Center Channel Speaker Design Additional Considerations

The article previously published on Audioholics.com “against” horizontally-oriented center channel M-T-M speakers is straightforward and espouses the conventional thought as to why this is a usually an unwise design approach—limited horizontal dispersion and driver interference and “picket-fencing.” Turning them vertical and remeasuring them was a good idea, and it showed that with the dual midranges vertically-oriented, the horizontal “suckouts” pretty much disappeared, as one would expect it to. (The Klipsch horn’s behavior when rotated 90 degrees can certainly be excused, and it wasn’t germane to the experiment.)

The interesting thing about the issue of vertical plane vs. the horizontal plane is that horizontally, people can be arrayed throughout the room over distances of several feet, from one side of the room to the other. While, yes, most people sit on the sofa in more-or-less a “prime” listening position and aren’t more than 15-20 degrees off axis of the center speaker—a few feet one way or the other—it is certainly not uncommon for there to be a group watching the movie (more than just 2 or 3 people), and a few of that larger group will, in fact, be well off horizontal center axis: several feet, more than 30-45 degrees.

Therefore, horizontal dispersion from the center channel speaker is important.

Another thing to consider is this: While people might be arrayed around the room horizontally separated by 6 or 8 or 10 feet or more, the interesting thing is that regardless of a person’s standing height, the vast majority of peoples’ ears end up being within a vertical window of only 10 inches or so. Sitting on a standard-height chair or sofa whether a person is a 5-5 female or a 6-2 male, their ears are very close to the same distance off the floor. That’s why vertical dispersion is nowhere near as important as horizontal dispersion in a center-channel speaker. As long as the center channel speaker “hits” that window vertically, you’re good to go. That’s quite a bit different than the horizontal consideration.

A few other things to mention and ponder: While it’s true that dual horizontal midrange drivers covering the same frequency range will overlap and “picket-fence” with each other at some angle off-axis, what is also true is that in the crossover region, where the midrange driver is crossing over to the tweeter, their output also overlaps and causes interference in the horizontal plane. So it’s not just the dual 5-inchers, it’s also where the 5” crosses over and overlaps with the tweeter. Even if it were a single 5” mid crossing over to a 1” tweeter, if they were mounted side-by-side, there would be a lot of crossover-region interference in the horizontal plane.

So the interference between dual midrange drivers of an M-T-M center is only half of the reason that horizontal centers have problems. The side-by-side positioning of the tweeter and midrange is the other half—and maybe even worse.

Check out our recently added YouTube video discussion for further perspective.

Center Channel Speakers: Which Design is Best for Home Theater?

That’s why in 1978, AR introduced the AR-9, which they called their “Vertical” speaker. It was a 4-way speaker, with all the mids and tweeter in a precisely intentional vertically-aligned array. Up until that time, the speaker industry didn’t recognize the relationship and behavior of side-by-side drivers. The AR-9 was the first speaker in the industry that was designed to eliminate the horizontal interference between drivers in the crossover region, and instead, throw that interference into the far less important vertical plane. (See figure 2.) Now look at every serious audiophile music speaker from every single good speaker company—Aerial, Paradigm, Polk, BA, Legacy, Revel, B&W, KEF, Golden Ear, etc. Fill in the blanks and name your favorite speaker.

Figure 2.  AR9 Vertical Drivers

All serious bookshelf and floorstanding audiophile music speakers have one thing in common, regardless of what kind of bass loading they use, or their cone materials, or their crossover types, or cabinet bracing, or anything else: All of them have vertically-oriented midranges and tweeters. Nice and straight, in a line, up and down.

The reason? All of those companies’ engineers are good and they all know the same basic laws of acoustics. One of which is that side-by-side drivers that overlap in the crossover region will interfere with each other in the horizontal plane….so it’s much, much better to throw that unavoidable, inevitable interference into the vertical plane, where it’s far less critical and detrimental. (Remember, it’s only about a 10-inch window, vertically, that the speaker has to deal with, not 4 or 6 or 10 feet, like it is horizontally.)

In 1992, Boston Acoustics introduced the 3-way VR12 center channel speaker, with a vertically-aligned midrange and tweeter. This was the industry’s first 3-way center speaker and it completely avoided the horizontal interference/overlap problems of all the previous 2-way M-T-M center-channel speakers.

Figure 3. Boston Acoustics VR12

To gain a broader perspective on the vertical vs horizontal center channel argument, Audioholics.com authored a "counterargument" article for horizontally mounted MTM center channels.

Yes, perhaps a dual-mid driver M-T-M center will have higher sensitivity because of those two big midrange drivers operating in tandem and moving a lot of air, but in a home theater system, it has to be channel-balanced with the other speakers anyway, so if the C is 3dB more sensitive that the LR channels, you have to reduce the C-channel level by 3dB. So the C’s increased sensitivity may be a false “advantage.” (Not having to drive the C as hard may give you somewhat lowered distortion—from both amplifier and speaker—all other things being equal, so increased sensitivity in the center speaker is not entirely without merit.)

While the data showing that listeners are not usually too far off center axis of the center channel speaker and therefore are within its “good” horizontal dispersion zone is interesting information, that data may not capture all the relevant sonic considerations. A horizontal M-T-M center’s reduced-angle horizontal dispersion will also affect its far-field power response, because its narrower dispersion will mean that it is delivering less total upper-mid energy into the far field than would a wider dispersion design. Remember, the sound of a speaker at a listener’s ears is determined by a combination of first-arrival (near-field) energy and far-field total energy output.

