Why single center channels?

[abstracta]

This past weekend I helped a co-worker 'tweak-out' his new DLP HT system with a trick I learned from a friend years back who used to install very high custom systems for wealthy clients. Basically you eliminate the need for a single center channel speaker by using dual speakers run in series off the center channel speaker-out and flipped out phase. You can then position those speakers on either side or above and below a large projection screen.

My co-worker was ecstatic at the sound improvement along with his family. No more localization coming from above the screen on the ceiling, or below the screen. A split mon center (done correctly) makes the center channel sound like like it's radiating from the area between the speakers, and hence sounds more seamless. Any phase cancellation issues seem to be restrcted to non listening positions.

For me this is a hobby, but at last count this was the fifth system I've 'teaked' in such a fashion for friends that resulted in rave reviews, and as far as I'm concerned sounded superior to the conventional 'three up front' 5.1 config. It's especially helpfull for large projection sets where you are otherwise forced to mount your center on the ceiling or below the screen.

I don't recall ever being in a state of the art movie theater using a single center channel, so why are we forced to use a single center channel speaker above or below our screen? Because the guy at Best Buy says so?

Next, from my knowledge of speaker designs, MTM speaker designs have pretty crappy vertical disperision lobing characteristics (by design), and when we flip them on their side it now becomes a horizontal dispersion issue. Yet, most center channels speakers I see are MTM designs, and frankly don't sound that good except for some very large ones costing several grand.

Anybody else using dual centers with an AV Receiver not designed for doing such?

[Worf101]

I've installed/hooked up about 7 systems over the years but never in a room large enough or one with a large enough screen to make a "single center" channel an issue. Good looking out though. I think your idea is worth noting.

Da Worfster

[musicman1999]

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How big was his room?It must be very,very large.

bill

[kexodusc]

Basically you eliminate the need for a single center channel speaker by using dual speakers run in series off the center channel speaker-out and flipped out phase.
Not sure what "flipped out phase" means. If you mean out of phase with the other series speaker, why? Much like 2-channel system this would seem to be counterproductive. Can you elaborate?


I don't recall ever being in a state of the art movie theater using a single center channel, so why are we forced to use a single center channel speaker above or below our screen? Because the guy at Best Buy says so?
No, Best Buy has nothing to do with it. In fact, I'm sure they'd rather sell you 2 center speakers instead of one. 1 center channel is the convention because currently soundtracks are engineered with this configuration in mind - ie, they are optimized to a single center channel. It's a most-for-the-least compromise of course. I remember reading modern HT theory suggesting a 22.2 speaker configuration or something ridiculous is the optimal configuration - that just ain't gonna happen though.


Next, from my knowledge of speaker designs, MTM speaker designs have pretty crappy vertical disperision lobing characteristics (by design), and when we flip them on their side it now becomes a horizontal dispersion issue. Yet, most center channels speakers I see are MTM designs, and frankly don't sound that good except for some very large ones costing several grand.

You are correct in mentioning the horizontal lobing characteristics of an MTM center channel. The argument that MTM's are bad for center channel duty, however, is a fallacy that's been refuted over and over again.

Poorly designed MTM's aren't great for home theater. Modern MTM's dictate 4th order acoustic slopes or higher (some still like 3rd order), pay attention to off-axis response, and have reasonably good horizontal dispersion. And that's the crux of the argument against MTM centers - the horizontal dispersion.
Sure, dispersion sucks for an MTM as you move 30 or 60 degrees off axis. Who the hell sits 6 feet off center in a 12 X 18 room? You're up against a wall!!! Most other speaker alignments have severe phase or dispersion issues as you move that far off axis too. Off axis issues aren't unique to MTM's.

Besides, it's excellent on axis, which is where the sweet spot is, and where the majority of people sit when listening to home theater. What's more, in most home theaters, it's impossible to put the center channel in the same vertical plane as the listeners ear (unless it's behind an acoustically transparent screen - most home theaters don't have this). That veritcal dispersion is nice when you set the speaker on top or below at TV!!!

The lobing is a non-issue for MTM's at 15 degrees and below - Even up to 30 degrees it's not necessarily worse than a lot of other options. No matter - 8 feet away from the screen, you've still got a good +/- 2 - 3 feet. That's enough for 2 large Lazy-boys or a decent sized sofa to accomodate 3 people. At 10 feet, it's even wider.

Most importantly, let's say you still want to sit 4 or 5 feet off axis when your TV is 10 feet away from the sweet spot. Sure your into an MTM's lobing areas now, but that's the least of your worries. MTM's aren't designed to work alone in a HT. At that distance you're so far off axis from the center point that you're now experiencing such a huge tilted bias in the imaging and soundstage to whatever side of the room you're on as to render center channel lobing a non-issue. MTM's in a properly setup HT, will not reach their horizontal limitations before throwing the rest of your surround field extremely out of balance. Still, a lot of speaker builders still get hung up on the theory without looking at the application - that's why you're seeing 2.5 way MTM Center Channels built more these days.


Anybody else using dual centers with an AV Receiver not designed for doing such
I have a pair of bookshelf speakers mounted on either side of my TV right now acting as my center channel while my full-time center is getting modded a bit. But they're setup in phase, and parallel, the impedance isn't too low for my amp. It works well - but I don't find it any better. It's a lot more hassle too. I doubt the average HT owner will go to that trouble.

[Rock789]

speculating on the "out of phase" part...

typically for movies, I run my center channel with a high pass crossover turned on in the preamp/pro... I find when I run the center full range, sometimes the bass coming through the center channel makes it difficult to hear the voices which come through the center channel.
perhaps by running the two center speakers out of phase with eachother, this cancles the lower frequencies... similar to what I have done by using a high pass crossover?

(fyi, I have had this issue with several center channel speakers, including my chorus s center...)

[GMichael]

Hmmm... very interesting. The increased resistance doesn't screw anything up?

[blackraven]

Magnepan makes dual center channel panels to use instead of a single center channel.
My JVC AV receiver has a nice feature. It has a separate tone control for the center channel. I love this feature and don't know why other manufacturers havent embraced this feature. On some movies the voices are hard to hear and the tone control makes all the difference in the world. In fact I prefer watching movies on the JVC vs my Adcom.

[kexodusc]

Hmmm... very interesting. The increased resistance doesn't screw anything up?
At first glance the added resistance should be offset by the added speaker (effectively adding +3 dB to the output)...kinda like +3 boost in sensitivity? No?
Your amp powers fewer watts into a 16 ohm load, but your speakers play louder per 1 watt played.
Should offset each other.

[GMichael]

At first glance the added resistance should be offset by the added speaker (effectively adding +3 dB to the output)...kinda like +3 boost in sensitivity? No?
Your amp powers fewer watts into a 16 ohm load, but your speakers play louder per 1 watt played.
Should offset each other.

That's great. Sounds like it's at least worth a shot.

[Dusty Chalk]

I just configure my receiver to "no center channel" and voila, problem solved. The imaging on my speakers is sufficient for an "interpolated" center channel.

But all the same -- thanks for the advice. If I had a bigger room and a bigger setup, I might need this.