A subwoofer is a special kind of speaker that reproduces the thumping bass sound, and you add the subwoofer to your existing set of speakers . Keep in mind that if not correctly adjusted, subwoofers can distort the sound - remmber the huge shaking sounds coming from that car filled with teens? 

Wherever you put a subwoofer, the thumping bass sound is more or less the same. Subwoofer spekaers thus can be put anywhere.

The best affordable subwoofer: Monoprice 9723 (12-inch, 150 watts power)

The second best affordable subwoofer: BIC America V1020 (compact, great bass sounds)

The best small, affordable subwoofer: Pioneer SW-8MK2 Subwoofer