Axiom Home Page Forums General Discussion Home Theater Dual M22's as a center channel

Hello everyone! I've recently puchased a couple of M60's and am thinking about completing the 5.1 system. My question is I saw someone in the forums that had an awesome set-up that had dual M22's. I was just wondering what the advantages would to this as opposed to a vp150? Also, do you need a special reciever to accomplish dual centers?

Thanks,
Tenacious_C

Tenny, welcome. In general the advantage that a vertically oriented speaker has over a horizontal MTM arrangement is a wider and smoother horizontal dispersion of sound. The VP150 has a unique TMMMT arrangement which is said to be significantly better than an MTM in this respect. The advantage of using dual verticals, either directly at the side of the screen or directly at the top and bottom edges, is that this arrangement makes the center channel sound appear to come from the middle of the screen.

No special receiver is necessary; the two speakers are simply connected in parallel to the center channel output terminals of an HT receiver. This parallel connection results in cutting the impedance of the center speakers in half, compared to their usual impedance. This possibly could result in excessive current being required at some extremely high sound level and the protective circuits shutting the receiver off. In most cases, however, this problem doesn't occur and shouldn't be assumed.

Incidentally, if when you speak of "completing" your 5.1 system you mean that you don't have surround speakers either at this point, it's strongly recommended that you add QSs before the center speaker if the budget won't handle both at this time.

Whether or not the dual centres are better will depend on where you sit and how much of the resulting comb filtering will be audible. Depending on frequency and where you are located the sound from the two radiators will be out of phase so accuracy of the sound will be compromised at other than the sweet spot. Listening even slightly off vertical axis of the pair will have audible effect. At some frequencies amplitude will be reinforced and at others there will be cancellation.

In some rooms such a setup conveys a greater sense of spaciousness, but I've always found the clarity of speech suffers especially as you move off the vertical axis. One way to visualize it is to think of a pond of water where two stones are dropped simualtaneously at opposite ends of the pond. Depending on where you are in the pond the waves will be of different amplitude.

If you are only concerned about the one sweet spot on vertical axis then such a setup can work, but don't expect guests to hear what you are hearing. Even well designed horizontal centres are a compromise and can suffer from some comb filtering, though the better ones have less of this problem. By aiming the speaker directly at the listeners, at least those in the front row hear more or less the same sounds because as you would expect a good horizontal speaker has better dispersion on a horizontal plane than a vertical speaker.

Ultimately you should try both to determine what sounds better to you, especially if you are just concerned about the sweet spot. The sense of spaciousness of the dual M22s may be worth the comb filtering. The best option is a single identical speaker as the mains on the same plane halfway between the left and right, though like most of us that's not possibe which is why you need to check out these less than ideal alternatives.

In my HT one of my objectives continues to be to try and get the same sound from all seats which is a daunting task and one more easily accomplished in the lower frequencies. Dual vertical centres sounded too different depending on where I sat and dialogue wasn't as clear so I opted for one horizontal centre aimed at the front row. One day I will likely go the perforated screen route and mount a single vertical in dead centre behind the screen. For now its the single horizontal below the screen.

Hi,

While this is an excellent explanation of comb filtering, I would suggest that for most listeners using common source material--movie soundtracks and multi-channel music playback--and not with pink-noise test signals, comb filtering does not produce significant audible effects of any consequence.

After all, John, comb filtering is always taking place even listening in 2-channel stereo. Every time you move your head an inch or centimeter, there are comb filtering effects--altnernate reinforcement and cancellation of frequencies.

For those who are interested in learning how to hear comb filtering with pink noise, set your playback system to 2-channel stereo, play a pink-noise signal, sit exactly in the sweet spot and gradually shift your head to the left and to the right while facing the front soundstage.

Listen to the higher frequency portion of the pink-noise signal and you'll hear the hi-frequency content vary slightly as the high frequencies are reinforced and cancelled. That's comb filtering, and it's always taking place.

As far as expecting guests to hear what you are hearing, that will never be the case for any guest sitting off-axis whether you use a single center channel speaker or dual centers. Certainly using dual centers above and below the screen is a practical and effective solution for large screen projection when a single center results in the dialog located away from the screen. The latter, in my view, is far more distracting than any theoretical problems of comb filtering with program material.

