I understand the theory perfectly well.
What I am saying, and as Axiom stated, is that in the real world a speaker array is not a perfect point source, your ears are not a single point microphone, and your room is not reflectionless. The combination of all that should smooth the response. So, while your examples are pretty, they don't represent the complex reality that is audio engineering.
The point you're missing is that even though you're correct when you say that external factors will have an influence, you should always strive for the speakers
to be as accurate as possible. Even if there's room reflections which will create some issues, it doesn't mean that you should use a poor design because hey, there's room reflections so in the end, the issues which this poor center design will create will happen in your room anyhow... Even if there's issues out of the control of the speaker designer, that doesn't mean that the speaker designer shouldn't aim to make speakers as best as they can. Comb filtering will create serious FR issues for listeners out of axis, that's a fact... So why use a design which will have severe comb filtering issues then?
I'm sure multiple drivers create some issues, but could they also create some advantages?
I guess they could... Like I said, efficiency, max output, etc... But just take your statement, you're
sure they create some issues, issues which have been measured, and you say that they
could create
some advantages... You have a definite negative, and unknown, theoretical, maybe non-existing, irrelevant/inaudible advantages... Unless you know exactly what the advantages are and can weigh them vs disadvantages, then you shouldn't go for a design with known issues because you think that there
could be some advantages...
The Axiom article is the only place I've heard someone present advantages for multiple tweeters on a center.
And what were they? He claims that it's not as bad as most think, but the only advantage he mentions is
"at worst, it's irrelevant, at best it actually adds a pleasurable element of spaciousness to stereo". That's it...
And even if it's true that it adds 'spaciousness', so would creating a big null at 1khz, so are we to consider this better than a flat FR? And even, his article makes absolutely no sense in many ways:
The cancellations (dips) are what the single measurement microphone "hears" and measures using a full-frequency test sweep when the signals from the two M2 speakers don't perfectly overlap. This seems like an acoustic effect that may be potentially nasty in nature and should be avoided. These are pronounced cancellations, yet when we play music or speech over a pair of M2 speakers, we don't hear these comb filtering effects. Why is that?
Well there you go... The bolded part is exactly why you want to avoid using a driver configuration like on their center... He claims that we don't hear it... Huh, look how nasty that measurement is, I seriously doubt it's not audible... No matter how the brain deals with duplicated delayed sounds, issue here is the cancellation/reinforcement, and as he said, it's quite
" potentially nasty in nature and should be avoided."
Note that in the two rooms where the VP150 exhibited problems when I realigned the VP150 vertically the “beaming” effect, lobbing and male voice tonal anomaly disappeared. However, it still didn’t match the M80s very well IMO, which is why back in the first room I just ordered a 3rd M80 to use as a center instead.
Note that my experience with the VP150 is in the minority. Few people have reported the sonic anomalies I have. However, now that the VP180 had come out many people who previously remained quite are now coming forward and saying that they never did think the VP150 matched the M80s very well.
That's the thing, even if most people don't notice issues, or the issues are not apparent in all rooms, it's a bad idea to start off with a flawed design which will have evident issues. It's also ridiculous to dismiss those issues by saying people won't/can't hear them... Really, it really seems hugely close minded, like the ostrich putting its head in the sand thinking that when it does, all its problems go away...
If you understand comb filtering, then you (autoboy) should realize that their center designs are horrid (ok bit too strong, that they have issues... Well one at least, comb filtering...) because they will invariably create comb filtering issues... It's as simple as that. You can try to dismiss it, and say that they're not that important, but they're there, none the less... And most importantly, putting two tweeters vertically, if two tweeters are so important in the design, would yield the same benefits that two tweeters provide (SPL, sensitivity, etc.), with far less comb filtering issues. It really seems like a no brainer to me...
Have you notice that there's no off-axis measurements for their VP180 center? I certainly did... There's none for the VP150 either. That says a lot.
What would you think of the 2 tweeters placed diagonally in the middle instead of a direct vertical arrangement of the tweets?
It would seem 50% better than horizontal, but 50% worst than vertical... The worst comb filtering would be at like 45 degrees now, so it could be marginally better for those sitting off axis, might create issues with the mids, I don't know, wouldn't make much more sense to me... Energy has a center similar to this:
http://www.energy-speakers.com/na-en/products/rc-lcr-overview/
I'd have preferred to see it vertically, but in that case, they would have needed to make the center quite a bit bigger to fit that driver configuration W(vertical MTM)W, so as a compromise, they put it diagonally, but I don't think it's such a great idea... They say:
An Energy exclusive diagonal tweeter/midrange design that disperses sound identically in both planes.
And in one sense it's true, 90 degree up, down, left, right, will have the same response. The issue is 45 degrees top right and bottom left, and top left and bottom right. One will be like an horizontal vs vertical MTM, meaning that one will have very little comb filtering and the other will have big issues with it...
So again, for a center placed horizontally, you want comb filtering issues to happen vertically (2 tweeters, or tweeter + mid placed vertically), and not horizontally (TT or TM placed horizontally). Going diagonal doesn't fix the issues, might be a compromise, but it has its own sets of issues, should definitely try to avoid it if you can...
Btw, 2 vertical tweeters would still create comb filtering issues with the two mids on both sides... So it might not be as good as a single vertical TM... But, I don't know... Still should be better than WTMMTW imho... Might just require steeper crossovers... Or maybe it really doesn't work (because of mids on their side creating comb filtering) and Axiom's centers end up working better... No idea, but any way, obvious comb filtering issues on that VP180 center it seems to me... It would be interesting to see the off axis response.
