Why add a sub?

Subwoofers are speakers dedicated solely to reproducing low frequencies. No matter what kind of music you like, or how softly or loudly you like to listen, subwoofers are a crucial part of your overall listening experience. Small car speakers have trouble producing enough low-frequency sound to give your music realism and depth. A subwoofer can make the difference between a good-sounding and a great-sounding system.

Some key specs to consider

Once you've got an idea of what type of subwoofer or system you'd like to buy, comparing specs can be helpful in making your decision. Here are some key specs to consider:

1. Power — If you want a system that really booms, there's no substitute for plenty of power. Pay attention to RMS power ratings, not peak power ratings. RMS ratings measure continuous power handling or output and are a much more realistic measure than peak power. Make sure you match the sub's power handling to your amp's power output.
2. Sensitivity — Sensitivity goes hand-in-hand with power to achieve high output. A sub that has a higher sensitivity rating requires less power to produce the same amount of sound as a model with a lower sensitivity rating.
3. Frequency range — Frequency range gives you an idea of how low a sub can play. Keep in mind, though, that the actual performance of the sub can depend on a lot of variables, like the box type it's mounted in.
4. Enclosure type — The type of enclosure a sub is mounted in will have a big effect on the type of sound it produces. In general, sealed boxes give you the deepest, most accurate sound, while ported and bandpass enclosures produce more volume.
5. Number of voice coils — Dual voice coil subwoofers are a popular choice among car audio enthusiasts who want more flexibility in wiring their sound systems. While typical subwoofers have a single voice coil, dual voice coil (DVC) subwoofers use two separate voice coils, each with its own connections, mounted on one cylinder, connected to a common cone.
6. Size of the woofer — It's a never-ending question — what size subwoofers play loudest and lowest? It's not an easy question — you need to consider sensitivity, enclosure type, and available power. If your ultimate goal is to have a system that plays loud and low, and space isn't an issue, go for the biggest subs. But don't underestimate smaller subs. Properly powered and in the right enclosure, smaller subs can put out plenty of sound.
7. Impedance — Most subwoofers are rated at 4 ohms impedance, but 2-ohm, 8-ohm, and dual voice coil subwoofers have become commonplace. When you've chosen a subwoofer, look for amplifiers that will match up to your sub in terms of power rating and impedance. There are a wide variety of amps available, so you shouldn't have any trouble finding one that will bring out the best in the sub you've chosen.