A specialized speaker, known as a subwoofer, is responsible for this experience. The subwoofer is designed only to reproduce the lowest of audible frequencies.
Passive SubwoofersPassive subwoofers are powered by an external amplifier, in the same fashion as other speakers in your system. The important consideration here is that since extreme bass needs more power to reproduce low frequency sounds, your amplifier or receiver needs to be able to output enough power to sustain bass effects in the subwoofer without draining the amp. How much power depends on the requirements of the speaker and the size of the room (and how much bass you can stomach!).
Powered SubwoofersTo solve the problem of inadequate power or other characteristics that may be lacking in a receiver or amplifier, powered subwoofers are self-contained speaker/amplifier units in which the characteristics of the amplifier and sub woofer are optimally matched.
As a side benefit, all a powered subwoofer needs is a line output from a receiver. This arrangement takes a lot of the power load away from the amp/receiver and allows the amp/receiver to power the mid-range and tweeters more easily.
Front-Firing and Down-Firing SubwoofersFront-firing subwoofers employ a speaker mounted so that it radiates the sound from the side or front of the subwoofer enclosure.
Down-firing subwoofers employ a speaker that is mounted so that it radiates downward, towards the floor.
Ports and Passive RadiatorsSome subwoofer enclosures also employ an additional port, which forces out more air, increasing bass response in a more efficient manner than sealed enclosures.
Another type of enclosure utilizes a Passive Radiator in addition to the speaker, instead of a port, to increase efficiency and preciseness. Passive radiators can either be speakers with the voice coil removed, or a flat diaphragm.
CrossoversThe crossover is an electronic circuit that routes all frequencies below a specific point to the subwoofer; all frequencies above that point are reproduced the main, center, and surround speakers. Typically, a good subwoofer has a "crossover" frequency of about 100hz.
Gone is the need for those large 3-Way speaker systems with 12" or 15" woofers. Smaller satellite speakers, optimized for mid-and-high frequencies, take up much less space and are now common in many home theater systems.