A sub-woofer is a speaker optimized to reproduce non-directional low frequencies. These are usually the sounds made by exploding space ships, (never mind there is no sound in outer space) the rumble of machinery etc. While this speaker will greatly enhance your system it is not necessary. Good bass response in the front speakers can take up the slack of not having a sub-woofer. If you do get a sub-woofer, maker sure your amplifier or decoder has a separate jack for one. In any case, make sure your amp or receiver can handle sub woofers properly. This means dedicated controls to tailor the characteristics of the sub woofer to your room's acoustics. Also, if you don't have a sub woofer yet, be sure the decoder and/or amp can by-pass it. When you add the sub woofer you can then set the controls to see it. While you can go without a sub woofer if you can afford one get it. It takes the sound track from just hearing the movie to feeling it. Also, many receivers can take the very low frequencies from audio sources like music and TV programs and run them through the sub woofer for better sound quality.
There are two basic types of sub woofers, the passive and the active or powered. The passive sub woofer takes the full signal from the receiver just like any other speaker. The powered sub woofer has it's own amplifier in it and requires not only a connection to the amplifier but also one to the wall outlet. Both provide extremely low bass. With the powered sub woofer the amplifier is designed only for the low frequencies it must handle. This provides a crisper, clearer sub bass response. The powered sub woofer usually has some degree of control such as the crossover frequency. This is the frequency that the sub woofer will take over. This control is more important with Dolby Prologic mode and other non-Dolby 5.1/DTS modes since in DD 5.1 and DTS the sub woofer has it's own independent channel. One little installation tip here. Make sure your sub woofer has four rubber feet on the bottom. This may seem trivial but its not. This will prevent the speaker from rattling when in heavy use. If you didn't get these feet with the speaker go to a hardware (or some computer) store and buy a set. They are cheap and will improve the performance a lot. This is especially true for rooms with hard floors such as wood or tile. In Prologic mode most sub woofers are provided some audio information. Unlike the 5.1 modes this information is sent to the sub woofer only on the basis of frequencies. To test this tune into a music video channel or put a music CD on. The sub woofer will pump out the very low frequencies. The frequency where the sound is sent to the sub woofer is called the cut over frequencies. Audio information at or below this frequency is sent to the sub woofer. Frequencies above it are processed by the woofer of the regular front speakers.
There are two ways to wire a sub woofer. The first is direct to the sub woofer output from a six channel receiver or amplifier. This will permit you to enjoy full six channel sound form sources like DVDs, Laser Discs and the new 5.1 cable channels. Most mid to high end receivers have cross over circuits included that will automatically process the very low frequencies to this output. The frequency used as the cut off can usually be adjusted. Some powered sub woofers also have such an adjustment of the cross over. If it does it is important to make sure the frequency set on the receiver matches the sub woofer. The second way to wire a sub woofer is to run the outputs from the two front speakers to the sub woofer and then out to the left and right speakers. In order to wire the system this way the sub woofer must have both two inputs and two corresponding outputs. If this method is used you will not have sound coming from the SFX or sub woofer channel of digital six channel sources. I have found it best to always use the discreet sub woofer output. Even if you don't have a six channel source let the receiver handle the cross over for the sub woofer. This method provides greater control and needs a lot less wires. In the second method you have wires running from the receiver to the sub woofer and then back to the speakers. This can get messy.
The direction of the speaker in a sub woofer can also make a difference. There are two basic types with a few variations. The first is the front direction. This means that the speaker points to the front of the sub woofer's cabinets, much like any other speaker. These subs are often ported. A ported speaker has a solid front with a hole in it. The sound is focused out of this hole. If you have this type of sub you can feel the rush of air during heavy use. The next main configuration of sub woofers is the downfacing. Here the sub woofer's speaker is pointed to the floor. The ports on this model typically are at the bottom at the front and two sides. This distributes the sound more evenly throughout the room. I have found that a downfacing sub is better placed in a more central location to the TV screen. The downfacing sub also works best on a hard floor. Don't choose one if you have carpet.
In any case, make sure the sub woofer you choose is magnetically shielded. Even if is placed no where near your TV screen this is important. First, you may want to move your speakers around a bit and may forget and place the sub near the TV. Second, you may have to place the sub next to your video tapes. It has a powerful magnet and will damage them. Since a sub woofer requires a power cord this will somewhat limit your placement. I have found that plugging the sub woofer into one of the switched power outlets or your receiver. This way the power to the sub woofer goes off when you turn off the receiver.
The last thing to consider with a sub woofer is the cross over control. As mentioned above, this is the setting for the frequency that will decide if a sound is sent to the regular speakers or the sub woofer. This is of little importance with a six channel system since for those the information for the sub woofer is coded into a channel sent directly to the sub. The cross over frequency only matters for watching non-six channel sources like cable or CD music. There is usually a cross over control on the receiver and the sub woofer itself. For best results set them both to the same frequency. The range of frequency will usually range between 50 and 200 Hz. If you set it too low, towards the 50, your audio will sound dark and murky. Too high and you will get little help from the sub and the sound will be overly bright. I have found a frequency of about 90 Hz works best for me.