The size of the enclosure is determined by the size of the woofer or woofers. Be sure to listen before you buy, since some woofers are better optimized for small enclosures than others.

Because woofers move a lot of air, they generate what’s called a “back wave” behind them. If you mount a woofer in free space without an enclosure, then you will get almost no bass because the back wave will cancel out the sound from the front of the woofer.

Here are some common enclosure types:

Free Air

You can mount a “free air” woofer to the rear deck of the car and using the trunk as a big box. Some BMW owners use the ski tube as a port, or remove the speakers from the rear deck. A side benefit of this method is that you do not lose trunk space from a large enclosure box.

Ported

There are many types of enclosures for woofers to handle this back wave. A popular type of enclosure is a ported box. This enclosure has the woofer mounted in a box with a hole in it and a tube or “port” attached to the hole. The port is made a specific size and depth to cause a bump or rise in the frequency response at that point. This makes the overall system more efficient but can cause the bass to be somewhat boomy.

Bandpass

The “bandpass” box has the woofer mounted inside the box, but the sound fires into another chamber within the box that is ported to the outside. This technique increases efficiency greatly, but only at a certain frequency, which can make the system very loud and boomy.

Sealed Box

The most popular type of enclosure is a sealed box. This method has the woofer positioned to fire into the car while the back wave is suppressed inside the box. This method usually produces tight and accurate bass, but it is not as efficient as other methods. The sealed box typically requires a large box to work well, and because of the lower efficiency of this design, you will need more powerful amps and woofers to play loudly.