There are a few key things you need to know when choosing the right subwoofer system for your car and one of those items are the design of your enclosure system:
A - Sealed: Sealed systems use a single (or multiple) subwoofers in an enclosed box with no bass ports. In this kind of design, the subwoofer itself and its motion is the main sound generator as the subwoofer moves forwards and backwards creating sound pressure waves. The advantage of these kinds of car subs is that the sealed enclosure helps the dampen the subwoofer allowing it to return to its resting position as soon as possible. What this means is that a sealed system, especially with a smaller sized subwoofer, gives the best bass response of all systems and that is why typically you'll find that director monitor speakers used by sound professionals in pro studios will use smaller 6.5" or 8" subs in a sealed bookshelf speaker system.
B - Vented: Vented systems use a single (or multiple) subwoofers in an enclosure that is specifically tuned to have a specific resonant frequency. In these types of systems the function of the subwoofer is to move and pressurize the air inside the subwoofer box, and the primary sound generator is the resonant frequency of the box itself, while the sound exits the box from tuned bass ports (or holes). These types of boxes don't play all bass frequencies equally and are typically tuned around a frequency that sounds better to the listener, such as a nice 40hz wave, and they give louder bass at that specific frequency than any other. Although these types of systems are not as responsive or accurate as a sealed system, they have the advantage of being able to fill a larger space (such as a club, venue, or large SUV) with more bass from a smaller driver or speaker. A good example is that if you look at most 2.1 sound systems for desktop PC's you find that the small sound system uses a single 6.5" or 8" subwoofer in a vented box. The vented box helps fill the whole room with bass from a smaller box powered from a lower power amplifier.
C - Free Air: Free air subwoofer systems are used more for show. Since the subwoofer basket is exposed and visible, free air installs are very showy and fun. However to make a good sounding free air system, the subwoofer itself needs to have a more rigid suspension. The reason for this rigidness is that the is no trapped air in the enclosure to help dampen the subwoofer's movement and prevent distortion. Thus free air or infinite baffle subwoofers typically require a Qts of 0.7 or above. (Qts is the total suspension dampening factor for the subwoofer at it's resonant frequency Fs).
D- Band pass: In a band pass system the car subs installed on a mounting surface INSIDE the box with the a side facing port or vent. The subwoofer creates sound pressure waves inside the boss due to the subwoofer cone's motion. These waves are reflected and amplified inside the box, but only a certain narrow band of frequencies is allowed to exit through a strategically located bass port. Unlike the vented system that has a typical 3-4dB gain over a sealed system at most frequencies (if installed correctly), a bandpass also amplifies a single frequency that the port and box are tuned for, but it also mutes all other frequencies preventing them from passing. This type of system requires a good complimentary set of mid-bass drivers to fill in the rest of the sound range but it does have great bass characteristics around a single choice patch of frequencies.
