Surround Speakers- Explained.
© 2010 Steve Feinstein, Atlantic Technology
The surround speakers in a home theater system play an extremely important role: they convey the three-dimensional sound effects (like the sonar pings in a submarine’s hull, the wind whipping through the desert sands, or the hustle and bustle of a crowded city street) that make a movie’s soundtrack lifelike and believable. Without good surround speakers, home theater is just…plain ol’ television.
Next time you go to a good commercial movie theater, look around at the side and rear walls before the lights go down. Mounted on those walls are lots of surround speakers, probably dozens, high above the audience’s head. These multiple speakers play all the ambient information on the soundtrack, and because there are so many speakers, the listener is not aware of any one individual speaker. Remember, you can’t point to an echo and say, “Aha! There it is!” The surround effects are simply there, all around you. (See Figure 1)
The challenge with home theater is how do you get one pair (maybe two pair, if you’re really lucky) of surround speakers in your living room to sound like 24 pairs in a movie theater?
The best way is with dipole surround speakers. Dipole speakers have two sets of drivers, facing opposite directions, that play out-of-phase with each other. That is, one set of drivers is moving out (“+”) when the other set is moving in (“-”). Where their output overlaps directly in front of the speaker, it cancels out. (Plus added to Minus = Zero, right?) That’s good, because it makes the speaker’s location very difficult to pinpoint (engineers call it “localize”) by ear.
Instead, the sound gets away from the speaker, and is reflected all around the room before finally reaching the listeners’ ears. These multiple sound reflections fool the ear into thinking that there are lots of surround speakers in the room—just like in those fancy commercial movie theaters. (See Figure 2)
Yes, you can use “regular” speakers for surround, if you absolutely must. Just try to point them away from the listeners so their sound has a chance to reflect around the room first.
Nowhere near as effective as dipoles, but not too bad. But don’t make the mistake of pointing regular speakers right at the listening location, because if you do, the listeners will easily localize on their position and that nice, three-dimensional, ambient surround effect will be ruined.