Regards,

This is a great topic, I wish I could hear more. I feel conflicted about whether to just add another VP150, or change it for two M22's or go for a pair of M60's(matching my mains). Some have said that the four M60's would be ideal, but some say two m60's at the center will cancel out my m60 mains.

To add to your choices, you could go with 3 M60's across the front instead of dual M60 centers. Axiom does sell speakers individually.

"As far as expecting guests to hear what you are hearing, that will never be the case for any guest sitting off-axis whether you use a single center channel speaker or dual centers..."

Alan,

wouldn't the Audyssey MultEQ take care of this problem. I don't know how the cheaper version of this works (the one that only has two microphone placements), but people seem to like the one Offered in some of the midpriced Denon's (they have 8 microphone placements). Well, some say that it may make things sound a little artificial. But I had my eye on this one: http://www.audioholics.com/reviews/acoustics/audyssey-multeq-pro-sound-equalizer
With the Multeq pro it just enlarges the sweet spot so that everyone is hearing that sound no matter what seat they are in, but without sounding overly processed or artificial.

Many people around here are talking about how they want to buy a new and expensive amp but they don't even worry about how the room is affecting the sound. The Audyssey MultEQ is supposed to make every seat sound like the sweet spot as long as the room isn't too far off of ideal. I bet you'll notice much more of a difference from the MultEQ than you would a new expensive amp. I'm planning on running the MultEQ Pro between an Outlaw pre-amp and Outlaw amp. But, it will be a little while before I can get the money together, since that is about $4,000 total.

- Nick

Sooo, thinking ahead to when the dedicated HT room gets built and I move from a 46" TV to a projector....

Would adding a 2nd VP150, (so there are two, one above and one below the screen,) be a reasonable option or would it be preferable to have two vertical projection type speakers such as the M22s ?

I also am thinking about building the screen out from the back wall so my existing VP150 could sit directly in the center behind an 'acoustically invisible' screen but I'm not sure yet just how truly invisible these screens are so more research is needed there.

Anyways, not an urgent question as I have a couple of years minimum before I can afford to move the HT out of my current, nasty square room.

Just one? How would that work? I do not have a perforated screen, I have a 52'' DLP. If there is a way to accomplish that, it would certaintly be my first choice.

Interesting points Alan, though the question remains why introduce comb filtering artifacts unnecessarily when care should be taken to reducing comb filtering through treatments or judicious speaker placement. The main problem with dual identical centre speakers is the havoc they play with dialogue because of the cancellations and reinforcement at various frequencies right across the room not just on axis.

Audibility of comb filtering depends on several factors including frequency, delay, phase, reflection etc. While in some rooms the two speakers may act to anchor sound in the middle of the screen, in others there could be directivity to one or the other speaker because of the comb filtering. Duals will not always centre the channel in the middle of the screen because of these artifacts. The ear will tend to localize the sound to the nearest loud source particularly HF sounds.

What I've always liked about dual centres is the extra spaciousness from the centre channel which can be pleasing. It seems to me that whether one gets an overall sonic improvment depends on seating arrangement and room acoustics and how much you want to trade off dialogue clarity for a more "airy" sound from the centre channel.

I've found a well designed, properly placed single horizontal works better in most rooms. Though that configuration also has issues depending on the quality of the speaker, which make it less than favourable than a single speaker on plane with identical mains.

Hi John,

I would certainly agree with you that the degree of sonic improvement of using two centers is very dependent on the room setup, seating locations and the vagaries of the room itself.

But my proposal of using two centers wasn't to deliver sonic improvement; rather, it was to remove the extremely off-putting sensation that may occur with some large-screen front projection systems where the dialog gets localized at the single center channel speaker because it's too far away from the visual screen center. That is what using two centers (above and below) is meant to correct, and it does that quite well in the installations I've heard with no audible downsides.

You also said:

"The main problem with dual identical centre speakers is the havoc they play with dialogue because of the cancellations and reinforcement at various frequencies right across the room not just on axis."

I have not noted any of these effects in several installations nor in my own experiments with single and dual centers, and I would point out that "cancellations and reinforcements" are always occuring in any multiple speaker installation in any given room. Despite these effects, multi-channel music reproduction and home theater can be enormously satisfying and highly realistic.

You also noted:

"Audibility of comb filtering depends on several factors including frequency, delay, phase, reflection etc. While in some rooms the two speakers may act to anchor sound in the middle of the screen, in others there could be directivity to one or the other speaker because of the comb filtering. Duals will not always centre the channel in the middle of the screen because of these artifacts."

