder but the type x does extremly well in small airspaces... my buddy had 2 in a sealed box and it was very nice sounding i would go with sealed IMO
2 answers here from an all things equal in the install standpoint the answer is the ported box plus 3 db over sealed....The second answer is the box tuned to the cars resonant frequency and theoretically sealed and ported could end up equal in spl if it was at the cars threshold for maximum spl ...but if the threshold wasnt reach in this case ported wins again (smiles)
the easiest approach is to test the subwoofer in a sealed encosure with a frequency sweep cd and a spl meter in the car where its going to be installed then do your testing again with the box out side the car at the same volumned you tested in the car and measure the spl again at each frequency and map both test results out on paper and compare and youll find your cars cabin boost on each frequency and you can build your box around those results whether it be from a sq stand point (meaning balanced spl across the frequency spread) or spl...also make sure your amps bass boost is all the way down to make for accurate test results
^thats a good and accurate approach .. but honestly its to much work and playing around for most people.. if your willing to do all of that , do it of course... but back to the question ported will most likely always be louder...and as chauncey said the type x is designed to work with sealed enclosures... id go about 1 cu ft