written by Mike Shea on 21 November 1999
One of the most popular subjects in home theater is the subwoofer. Even with large tower speakers, an added subwoofer can bring a whole new dimension to a system, giving it that extra "kick". Adding a subwoofer to a system allows your mains to be driven higher, instead of forcing them to perform at the lower frequencies which they generally aren't good at. Dolby Digital makes it even more important.
First a crash course in speaker sound. The common frequency range that is available on movie soundtracks is 20,000 hz (very high pitch) to 20 hz (very low pitch). Speakers are able to produce the high frequencies (20khz to about 80hz) without a problem, but frequencies below 80hz are much more difficult to produce. Since often a single amplifier and two drivers (tweeter and woofer) are asked to reproduce this sound, there is a lot of strain on both the amplifier and drivers. Even with a good powerful receiver, the lowest levels of sound force the receiver to put out more power than it has available, and thus neglect the higher tones which ironically, it would be much better at achieving. A subwoofer is a single cube-like cabinet that contains one or more large drivers (from 8" to 18") and a large dedicated amplifier (from 60 watts to 1400 watts). The larger the driver and amplifier, the lower and louder the sub.
It is important to know that the power and size of the sub are not the only thing that makes it good. Control is also very important. With such a large driver being moved by such a large amp, it is possible for the unit to misfire and cause a beat to skip or come early. It will vibrate or boom too much. This is the distortion of the subwoofer. It isn't so much a problem with movies (a good rumble is important for a meteor) but with music it can make Mozart sound like you want to put on your size 57 work pants and go cruise in your low rider for chicks. Many subwoofers have tricks to gain control over this problem. Velodyne has a device called an Active Servo Control that uses an extra magnetic field to control the subs behavior. M uses a second vented driver to make sure that the first doesn't lose control. For this reason, it is difficult to judge a sub by it's statistics alone. That is the purpose of the table at the bottom of this article. Name does matter a lot of times.
A subwoofer can be set up a number of different ways. It the most popular configuration it can handle all the base from all five channels, thus letting those channels only handle signals roughly 80 hz and above. This works best with bookshelf or satellite speakers which can't properly reproduce lower signals. With a system that has tower speakers, it can work in conjunction with the base from those speakers, or handle the base for the center and surrounds if they are still smaller speakers. With Dolby Digital and DTS, there is a dedicated low frequency effect (LFE) channel that contains the lowest level effects (explosions, meteors, harleys). With Dolby Digital, the specs call for a full 20hz to 20khz on every channel, and 20hz to 100hz on the LFE channel. This would require a separate subwoofer for every channel, plus the LFE. Without the use of active speakers (see below) this is pretty unrealistic. However, with a good single subwoofer and all channels set to small, all of the base will be properly reproduced by the subwoofer.
One of the newer debates within the home theater community is whether it is necessary to have two subwoofers. Articles in Home Theater Magazine and Steriophile's Guide to Home Theater point out that placement of a subwoofer is very difficult and by having two you are more likely to have one placed right for any position in the room. On the Video Essentials DVD it shows placement up front (even directly underneath the mains), placement to the sides of the audience, or even behind the audience. Two subwoofers can more accurately represent the lower signals than one, which is why many THX certified systems are only certified if they come with two subwoofers. The Atlantic Technology 350 and the discontinued Boston Acoustics THX systems both included two subwoofers. Now whether it is better to purchase two lower priced subwoofers over a single high powered subwoofer is a different question, and is one I have no answer to.
Active Speakers
One of the latest trends in speakers is the active speaker. Many high end companies such as Polk, Definitive Technologies, Boston Acoustics, and Energy. Definitive went so far as to release the first active center channel. What are active speakers? Basically they are a speaker containing its own internal amplifier which can power the woofer or subwoofers contained in the cabinet. The advantage of this design is both in ergonomics (no need to have a big old box) and in quality. Since a speaker is being built from the ground up by a single manufacturer, the amplifier, crossovers, drivers, and cabinet are all designed together creating a speaker with optimum performance for it's components. For instance, you don't have to worry about where your subwoofer crossover should be set to reduce stress on your midranges, it is all done for you in the speaker itself. All you have to do is set the gain of the subwoofer to your liking. Another advantage is to take the receiver or decoder out of the loop for speaker setup. You know that your main speaker can handle a full range signal and you can set it as such. You could also set the LFE (low frequency effect) track of a Dolby Digital or DTS soundtrack to go to your active main speakers. Many of the reviews of active speakers have been very positive. Both Home Theater Magazines and Stereophile's Guide to Home Theater gave the Definitive BP3000 and 2000 series active speakers good reviews, saying that the speakers performed very well with music and movies. Click for more info on the Definitive BP3000TL and C/L/R 3000 active center.
What are the disadvantages of active speakers? Mainly price right now. The powerful BP3000 Definitive Tech active bipolar tower speaker can set you back $4500 a pair. They contain a massive 18" woofer and 1000 watt amp and are flat from 20hz to 20khz (the full range audio spectrum). The more modestly priced (and powered) Definitive 2000 run about $1500 a pair with a 15" woofer and 300 watt amp. The other disadvantage is they will more likely than not still need a sub for a full home theater setup. The cabinets of the towers are just not large enough to outperform a large dedicated subwoofer. But when you do add in that extra LFE sub, the overall performance of your system would be astounding. A final disadvantage is location.
It is known that the more subwoofers one has, the better. For one example of a true system able to fully recreate the sound required to mimic the sound of a theater, take a look at the high end theater recommendation. It is widly accepted that adding a subwoofer to a home theater system is one of the largest bangs for the buck.
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