In the beginning, there was stereo.  Two speakers for two ears.  (Well, before that, there was mono but let’s not go there).  In the 70s, theaters added a few more channels of sound to make the movie-going experience more exciting This started an “arms-war” between the theaters and homes that goes on to this day (some theaters use as many as 13 different channels!).  The first few expansions made sense:  Two channels behind you, then a front center speaker for dialogue, and then finally subwoofers, which were specially designed to re-produce those essential sonic booms.  The home version of this six-channel setup, which had become fairly standard, is referred to as “5.1” sound.  The “.1” suffix refers to the fact that the subwoofer reproduces only a small portion of the frequency range. 

But as with most things, more is better.  Current high-end systems include “6.1” sound (just like 5.1, but add a center rear speaker), or even “7.1” sound, accomplished by feeding two rear speakers with the same channel from a “6.1” source. 

However, the true “state-of-the-art” in-home reproduction of theater sound today is currently know as “7.1 Surround”.  This takes the standard 5.1 setup and adds two more channels, one immediately on each side of the audience.  In a theater, this can be used advantageously.  The three “front” channels are behind or below the screen, the two rear channels are in the back of the theater, and the side channels run down the side walls.  The side channels add ambience, and emphasize panning effects (the movement of sounds from the front to the back of the listening space).

At home, the usefulness of these extra two channels is less clear.  The first question is:  Will you be listening to sources that produce 7.1 sound?  Because of storage requirements, 7.1 sound is currently limited to high-definition DVDs (HD-DVD or Blu-Ray) and video games.  The second question is:  Is your listening space large enough that your ears will be able to distinguish these two extra channels?  In a 5.1 system, the sound that would normally go into the 2 new side channels is distributed between the front and back speakers, so you’re not actually “missing” anything, and if the extra set of speakers will be only a few feet from the existing ones, you may not notice the improvement.

Finally:  Do you want to pay for it?  Since ambient effects typically aren’t that loud, your side speakers can be small, (just like the rear speakers), but you still need to purchase them, and, depending on your situation, have them properly installed.  You also need a home theater receiver (or for your PC, a sound card) that is 7.1-capable.  This isn’t an add-on you buy, but a replacement of your current equipment.  Home theater receivers with 7.1 surround can cost several hundred dollars, or more. 

The answer is probably this:  If your current home theater is big enough that you’re sure you will notice the difference, you probably won’t notice the cost of upgrading.  But for the rest of us, we may wait until we replace our equipment for other reasons, or at least until 7.1 surround becomes more ubiquitous, or at least less expensive.