basically there are two different subs, enclosed and free air.
Enclosed need a box to maximise air movement, this box needs to be air tight
Free air, as the name suggests can be mounted in free air.
These 15 inch bad boys are free air subs which can also be used in a enclosed box, <img border="0" alt="
" title="" src="graemlins/cwemotions051.gif" />
they can run in either condition.
Free air subs tend to be a bit more of a bank account killer, more expensive
<img border="0" alt="[does this help]" title="" src="graemlins/cwemotions041.gif" />