Please explain the MtM "issue" with centers

To work properly MTMs must be vertically placed. Doing so gives wide dispersion on the horizontal axis, narrow dispersion on the vertical axis. When placed horizontally, as is usually the case with centers, the dispersion axis is rotated 90 degrees, making it too narrow horizontally, too wide vertically.

In spite of the scientific evidence to the contrary, virtually ALL speaker manufacturers sell HORIZONTAL center speakers because most people don't have the luxury of being able to place vertical center speakers in their home theater setups and speaker manufacturers are in the business to make money.

If you have the room and inclination to pursue the ideal setup ... go for it!

Bill C

And another comprehensive overview of the issues:

http://forum.blu-ray.com/speakers/89...d-avoided.html

Designers of multi-driver systems and even large planar systems should always take off-axis frequency response into account. That this is not always done especially in inexpensive MTM centers is well documented in the Audioholics article referenced above. Those of us who have the space for three large speakers in our home theaters will of course choose that option, but many of us have living spaces where a horizontally placed MTM center is what fits. The advantage in home theater of having two mid-woofers in this price range is significant, and what it boils down to is a tradeoff between ability to cover a wide seating area with the same quality as the central seating area, and having the advantages of higher output and lower distortion that two-midwoofs bring.
The number of well reviewed horizontal MTM centers from highly regarded manufacturers such as B&W, Epos, Dali etc. is large, as is the number of speaker design posts where you could learn more about the effects of crossover design on off-axis frequency response, but the best overview of the compromises involved is here: http://forum.ascendacoustics.com/sho...7&postcount=10

That's the key phrase. As the OPs question proves the average consumer is unaware of what an MTM is and why it should not be placed horizontally. Most assume that a wider driver orientation gives wider dispersion, not narrower, giving them the inclination to buy an MTM rather than a single midbass driver speaker. Manufacturers are in the business of making money, and if that means giving the customer what he wants whether or not it works well then so be it.

Here's what I don't get: how much "taller" does a cabinet have to be to house a 1" tweeter atop a typical 4-6 1/2" midrange?

1-2 inches? Maybe a bit taller? So couldn't manu's just as easily continue to make a "wider" cabinet just slightly taller? Then the real sonic benefits are realized while joe six-pack gets the wider center he perceives to be "better" in the role of a center channel?

I realize that 2 more inches may not fly in some cabinets/situs, but surely most apps that would take a 7" speaker would house a 9" unit?

Seems to me that typical bookshelf and centers with a T/M design have the tweeter pretty darn close to the top of the midrange driver. ???

Where am I going wrong?

James

The OP is asking about an MTM. There's no major problem placing a tweeter and single midbass horizontally if crossed over properly based on the CTC distance. Vertical is better, but horizontal isn't a deal breaker. It's the dual midbass configuration of the MTM that's problematic unless extremely well engineered, which few center speakers are.

^ gotcha on the dual midbass point.

So then, what about continuing the dual midbass' (which I imagine helps in the ouput/ extension departments) and simply integrate the tweeter(s) above either of them and inch or two?

I'd imagine this has been looked at already and folks with millions invested in this area have found 9" cabs to be too tall or the tweeter integrated (as I suggested) to be a step-down, sonically?

Thanks
James

Good responses, ...fwiw, in my opinion, this is one of the biggest and most prevalent mistakes in all of HT audio; screwed up center channel implementation. To me, three identical mains, all across the front in an even and level horizontal line, and aim them at the primary listening position. This is 101 level stuff. If this means the three are slightly above, or slightly below the display, so be it.

Line 'em up, keep 'em away from adjacent surfaces, aim 'em at the listener.



Good luck

The blu-ray.com forums link mentions that a MTM where the tweeter is off the same horizontal axis is better than one where its inline. If that's true why don't we see more speakers like that? Something like this doesn't seem to add any additional size or electronic complexity to a horizontal center.

Additionally; horizontal designs can also have many more points of diffraction, creating VER and multiple virtual components delayed in time, in the more discernible horizontal axis. Theoretically, these same VER would be less destructive in the vertical axis.

^ precisely my point, moose. Is that acceptable tweet positioning or is a higher point even more desirable with most designs?

Cuz if THAT'S ok, then you can throw out my entire bigger cabinet deal, as it would be nearly moot with a design like this.

James

Friends;

If I may interject my 2cents for a moment. I often see our MTM article links above referenced here but never our counter article that gives the other side of the story.

I hope all consider both sides of the argument when making a decision on purchasing a center channel that is right for their installation.

Please see:
http://www.audioholics.com/education...hannel-speaker

Enjoy.

Not really. The MTM was invented by Joe D'Appolito to address the specific issues of horizontal dispersion, vertical pattern control and baffle step compensation, and to do so the drivers must be vertically aligned. No matter how well designed a horizontal alignment will be inferior to vertical. The benefit to a well designed horizontal alignment is that it will at least work better than one that's poorly designed. IMO the only reason for using a horizontal driver array is when the required system sensitivity and output cannot be achieved any other way, in which case you accept the compromises involved to reach the desired goal.

That tweeter positioning MIGHT provide for better dispersion and MIGHT lessen the effects of some undesirable interactions between the tweeter and midwoofers in the crossover region but it doesn't address the lobing issue caused by the dual, horizontally-aligned midwoofers at all, really (save for perhaps moving them a little bit closer together).

