The subwoofer is the crown jewel of the car stereo system. This is what creates that beating sound that you hear on cars as they pass you on the street. These are what get you noticed and these are what so many car stereo system enthusiasts will spend their money on. Having a great subwoofer completes the car for many.
However, what exactly is the subwoofer? Surprisingly, many do not know.

A subwoofer is a woofer that is dedicated to the production of bass audio frequencies that go as low as 150 hertz, or in some cases, 20 hertz. There are even some subwoofers that go down even lower. The reason they are called subwoofers is because they offer sub, or lower frequency performance than main loudspeakers. Subwoofers are created by mounting one or more woofers in a brace of wood or plastic that measures between eight and 21 inches.
The subwoofer has been in development sing the 1960s when people began complaining about the lack of bass in their speakers. By the 2000s, subwoofers have become universal in sound applications, from churches and nightclubs, to parks and cars.

Amplification for subwoofers comes in two varieties; active and passive. In active subwoofers, there is a dedicated amplifier to it. This allows the user to boost or reduce the output of the frequency of the subwoofer. Passive subwoofers have a subwoofer driver and enclosure, but there is no amplifier. They will sometimes have internal passive crossovers with the filter frequency determined in the factory of its construction.

More often than not, there will be a subwoofer that is hidden in vehicles because of space limitations. In these custom car stereo systems, the subwoofer will be in a backseat space, or in the trunk. Sound pressure levels in the cabin of the car can get very high because of the small volume of the car interior. You need to be very careful with this because high sound levels can cause hearing loss and tinnitus over long period of times.

Despite this, people will often choose to have large subwoofers that will create a very large booming sensation as they go down the road.
Subwoofers, therefore, have become in integral part of the experience of having a great car stereo system, and a lot of money goes into this.
Some people will actually have huge subwoofers in their car, and the vehicle will only be used in competitions, and never driven because they are no longer practical.