The speakers that come in most cars are usually not quite top of the line, although many automobile manufacturers have recognized the importance of good speaker design, so they are using some really good components as an incentive for the entire automobile purchase. Subwoofers are the most immediately evident improvement one could make to an automobile's sound system. Subwoofers are available in two forms--raw and enclosed. An enclosed subwoofer consists of a driver mounted in a box, or it can refer to a band-pass box. A raw subwoofer is just the driver--a speaker cone suspended in a "spider," the metal frame that holds the speaker. The motor consists of the voice-coil and magnet. Enclosed subwoofers are typically unpowered, so you'll need a channel of amplification. At NuGadgets.com you can find high-quality car amplifiers by your needs. Replacement speakers come in two basic designs: coaxial or component. A coaxial speaker has the tweeter suspended over the woofer. A component speaker system consists of a separate woofer and a tweeter that can be positioned for a more accurate high-end response and a wider stereo image. They usually come with the necessary crossover electronics. Although usually considered a separate upgrade, you can also add a subwoofer.