So even if a listener is sitting within the so-called “acceptable” horizontal dispersion region of a horizontal M-T-M center speaker, that center speaker’s far-field energy response will be quite different from the vertically-arrayed LR speakers that it is matched with. Because of that, the C speaker’s tonal signature will be different from that of the LR speakers, perhaps by enough to cause a distracting discontinuity in the L-C-R sonic imaging as the sound pans across the screen.

The ideal theater situation is to have three identical vertically-oriented L-C-R front channel speakers. With the identical radiation pattern and identical tonal balance—coupled with the wider horizontal dispersion of a vertically-arrayed speaker—the sonic texture will be exactly the same as the sound is “handed off” from channel to channel and the horizontal coverage will be the widest, assuring everyone of the best intelligibility and detail.

The horizontal box center speaker as we know it today stems from the advent of Dolby Pro Logic in the late ‘80’s, which synthesized a separate “center channel” track to reproduce the dialog and on-screen effects from movie soundtracks. Since it would have looked strange and been impractical for a vertically-oriented center channel speaker to sit tottering atop a CRT television, speaker company’s marketing departments decreed horizontal boxes to lay flat and safe on top of the 27” or 32” sets of the day—to the outraged howls of many an engineer who knew better! That’s where the “habit” of seeing a horizontal center speaker comes from. But for in-wall speakers—especially behind an acoustically-transparent screen, in which case the speakers aren’t even visible—there is simply no need for the center speaker to be horizontal. Indeed, a horizontal in-wall center speaker is a monument to ignorance.

A Compromise to Improve Horizontal MTM Performance

An interesting compromise is the approach that some companies take with their center channel box speakers. (See figure 4, an Atlantic Technology center channel speaker.) They use an “optimized” M-T-M-type of design, whereby the tweeter is raised well above the horizontal centerline of the two midranges. We’ll call this the “raised-tweeter” approach.

Figure 4.  Atlantic Technology 4400 C Center Channel

The raised-tweeter approach to center-channel design preserves the few advantages that a horizontal MTM has (higher sensitivity, increased power handling, etc), but significantly improves on a horizontal MTM center's greatest shortcoming—its very restricted horizontal dispersion. Raised tweeter centers are much better than a conventional MTM center in that regard, both because the tweeter is not in the same horizontal plane as the woofers and because elevating the tweeter allows the woofers to be brought in significantly closer together. (Remember, two identical drivers covering the same frequency range behave as if they were an oval driver of their combined outer distances apart. Driver directionality is primarily a function of the relationship of the radiated wavelength to the effective piston diameter of the driver. Other factors can also affect directionality, such as cone shape, cone stiffness, breakup modes, among others, but the first determinant of directionality is driver size.) Bringing the dual midranges in much closer to each other minimizes the distance between their acoustic centers, for an effectively smaller radiating dimension, which maximizes their horizontal dispersion.

It is true that that merely elevating the tweeter in a horizontal MTM center is nowhere near as good as a true 3-way with a tweeter above the midrange (with that array between two woofers) with the woofer-to-midrange crossover low enough so the woofers aren't directional in the horizontal plane—assuming the 3-way design uses components of at least the same quality as a 2-way center. This is critical—a “cheap” 3-way that uses inferior drivers and poorly-selected crossover points will sound markedly worse than a well-designed 2-way center that uses high-quality components.

But....the "counter-argument" article posited that a 2-way MTM horizontal center did have a few things going for it, like higher sensitivity and very good power-handling (and hence low distortion) because you had two beefy drivers handling the midrange as opposed to a single smaller midrange in a 3-way center.

Those things are true as well.

There is also the real-world cost consideration to take into account. A raised-tweeter center does not cost any more to manufacture than a conventional MTM center, because the number of drivers, the crossover complexity, and the cabinet routing and construction are all essentially identical to a conventional MTM center.  However, it does require the manufacturer to stock two different cabinet designs and product skus (one for the standard MTM to be used vertically, and the raised tweeter MTM to be used horizontally).

Three-Way Center Channels to the Rescue

A true 3-way center [W(T/M)W] with a vertical midrange and tweeter can undeniably be superior (assuming the same quality components as a good 2-way, as stated above), but it costs a lot more to manufacture than a 2-way MTM.   So, if the manufacturer is trying to hit a price point, a raised-tweeter center clearly outperforms a conventional MTM center, but for the same cost.

A well-designed 3-way crossover typically has at least double the number of elements of a 2-way crossover, and those elements (especially the woofer chokes) are a much larger value and hence more costly. That means that a 3-way crossover is usually more than double the cost of a 2-way crossover. Then you have the extra driver (a separate midrange), and you have a much more complicated cabinet to assemble (the routing, the internal midrange sub-enclosure, more bracing to reinforce the routing, etc). That's why a good 3-way center is always at the higher end of the price spectrum.  The picture at the beginning of this article (courtesy of Snell Acoustics) combines the best of both worlds yielding a true 3-way design while also implementing a vertical MTM for better horizontal dispersion and higher output capability than a typical 3-way single midrange alternative.  This is typically not a very popular option because it also requires the cabinet height to increase which is often a deal breaker for those attempting to squeeze in a center channel below a display or screen without obstructing the view.

Conclusion

If, as a manufacturer, you're trying to hit a low-to-mid price point and you're limited by your budget to a 2-way design, then you're best off doing an elevated tweeter design and it'll be better for the same cost than a conventional 2-way horizontal MTM.

The subject of proper center channel speaker design is not a simple one. There are many considerations—price, desired coverage area, aesthetics, and others. A manufacturer has a daunting task trying to balance many seemingly conflicting requirements, while for the consumer, education and information are the keys. Being aware of the strengths and weaknesses of the various design types gives the consumer the best opportunity to make the most satisfying choice.