Dual Center Channel Speakers !!!

[drumdrum]

hello folks - This is my first post so I'll start at the begining...
whether that be the top or the bottom...
((( OR IN THIS CASE THE TOP AND THE BOTTOM !!!)))
I have two center channel speakers in my system, and I'm wondering if anyone else here has experimented with this set-up ?

Mine are custom built cabinets using the same "Greater Heil air motion transformers" that I have in my mains. In order to both "keep things kosher" across the front, and to attempt to improve the shortcomings of the original ESS amt's. ( they are paired with 6 1/2' Isophone woofers and use the original Xovers)

The twin center channel speakers are mounted both above and below my Pioneer monitor ( and run in parallel on a NEC monoblock.)

I essence what this setup give me is a phantom center sonic image that is not localized to either speaker...which therefore appears to come from the center of the monitor itself. ( VERY NICE !!!)

Simply stated - I'd never go back to a single unit to cover the critical CENTER position !!!
1) increased dynamic range
2) eliminates localization
3) dramatic increase in vertical spread - compliments front LR spread
4) EVERYTHING IS MORE BELIEVABLE !!!

WHO EVER DECIDED THAT CENTER CHANNEL = ONE ??

[Kevio]

That's a cool idea. One general hazard of using multiple speakers wired to the same amplifier channel is phasing or comb filtering due to interaction of the two signals. With this setup, you'll probably notice that the timbre of the center channel is different whether you standing or seated. Probably not enough of an issue to kill your buzz though.

[N. Abstentia]

I've always thought it was a great idea (even when we did it 12-13 years ago just for giggles!) but it's just not practical for most folks.

[kexodusc]

I've done it and it worked well enough.

However, I've always found for the price of 2 I could buy or build 1 that would sound even better.

I suppose at some point there is a technical advantage to using 1 speaker too - especially if the centers are placed horizontally instead of vertically...multi-channel tracks are engineered and mixed with 1 center channel in mind...if the front right and center channels are used to image a sound in a position, physics dictates a dual center setup would "pull" the image a bit further to the left than it should be. How much would depend on the disperion of the speakers and how wide apart the 2 centers are. Even a perfect phantom setup has limitations, but we're splitting hairs here if some highly directional sound cue is off an inch or two.
When I did it, I had them above and below my set. Wife hated it.

But if it works, use it

[canuckle]

The interference pattern that gets set up is a pretty well-researched phenomena that is more than enough reason to avoid this kind of set up. In short, having two speakers has done nothing for your system's dynamic range (you should research the term I suspect), human hearing isn't designed to localize things vertically very well and a properly set up system with one speaker will not have any issues in this area, and a "vertical spread" means nothing since the centre channel is a mono line (unlike the front L/R to which you compare it). But since you like it, enjoy

[Kevio]

There are plenty of expensive speakers with dual vertically-arranged drivers. I personally am not convinced that it's a great idea but clearly it can be made to work.

[pixelthis]

The only rationale for two centers is an extremely large screen, where two horizontaly
placed speakers can spread the sound out better.
Mostly the conflict between the two isnt worth the trouble
Basically something these guys would do

[canuckle]

There are plenty of expensive speakers with dual vertically-arranged drivers. I personally am not convinced that it's a great idea but clearly it can be made to work.

Yes, and the #1 engineering challenge in getting those speakers to work is finding a way to keep the 2 drivers from screwing each other up. There are many electronic tricks that help this work. Can that working system be replicated by placing two speakers above each other rather randomly by hand? I doubt it.

[kexodusc]

Yes, and the #1 engineering challenge in getting those speakers to work is finding a way to keep the 2 drivers from screwing each other up. There are many electronic tricks that help this work. Can that working system be replicated by placing two speakers above each other rather randomly by hand? I doubt it.

Screwing each other up? Care to elaborate?

