The best of the new subwoofers are alluminum subs. I use a single aluminum Audiobahn 12" in my Pathfinder off of a Kicker amp, and I love it. The sound quality that it gives me is very clean and smooth. The surrounds around the voice coils are usually rubber. Other brands may use other types of surrounds, but rubber is still the most commonly used and most durable.
-Voice Coils-
*The more layers and space that you have, the better sound quality you get. There are two kinds of bass, clean bass and "ghetto bass". Clean bass subs include brands such as Nakamichi, McIntosh, Blaupunkt, Audiobahn, Infinity, and other higher end brands. They usually have dual-voice coils and give very clean bass. There are also such things as "ghetto subs" which can handle enormous amounts of power, such as 1000 Watt+ RMS, but their bass is very dirty and distorted, it's not good for real music. I've seen very expensive subs that have quadruple voice coils (Sony Mobile ES), but a majority of the high quality subs will have dual-voice coils, but you can definitly do with one.
-Driver Sizes-
*Well, as you can see, in the car an 8" sub will make the car vibrate and do everything that someone would want a subwoofer to do. However, the larger the subwoofer, it can get: Louder, cleaner, hit lower frequencies, and handle more power. If you've got a tight budget, an 8" will do the job, but if you're an audiophile or are really into music, then bass is would be very important to you. I think the ideal size for any subwoofer is 12" because it can hit the lowest notes without using all that much power. A 12" can certainly reach lower notes than an 8". If you can hear the difference in frequencies or need that "bang", then you should get a larger subwoofer. However, if you just want to fill in some lower notes but aren't expecting that much, then go for a 8" subwoofer or something like that. It would save you money.
-Amps and Power-
*The power of your amp should depend on the size of the driver or how much bass you want. I use a fairly powerful amp to power my Audiobahn. I know people that are car stereo crazy, and they power over 1000 Watts to some of their subs, but that is over-kill. It can break the glass on a car, and if the subwoofer is poorer quality, there is a huge amount of distortion. You should never exceed the RMS rating on your drivers, because one day your sub will go "pop!", and there will be nothing left. I think something around 200 Watts per driver should be enough for most people. Most amps that have much more power than that are over-kill I think.
-In Conclusion-
*I hope that this review has helped you in finding the perfect subwoofer for your car. If you have trouble finding the right brand or anything, just look up consumer reviews or get advice from friends. My personal advice is that Audiobahn or Blaupunkt are both superb brands and you'll definitely get a lot for your money. But, most brands will make consumers happy. Well, good luck!
