If you are not using full range speakers, (floor-standing does not necessarily mean full-range) then you will need a subwoofer. Surround processors often have a subwoofer output jack, or alternatively, a “mono out”, to connect such a speaker. Subwoofers are designed to reproduce only the lowest frequencies in the music or movie sound track. There are 10 octaves in the range of human hearing. Each octave doubles the frequency. So, octave 1 is 20 Hz – 40 Hz, octave 2 is 40 Hz – 80 Hz, octave 3 is 80 Hz – 160 Hz, and so on, up to 20 kHz. Subwoofers reproduce the lowest 2 octaves, or 20 – 80Hz, sometimes slightly higher. Most of the speakers designed for surround sound will reproduce frequencies down to about 80 Hz, so this is where the subwoofer comes into play. One should note that in most modern surround sound receivers or processors, the setup is critical to ensure that the subwoofers get the range that they handle best, as opposed to passing low frequency information directly onto speakers incapable of reproducing the full range of bass. If in doubt, set up each channel (Left, Center, Right, and both surround channels) to “small” so as to redirect bass information to the subwoofer.
Many types of subwoofers are available, in fact, just about every speaker manufacturer has at least one powered subwoofer in their lineup. The drivers typically range from 8″ up to 18″ in diameter. Most have built in power amplifiers, and a few are just the driver in the enclosure (you need a separate power amplifier to use them). For those readers who need an inconspicuous, yet powerful subwoofer, there are several designs with small enclosures, but very long throw drivers and large amplifiers to compensate, like the one shown on the right, which has a 10″ driver.
The example on the left is a 12″ push-pull subwoofer. Depending on which type of subwoofer you choose, and you decide on one of the power amplifiers that has several channels of amplification (3 – 6) all in one chassis, you will need to consider how many amplifier channels to purchase. If you are not going to use the center channel (in which case, you are using the “phantom” mode), and the subwoofer is self powered, then you only need a four channel power amplifier. If you use the center channel and a self powered subwoofer, then you need a five channel power amplifier, and if you use the center channel and a subwoofer that has no amplifier of its own, then a six channel power amplifier is in order. Remember to make your choices up front. The majority of commercial subwoofers are self powered. Most “Digital” surround receivers have enough amplifiers for all channels except the subwoofer.
Interestingly, subjectively higher volume will be heard from higher distortion subwoofers at a given SPL because the harmonic distortion is higher in frequency than the ultra-low fundamentals, and therefore more audible. However, this distortion is not as irritating at low frequencies as it is at higher frequencies, so, if you want a subwoofer with additional “punch”, you might consider a subwoofer with lots of distortion. As always, listen and compare.