[Kevio]

Yes, and the #1 engineering challenge in getting those speakers to work is finding a way to keep the 2 drivers from screwing each other up. There are many electronic tricks that help this work. Can that working system be replicated by placing two speakers above each other rather randomly by hand? I doubt it.

Electronic tricks? Care to elaborate?

[Sir Terrence the Terrible]

The interference pattern that gets set up is a pretty well-researched phenomena that is more than enough reason to avoid this kind of set up. In short, having two speakers has done nothing for your system's dynamic range (you should research the term I suspect), human hearing isn't designed to localize things vertically very well and a properly set up system with one speaker will not have any issues in this area, and a "vertical spread" means nothing since the centre channel is a mono line (unlike the front L/R to which you compare it). But since you like it, enjoy

This is an excellent post, and exactly what I was going to say. It does not matter whether the speakers are set up vertically, or horizontally, the interference pattern will always exist. I bet if you measured that combination, the measurement would not look very good.

[Sir Terrence the Terrible]

Screwing each other up? Care to elaborate?

What he is stating is that two drivers playing the same information will have the same interference pattern as two spaced speakers playing the same information. In order to deal with the drivers interference pattern, you have to gradually roll off the output of one of the drivers as the frequency goes up, or the speaker will measure poorly, and will have intelligibility issues with dialog. These are called 2.5 way speakers, as opposed to two way speakers where both woofers handle the same range of frequencies.

[kexodusc]

What he is stating is that two drivers playing the same information will have the same interference pattern as two spaced speakers playing the same information. In order to deal with the drivers interference pattern, you have to gradually roll off the output of one of the drivers as the frequency goes up, or the speaker will measure poorly, and will have intelligibility issues with dialog. These are called 2.5 way speakers, as opposed to two way speakers where both woofers handle the same range of frequencies.

Oh ok he's talking screw up acoustically, not electrically...and really the drivers aren't screwing each other up...the sound waves are.

Just to play devils advocate here though, been a few years since I took physics but isn't the audible effects of the interference pattern directly related to where you are physically positioned relative to the drivers(s), and how far apart the acoustic centers of the speakers/drivers are?...and in real life, how audible is this going to be in a typical room, several feet back, even if the spacing between the speakers is going to be a few feet? It would seem to me the further back you are the less relevant it becomes?

Why isn't interference pattern an issue for stereo imaging (centered vocals etc?)...or maybe it is and it's just a given stereo will never be perfect?

[Kevio]

The interference occurs when one of the drivers is closer to you than the other. In this situation, due to the limited speed of sound, you get two copies of the same sound arriving at your ear at slightly different times. The delay translates into a phase difference and the two copies partially cancel each other out resulting in what we typically perceive as timbre change. The technical name for the phenomenon is a comb filter. But keep in mind that none of this happens when the distance to the two drivers is approximately equal as is the case for most of these ear height tower speakers.

It certainly is an issue in stereo playback - that's why we have a listening sweet spot (were the distance from our ears to each speaker is the same).

Also recognize that this comb filtering happens in our natural environments as sound bounces off walls and reaches our ears through multiple paths. Along with reverberation, It is part of what gives our hearing a sense of space. So as with 2nd harmonic distortion, a comb filter scenario can add to and enhance our enjoyment of recorded music.

[Kevio]

Just to play devils advocate here though, been a few years since I took physics but isn't the audible effects of the interference pattern directly related to where you are physically positioned relative to the drivers(s), and how far apart the acoustic centers of the speakers/drivers are?...and in real life, how audible is this going to be in a typical room, several feet back, even if the spacing between the speakers is going to be a few feet? It would seem to me the further back you are the less relevant it becomes?

Here's a mathematical picture that may help you visualize what's going on at different frequencies

Here's a pretty picture showing what's happening in three dimensions at a single frequency with two side-by-side drivers. You can cock your head sideways and imagine how drivers stacked vertically behave.

