Speaker Placement: Bipole, Dipole & Direct
Expert advice for anyone contemplating bipole, dipole or direct-radiating speakers in their surround sound setup.
Bipole, Dipole & Direct
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August 12, 2008 by Dennis Burger

“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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