The 6 things to know about car subwoofer speakers are:
The more subwoofer speakers you use in the system, the louder the audio and the more subbass frequencies you get. Commonly, a car audio system includes two subwoofers.
The size of the subwoofers vary from 8-inches to 12-inches. The larger the driver, the deeper the subbass. So if you are looking for the deepest and loudest bass possible and you are considering two 12-inch subwoofers or four 10-inch subwoofers, select the four 10-inch subwoofers.
The impedance load of the subwoofer refers to the resistance to the current traveling the wire from the amplifier to the subwoofer. This is measured in ohms and is also referred to as the ohms load. You can find speakers with an ohms load of 8, 4, or 2. However, how the amplifier reads the load depends on how the subwoofer speakers are installed. There are two methods for installing speakers. Consult with the installer of your system. Tell him what you are looking for as far as power is concerned and he will recommend the best method of installation.
You will see on the spec sheet of the subwoofer two listings for power handling. One will say nominal and the other will say RMS. This tells you how much power the speaker can handle continuously. This spec helps you determine what amplifier to mate with the subwoofers. If the power handling of the subwoofer is say 100 watts, then you should power it with an amplifier that delivers 100 watts. However, keep in mind that the manner in which the speakers are installed combined with the ohm load of the drivers being used could conceivably bring the ohms load down to 1 or even 1/2-ohm. If that is the case, then the amplifier will be delivering more power to the subwoofer than it has been rated to provide. The point is, make certain that the amplifier feeds the amount of power the subwoofer can handle.
Finally, no matter the size or how many subwoofers you use, they will have to be contained in some kind of enclosure. The most common used is the sealed enclosure. But there are also ported or tuned enclosures as well as bandpass enclosures. The ported or tuned enclosure uses a port and a plastic pipe to tune the box to a particular frequency. The bandpass enclosures are actually a series of different box designs that include chambers within a box and different configurations on how the speakers are mounted in. So you may want to consult with the installer about what style enclosure would be best to achieve what you want in your system.