[pixelthis]

Oh ok he's talking screw up acoustically, not electrically...and really the drivers aren't screwing each other up...the sound waves are.

Just to play devils advocate here though, been a few years since I took physics but isn't the audible effects of the interference pattern directly related to where you are physically positioned relative to the drivers(s), and how far apart the acoustic centers of the speakers/drivers are?...and in real life, how audible is this going to be in a typical room, several feet back, even if the spacing between the speakers is going to be a few feet? It would seem to me the further back you are the less relevant it becomes?

Why isn't interference pattern an issue for stereo imaging (centered vocals etc?)...or maybe it is and it's just a given stereo will never be perfect?

SIR TALKY GOT IT WRONG, AS USUAL.
Two centers will work, its a matter of distance, which requires a LARGE screen.
Stereo works of course because its two different signals.
Ture they share info, so the father apart the better.

[kexodusc]

The interference occurs when one of the drivers is closer to you than the other. In this situation, due to the limited speed of sound, you get two copies of the same sound arriving at your ear at slightly different times. The delay translates into a phase difference and the two copies partially cancel each other out resulting in what we typically perceive as timbre change. The technical name for the phenomenon is a comb filter. But keep in mind that none of this happens when the distance to the two drivers is approximately equal as is the case for most of these ear height tower speakers.

It certainly is an issue in stereo playback - that's why we have a listening sweet spot (were the distance from our ears to each speaker is the same).

Also recognize that this comb filtering happens in our natural environments as sound bounces off walls and reaches our ears through multiple paths. Along with reverberation, It is part of what gives our hearing a sense of space. So as with 2nd harmonic distortion, a comb filter scenario can add to and enhance our enjoyment of recorded music.

I'm well aware of comb filtering - in the dual center channel example, the acoustic centers of the two speakers in question is what I'm wondering about - while we may measure some artifacts, how bad is it really going to be at the listening position, and would other perceived benefits of such an arrangement offset the negatives? If you're head is "in between" the axis of each speaker? Vertical dispersion in this setup is going to be fairly poor, but you're not likely to drift higher or lower than either speaker. Your post even hinted at it - "when the distance to the two drivers is approximately equal". While not measuring perfectly equal at the listening position, in light of all the other interference and distortion sound picks up between the time it leaves the speaker and the time it hits your ears, I would contend the interference effects in the dual center arrangement are tolerable, and overall not that significant. (But still easy to avoid if you buy a ONE good center channel, which would of course be preferable).


I know in speaker building, we try to keep the acoustic centers as close as possible, less than a wavelength of the frequency of the xo as a rule of thumb. The D'Appolito arrangement (woofer, tweeter, woofer) is well documented for its minimal vertical dispersion, but so long as your head is in between the woofers (close to tweeter level, give or take several inches) you're ok. I'm half expecting a similar effect here, just haven't seen any measurements of such a setup.

[kexodusc]

Here's a mathematical picture that may help you visualize what's going on at different frequencies

Here's a pretty picture showing what's happening in three dimensions at a single frequency with two side-by-side drivers. You can cock your head sideways and imagine how drivers stacked vertically behave.

Nice pictures, but they don't tell us how far apart the sources are, or your relative position to each source, which I believe are key variables. I'd be curious to see measurements with two speakers probably 4-6ft apart at 8-10 ft distnace, taken from the sweet spot?

[Rich-n-Texas]

You aren't taking into account the Melvin Walker rule which states that the mains should always be placed with a 12' space between them.

[Rich-n-Texas]

Here's a mathematical picture that may help you visualize what's going on at different frequencies

Here's a pretty picture showing what's happening in three dimensions at a single frequency with two side-by-side drivers. You can cock your head sideways and imagine how drivers stacked vertically behave.

Yes, the picture is very pretty. I especially like the many shades of blue that were included.

[Luvin Da Blues]

Are y'all taking into account the Melvin Walker rule which states that the mains should always be placed with a 12' gap between them?

