Subwoofers are becoming more and more crucial to the home theater experience. When you go to the movie theater, you marvel not only at the images projected on the screen, but the sounds emanating around you. What really grabs you, though, is the sound you actually feel; the deep bass that shakes you up and gets you right in the gut.
A specialized speaker, known as a subwoofer, is responsible for this experience. The subwoofer is designed only to reproduce the lowest of audible frequencies. In home theater, this is often referred to as LFE (Low Frequency Effects.
With the popularity of home theater sound systems resulting in specialized speakers for center channel dialog, main soundtracks, and surround effects, the need for a speaker to reproduce just the deep bass portion of a movie soundtrack is all the more important. Although these subwoofers are not quite as "thunderous" as the subwoofers employed at the local movie theater, these unique loudspeakers can still shake the house down or annoy the downstairs neighbors in your apartment or condo complex.
Subwoofers come in two basic types, Passive and Powered.
Passive 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!).
To 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 configurations 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 an amplifier. 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.
For more on the differences and how to hook-up Passive and Powered Subwoofers, read my supplementary article: Passive Subwoofers vs Powered Subwoofers.
Additional Subwoofer Characteristics
Additional subwoofer design variations include Front-firing , and Down-firing, and the use of Ports or Passive Radiators.
Front-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. In addition, some enclosures 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.
Typically, a good subwoofer has a "crossover" frequency of about 100hz. The crossover is an electronic circuit that routes all frequencies below that point to the subwoofer; all frequencies above that point are reproduced the main, center, and surround speakers. 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.
In addition, since the deep-bass frequencies reproduced by the subwoofers are non-directional (as frequencies that are at or below the threshold of hearing). It is very difficult for our ears to actually pin-point the direction in which the sound is coming. That is why we can only sense that an earthquake seems to be all around us, rather from coming from a particular direction. As a result, the subwoofer can be placed anywhere in the room, however, optimum results depend on room size, floor type, furnishings, and wall construction. Typically, best placement for a subwoofer is in the front of the room, just to the left or right of the main speakers. There are more installation tips in the conclusion of this article.