It is very important to choose the right type of enclosure that will give you the best sound for the types of music you listen to. There are 4 main types of enclosures, each of them have their own pros, and cons for each. You should review this guide when purchasing, or building your own subwoofer enclosure. It is important to choose your enclosure before you choose your subwoofers.
Sealed: A sealed box it very tightly and well constructed, it is made to have no air leaks. They are best for deep, tight, and accurate bass. They are not as boomy, as the other boxes, and in fact have less response, giving more of a flat response. It usually requires more power than other boxes, so you should use a large amp, and powerful sub. These are often available in many shapes in sizes from various audio stores.
Ported: These boxes offer good low bass response. The response is much louder than a sealed box, and is perfect for rock, and heavy metal. The bass from these enclosures is much deeper than a sealed box, giving a good kick with the airflow through the ports. With the proper math, and calculations you can find where a port will go best to give you the best response for the frequency you are going after.
Bandpass: This is a combination of a sealed and ported box. Usually the cone of the woofer is on the sealed side, with the coils sticking out into a ported box. The sound travels out of the ported side, and delivers a huge hit that is very loud and punchy. They are best for rap, reggae, and hard rock. These boxes are usually sold at stores like Best Buy, and Circuit City with 2 subs already installed in them. They don't offer very accurate bass response.
Free-Air: People use these when generally they don't want a huge box taking up space in their trunk. The special free-air subwoofer is usually mounted to the rear deck, or on a separator for the rear seats. They use the air in the trunk as their box. Generally, they do not sound good at all.
Once you have decided you can get your subwoofers, some are made specially to have good response with a certain type of enclosure. It is important to know what your sub sounds good with, so be sure to be up on the specs. Some may only like sealed enclosures, or require a lot of volume. Be sure to know what you want and do research ahead of time. You don't want to have a 12" enclosure, but only a 10" subwoofer after all.