Hey Rich, I guess you didn't get the memo. The 12' rule will vary (+/- 6.27") depending on the prevailing atmospheric pressure and ambient temperatures.

[Rich-n-Texas]

So the MW rule was amended? I wasn't aware of that. I'll update my library of facts. Thanks LDB.

[Kevio]

Nice pictures, but they don't tell us how far apart the sources are, or your relative position to each source, which I believe are key variables. I'd be curious to see measurements with two speakers probably 4-6ft apart at 8-10 ft distnace, taken from the sweet spot?

Measurements taken at the sweet spot are going to be fine. Comb filtering only occurs for off-center listening positions.

The first null in the comb filter occurs when the two signals are offset by 1/2 wavelength. At 1 KHz, a wavelength is approximately 1 foot. So you'll get a 1 KHz null if the difference between path length to the two sources is 6 inches. So, for example, if your distance from the sources is 8 feet and the sources are 1 foot apart, trigonometry tells us the null corresponds to a listening angle of 45 degrees off center. If the sources are further apart, your listening position is closer or the frequency of interest is higher, the sensitivity to listening angle will be greater.

[drumdrum]

hello guys - ( my original post ) - appreciate the opinons & feedback.

I guess what we're really talkin' about here is psycho acoustics...
( actually, my neighbors all call me one of them ! ) - and as for comb filtering...I just pull my wool cap down over my ears, which filters all that out, plus then I don't worry about combing anything !

But seriously, let me restate the dilemma as I see it:
The stereo (phantom center) image is gonna be at odds with any "actual" center channel (mono) - whether it be one or two sources.
( those guys pixelthis posted). So do ya listen to music in stereo only, with no center channel ??? the folks that designed my Lexicon CP-3 (1995) surround processor don't think so ! They took took this whole exploration right to the limit & built in lots of great multi-channel surround modes, along with Panorama, which eliminates the center ch and gets into the whole cross-channel cancellation thing ( out of phase signals sent along with actual L R information ).

But the thing I really can't deal with ( now that we all live in a 5.1 world) is having the center channel sound ( largely dialogue ) emanating from a localized source that doesn't correspond to the picture ! Whether the speaker is above or below, its still from a place that doesn't make sense to the brain.

I mean...we can't do what the big theaters do with their acoustically-transparent perforated screens. I other words I want what they got !
( not to even mention IMAX ! )

So - to my ears ( and my eyes, when you bring a TV monitor into the mix ) dual center channel speakers produce a sound field that's hard to localize - which is what I was referring to as a " phantom center".
Vertical spread may be a poor phrase to use, but I don't have a better one. In any case, I might settle with interference issues more easily than with the obvious disconnect of a source that doesn't correspond to, and support the visual picture.

The other benefit, is to have twice the horsepower ( as far as moving
air ) for when the center channel action gets hot & heavy. It may not be increased dynamic range, but it surely is less bass- low mid distortion. With large full range mains, a smaller single center speaker often can't cut it ! especially when ya read that well over half the sound during a movie is assigned to center.

I sit 6' from my 42" monitor ( at 56 my eyes aren't great ) and to maximize the movie experience. But what I am l listening to is not even close to pure " near source ". The Heil air-motion drivers are still the only true bipolars I'm aware of..(?).firing equally front and back
( in phase !) from the same membrane. And I've got 4 running up front, so I'm hearing a lot of disbursed-reflected sound, even though I'm close. I'm sure it all factors in ( along with room acoustics ) but since we've all got differing rooms & systems...the best we can do is discuss - until one of us figures how to build a Star Trek transporter - at which point we could party down in various listening-living rooms...and still make it to work the next day.

[drumdrum]

Just got out my tape measure out...
L & R are 7' apart and 3' between my upper and lower center speakers...
I'm seated for a movie not quite 7' out
height of lower center Heil 18" - 20" & upper Heil at 48"- 52"