A partial solution would be to simply eliminate one of the midwoofers and utilize a horizontal MT. Yes, that would still not disperse sound as well as a vertically-oriented speaker and there would still probably be issues in the crossover region between the two horizontally-oriented drivers but you'd at least sidestep one of the pitfalls of the MTM completely. Some manufacturers DO make horizontal MTs for center channel use. There are also horizontally-oriented cabinets which feature coincidentally arrayed tweeter/midwoofer drivers; KEFs, for example. These perform quite well.

Another solution utilized by some manufacturers is a 2.5-way horizontal MTM where both midwoofers operate at the lower frequencies but only one of the midwoofers operates in the critical midrange region where lobing occurs. Several center channel MTMs are actually designed like this; most of Klipsch's, for example.

And, of course, there are 3-way horizontal WMTWs that add a single true midrange driver, usually vertically-oriented with the tweeter, between the two flanking woofers. Although these speakers still suffer from some other issues inherent to the horizontal orientation, the midrange lobing issue is eliminated.

Now, one compromise that is usually inherent in a standard 2-way horizontal MTM design is a lower crossover point between the midwoofers and tweeter than might otherwise have been utilized were the speaker not designed for horizontal center channel use. What this does is allow the tweeter to reproduce at least some of the critical midrange frequencies at which lobing occurs. Of course, this probably comes with some trade-offs as the tweeter may not be the best driver for reproducing those particular frequencies in quantity.

No, not really. Lobing isn't the only issue. And I think there probably is complete agreement. Horizontally-oriented center channel speakers, whether a simple 2-way MTM or not, are a compromise for the many people who cannot accommodate a matching (and not even necessarily identical) vertically-oriented speaker in their center position.

Just to add to this, most (if not perhaps all) horizontal MTMs designed for center channel use are not truly D'Appolito array speakers. D'Appolito array speakers are designed with very specific crossover characteristics and correspondingly very specific driver spacing (among other things). Probably a good thing, I guess, as the specific performance benefits afforded by a true D'Appolito array in the proper vertical orientation would actually be very undesirable in a horizontally-oriented center channel MTM.

(Prior thread on the subject here. Note Dennis Murphy's remarks on what appear to be two types of MTM interference.)


For the skeptical view of MTM designs this is an important point and one that can get insufficient attention. The trick to MTM's with very good acoustic symmetry on the long axis involves tight driver spacing, aligning their acoustic centers, and a specific type of crossover.


In the 80s, D'Appolito wrote up his theory in Speaker Builder magazine. He also presented "A Geometric Approach to Eliminating Lobing Error in Multiway Loudspeakers", interestingly.

D'Appolito included a lot of variables in his original and quite elegant 3-driver, 2-way solution to solving uneven directivity, although the difficult combination of spacing, first order responses, and acoustically-aligned sources is hard to include in all MTMs, which is where MTM skepticism can and probably should occur. When done correctly, however, the response was said to deviate a theoretical +/- 1.5dB on the long axis.

More conventional MTMs, as opposed to the true D'Appolito array, cause/allow lobing dependent on the combination of the array's deviation from those elements, although this can be seen as a pro or a con, depending on design goals. Lynn Olson's nearly legendary Arial project used the vertical MTM w/o the D'Appolito requirements, thereby reducing long axis response instead of actually improving on it.

It's worth noting that the third order implementation also sums to quite flat off-axis, although as with all MTMs, only within the constraints of the driver's raw responses and their relationships to one another.

PS: As sivadselim just said:

Even Joe doesn't hold steadfast to his original design parameters anymore. After a few dozen iterations he realized that there's a lot more leeway than he originally thought. But only when made vertical. When horizontal any deviation from the maximum CTC distance requirements will result in comb filtering.

Absolutely, IF you can accommodate that. And that's not just because of the pitfalls of using a horizontally-oriented center. 3 perfectly matched speakers across the front soundstage is ideal for other obvious reasons.

That said, if you can't accommodate that, then you just can't. Don't lose any sleep over it.

What about this design?

What about it? It's a 3-way WMTW. It doesn't exhibit midrange lobing. It will still have some other issues associated with it due to its horizontal orientation. Yes, the offset tweeter most likely improves the dispersion characteristics of those particular frequencies. Still, that speaker will not work as well at center channel duty as a 3rd matching speaker speaker that is identical to the L/R speakers it was designed to compliment. It is still a compromise.

Without knowing anything about it I'd venture the midbasses are way too far apart.


And if you aren't single or have a very small family and circle of friends? When there are centers that have good dispersion I see no reason to get one that doesn't.

I'd love to get another one of my mains for the center but then the speaker would be blocking 1/3rd of my display and raising the display isn't an option since I hate more than anything is staring up at a display (very uncomfortable). But what I was getting at is that type of design should be the least compromised of horizontal centers, no?

Well, I don't know where they are crossed-over, but since that speaker is equipped with a midrange driver, they are essentially just woofers.

Right. Which is why manufacturers offer 'center channel speakers' in the first place. And some are better than others. But don't think for a minute that just because a manufacturer markets a speaker as a 'center channel speaker' that it is somehow designed specifically to reproduce the info in the center channel better than the L/R speakers it was designed to match could reproduce that same info. They can't. They are a compromise.