Speaker Placement: Bipole, Dipole & Direct

Bipole, Dipole & Direct

Expert advice for anyone contemplating bipole, dipole or direct-radiating speakers in their surround sound setup.

Aug. 12, 2008 — by .(JavaScript must be enabled to view this email address)

For the obsessive home theater aficionado, it’s the ultimate horror: you enter a friend’s house and spot a home theater system. Everything looks reasonably normal at first—left and right speakers flank the screen; a center channel sits above or below; but then you notice the rear speakers, stacked on top of the entertainment center at the front of the room. Or on top of the front left and right speakers. Or maybe one is at the side of the room and the other is in the back, each of them pointed in wildly different directions.

“Cleanup on Aisle Geek!”

When pressed, your host invariably gives the same sort of answer: “Well, I paid for them so I’m going to use them, but I can’t go running wires all the way around the edge of my room,” or, “I just don’t have room to put them back there,” or, “I really just don’t care.”

Catch your breath, count to ten, and remind yourself that “Surround Sound Speakers 101” isn’t a pre-requisite class in most college curricula. Given the amount of bad information on the web about surround sound speaker placement and even the importance of proper placement, it’s no surprise that the uninitiated are often confused. What’s more, there are so many different types of surround sound speakers—direct radiators, bipoles, dipoles… quadrupoles, for goodness’ sake!—it’s amazing that anyone other than professionals and diehard home theater aficionados end up with complete surround sound systems to begin with.

Direct Radiating
“If you can’t get a decent placement for your surround speakers, just don’t use them at all.” That’s the advice of Anson Fogel, co-owner and chief operating officer of the multiple CEDIA award-winning architectural electronics firm Electronic Systems Consultants (ESC) in Aspen. “If you can’t put them in a reasonable position—which isn’t that hard—just don’t install them, because otherwise they’re either not going to do much, or even make things worse.”

For a 5.1 surround sound setup, that reasonable placement for direct-radiating speakers—in other words, the typical forward-firing speakers that immediately comes to mind when someone says “speaker”—is slightly behind you, 110-degrees from the screen. If that number doesn’t make sense to you, think of it like this: Directly in front of you, toward the screen, is 0-degrees. Directly to your left and right—where many people think surround sound speakers should be placed—is 90-degrees. Keep on going another 20-degrees or so, to a point just slightly behind you, and you’re headed in the right direction for proper surround speaker placement. And if at all possible, try to keep them the same height as your front speakers, or perhaps just a bit higher if necessary.

Why is that the correct position? Fogel explains: “What you’re trying to do with surround speakers is to set up a soundfield that comes as close as possible to the soundfield that the mastering engineer used in the studio when he mastered the movie’s soundtrack. And his speakers are behind him, 110-degrees off of axis, because that’s what’s prescribed by the Audio Engineering Society. They’re not above him, they’re not below him, and they’re not right beside him. They’re not sitting on the wall above the couch with the couch shoved against the wall.”

Of course, the surround sound police aren’t going to come knocking on your door if you deviate from this. “Many times in residential spaces—normal living rooms, family rooms—strict adherence to these guidelines isn’t reasonable,” Fogel says. “You can certainly put them a little higher, on a bookshelf or something, and they can certainly come forward or move backward a little. But the closer you get to the ideal, the better your sound is going to be.”

These rules fly completely out the window when bipolar and dipolar surround sound speakers enter the picture, though. But before we start hanging them on the wall, let’s quickly discuss what the words “bipolar” and “dipolar” mean, and how such speakers work.

Bipole & Dipole
Bipolar speakers work in roughly the same way as direct radiating speakers. Instead of projecting sound straight ahead, though, they feature two sets of drivers pointed in different directions, which fire in-phase—both pushing and pulling air at the exact same time. “That gets sound moving toward more reflective spaces in the room, introducing more sonic energy in more directions, so that you get more reflected energy coming off of walls, ceilings, book cases, tables, et cetera, than you would with a direct-radiating speaker.” In other words, you’ve got more sound coming at you from more directions, but most of the sound is coming from the speaker itself.

“Bipolar surround speakers are so rare and esoteric these days that I don’t know why anyone is still talking about them, though,” Fogel says. “The real choice is between dipolar speakers and direct radiators.” Like bipolar speakers, dipoles feature two sets of drivers firing in different, often opposite, directions. Unlike bipolar speakers, the drivers in dipoles aren’t moving in and out at the same time, though. They’re not in-phase, that is to say. One driver pushes air while the other pulls. So when the dipoles are placed properly, at 90° from the screen, directly to the left and right of the listener, they create a null zone—an area in which the sound coming from each speaker effectively cancels itself out, kind of like matter and antimatter coming together, but with sound and with less earth-shattering kabooming. The sonic information coming straight toward your ears is effectively dampened, and instead you hear virtually nothing but sonic reflections. So instead of perceiving sounds as coming from the speaker itself, the result is a diffuse soundfield that seems to emanate from the room itself.

“That sounds great, in theory,” Fogel says. “And when implemented well, it can be great in practice. We still use good dipoles when the room demands it. But many people have forgotten why dipolar speakers were created to begin with. Dipoles were originally design to result in a diffuse soundfield in rooms that were heavily acoustically treated. They were a part of the first consumer THX systems, which were trying to duplicate the effect of multiple direct radiating speakers in commercial theaters. It was expected that these systems would be installed in more professionally built home theaters. But today, even in very high-end home theaters we don’t dampen the room that much. So the advantage of dipoles is limited.

“Dipolar speakers were also invented in the old analog surround sound days, in a very different environment. These days virtually every DVD and even many TV shows have digital surround sound, with five discrete, individual channels. And the diffuse soundfield of dipole is really doing an injustice to these discrete digital surround soundtracks,” Fogel says. “Think about it this way: with stereo audio, you’re essentially creating images in space with two speakers. You can move instruments around in space, and the listener will perceive more than two speakers. Add two more speakers in the rear and you now have the ability to image between all four speakers, so in theory—in a perfectly implemented four- or five-channel system—you can place an instrument anywhere in 360 degrees around the listener’s head. But when you introduce dipoles, you destroy that imagine. You intentionally screw that up.”

And what about 7.1 systems? “If you can place four surround speakers in a room reasonably well, the value of dipoles and bipoles becomes even harder to justify. These speakers were designed to try to mimic the sound of multiple speakers. If you can actually have multiple speakers, you no longer need to fake it. So unless you absolutely cannot place your surround speakers correctly and unless you have the budget to spend on decent dipoles— because good dipoles cost $1000 apiece, at least, and less-expensive dipoles introduce all kinds of phase anomalies—I would never specify anything other than direct radiating speakers,” Fogel says.

“Luckily there are literally hundreds of good, inexpensive, small, direct-radiating speakers that can be well-placed in the average living room” he says. “You can get world-class surround sound with inexpensive speakers and a basic surround receiver for a few hundred dollars, as long as you put as much thought into your surround speakers as you do the rest of your system.”

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