So far, I have not heard any installations where the use of dual centers resulted in any degradation of center channel sound or intelligibility.

Regards,

Hi Nick,

I know Audyssey Multi-EQ makes these claims but I've not been able to participate in a really good comparison so I can't comment. I'm suspicious of multi-EQ and over-processing, but we live in an era of amazing chips that have enormous processing capacity so I don't rule out the possibility that it just might work well with no "artificial" artifacts.

Regards,

Hi Murph,

Heh, heh, heh, indeed. It's an interesting topic. From the evidence so far, there is no perfectly acoustically transparent screen material, and certainly conventional perforated screens can cause all kinds of nasty colorations. There seems to be some reasonable evidence that woven screens are superior to perforated in reducing various problems but I'll wait until I can see some indenpendent measurements and conduct some listening tests.

It's a subject worthy of investigation and one that I'm sure we'll consider in the future at Axiom. However, until really good evidence--not sponsored by a screen maker--is in, my advice is to avoid putting loudspeakers behind perforated screens if at all possible.

Regards,

Sorry, I meant how would a single M60 work as a center under my configuration. I have no intentions of replacing my TV.

Ahh, Sorry Drew, I misunderstood.

and thanks Alan
for helping me not feel so bad about my suspicions over just how well it could really work.

Edited: I was apologizing for not mentioning the link was sponsored by a screen film vendor, but upon going back, it appears to be written by the staff of "Home Theater"'s webzine but you just never know for sure. Anyways, it's still the best written, laymans' explanation I have read so far, as long as you take it with the obligatory grain of salt required.

Hijack victims released and unharmed....

Great comments as usual Alan, however this comment has me concerned.

Quote:

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But my proposal of using two centers wasn't to deliver sonic improvement; rather, it was to remove the extremely off-putting sensation that may occur with some large-screen front projection systems where the dialog gets localized at the single center channel speaker because it's too far away from the visual screen center. That is what using two centers (above and below) is meant to correct, and it does that quite well in the installations I've heard with no audible downsides.

---

No one can argue with trying to centre the channel in the middle of the display. I'm not so sure two identical speakers above and below the screen achieves that objective and it has the unfortunate effect of creating less linear frequency response throughout the room. Two identical signals in phase running through two identical speakers symetrically positioned would produce a lobe running down the vertical axis. And given the height of most screens the lobe would be fairly wide over several seats. Not my idea of good sound reproduction across the room.

On the other hand, if you have no other choice, and you sit pretty much on center, it may acceptable despite the acoustical problems dual centres create from comb-filtering, lobing, cancellations, and different arrival times.

Quote:

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I have not noted any of these effects in several installations nor in my own experiments with single and dual centers, and I would point out that "cancellations and reinforcements" are always occuring in any multiple speaker installation in any given room. Despite these effects, multi-channel music reproduction and home theater can be enormously satisfying and highly realistic.

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I can see why you would make these observations if you sit in one sweet spot and don't move. However, as you stated earlier, the whole point of a center is to anchor dialogue to the screen. By using dual centers the voices will move to the left or right depending on where you're sitting. In light of the acoustical damage done by duals I would lean toward not wasting money on dual centers, and instead either running without a center and use the left and right to derive the (phantom) center, or go with a carefully placed horizontal centre. I guess I've been in enough HTs using duals where the audibilty of voices is too heavily compromised by the resulting artifacts to blame on room acoustics.

My room isn’t very big, so the ‘sweat spot’ is really three but-spaces on the couch across the width of the room. With my two vertical centers I can’t detect this comb filtering or any other undesirable effects when I sit in the middle, right or left side of the couch. I suppose if someone had a pretty large room (20 feet wide or so), there may be some sonic anomalies…..but there are none in my 12’ wide room. I did have to experiment quite a bit with placing the M22’s. If I had them 12” apart or closer, the center channel was localized to the center of the screen. If I moved them too far apart, the center channel just sounded “off”. I’ve got them about 30” apart now and they sound great. I also tried towing them in and out and that didn’t do much. I leave them pointing straight into the room now.

One of these days I may try two VP’s under/over the screen, but I’ve got better things to spend money on for the time being.

I don't like sleeping in my own sweat spot, but sleeping in someone else's sweat spot is worse.

Those damn leather couches..... makes my ass sweat every time.

We know who GrammerBot really is now......NICK!!