Speakers in your car stereo or home audio system do a great job of reproducing your favourite music and movies, but can often leave a bit to be desired when it comes to low frequency reproduction. Adding a sub woofer to your system can make up for much of this, but the more bass you want, the larger of an enclosure you need to reproduce the sound from the same size speaker. Fortunately there is a simple technique that involves stuffing your speaker enclosures with fibre polyfill, which makes your speaker enclosure act as though it has more volume than it does, resulting in improved bass response.
Purchase some fibre polyfill at a fabric or craft store. Polyfill is inexpensive and comes in bags of 567gr or more.
Remove your speaker from its enclosure using a Phillips head screwdriver. If your speaker is mounted in a car door, the door acts as the enclosure. If the wires are long enough for you to reach your hand around the speaker and into the enclosure, you can leave them attached.
Stuff enough of the polyfill into the enclosure to fill the air space directly behind the speaker. Reinstall your speaker and tighten all the screws before testing.
Visit the source in the reference section for suggested amounts of polyfill to use based on your enclosure size.