It is not you, the sound really is getting worse

@erik_squires

Hi E
My experiences with this design have been negative. Dr Toole seems to agree. I am glad it has worked for you. I will try to attach a quote from him below from his excellent book, Sound Reproduction. If you have the book, he talks about it extensively in chapter 18. I have two AR4c in my closet eating dust if you are interested in buying. I usually feel bad selling things that don’t work well for me Stay safe. Thanks. 
“The simple one, often called the “midrange-tweeter-midrange” or MTM, arrangement is usually found in entry-level products but also, occasionally, in some expensive products. In its basic configuration of both woofers operating in parallel, crossing over to a tweeter—a two-way design—it is not optimum because of off-axis acoustical interference.”

— Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms (Audio Engineering Society Presents) by Floyd Toole
https://a.co/90gpvoL

My experiences with this design have been negative

Can you be more specific??

So, let’s talk Toole. Always happy to point out we are not actually disagreeing. I quote your quote:

The simple one, often called the “midrange-tweeter-midrange” or MTM, arrangement is usually found in entry-level products but also, occasionally, in some expensive products. In its basic configuration of both woofers operating in parallel, crossing over to a tweeter—a two-way design—it is not optimum because of off-axis acoustical interference.

Toole is not calling these poor performers for dialogue quality, by the way, he’s specifically calling out the issues inherent in off axis response of the design.

The key points in this discussion are the phrases:

• "not a good design in terms of dialogue clarity " (@spenav)
• "not optimum" (Toole)
• fine (me)
  

To be clear, the MTM is very similar to a D’Appolito on it’s side. The difference is that where the D’Appolito has issues above/below the center has to the side. Also, as D’Appolito has discovered, they work better with higher order crossovers. I think he recommends a 4th order Linkwitz Riley alignment (electro-acosutical). So, how the MTM is designed matters.

I agree with Toole that it’s not optimum, but I also think it’s a fine solution within the constraints of a normal home listening room. These are naturally space limited designs that don’t need very much horizontal dispersion.

I’ve made one, and it sounds fine on and off axis. The main issue, as I’ve noted, was NOT the driver to driver interference, but the placement location. It sounded boomy without EQ. Fix that and it is very serviceable.

So let’s talk about what would be optimum. Well, a normal tower would work better horizontally, as would a 3-way center with a vertical TM array in the center, BUT (and there’s always a but) ...

This arrangement only modestly improves things, and you have to make significant sacrifices in the strength and size of the motors for the tweeter/mid arrangement. In other words, either use 1 great tweeter or much smaller, possibly lower quality, T and M.  This design pushing down the Woofer/mid crossover point. OK, so instead of maybe 2 kHz you push this down to 500 to 800 Hz (no lower due to the still very small mid) and the crossover slopes still matter.

In theory a 3-way center could be better, but not necessarily and the crossover slopes used and the EQ after placement matter a lot more, as does the quality of the components. Design choices and room correction matter a great deal.

I haven’t purchased a commercial center channel, but I can say from my own experience building and listening to one that it is a fine, and sometimes superb solution. Definitely better than no center, and sometimes better than a 3-way. I certainly would NOT say that a 2-way is going to suffer in terms of dialogue quality, either on or off axis just because it is a 2 way.

Fortunately dear A’goners, there’s a very simple solution to this while auditioning. Listen. Listen to the dialog and move horizontally around the room. Don’t get stuck in theory when your own ears will reveal if there are any problems.

Best,

E

@erik_squires 

Hi E
I have used several iterations of that design in my system with little success. The last time I tried using two speakers in a kind of horizontal array with two AR4c.  It was better but still not totally free of distortion. From time to time I was still left wondering what was the actor saying. And my speaker was not in a cabinet by the way. In that quote I referred to, Toole was talking specifically of center channels. I am glad that yours sounds fine.  I thought about designing my own too but got discouraged at the end. Then I realized the solution was in front of my eyes all along. My front speakers are Audience 1+1, they have zero crossover distortion due to the fact that they have no crossover to start with and they are clear as a bell.  They use a single full range driver that can go down to about 100 hertz in my system where they crossover to my four ML subs. So I got one more and now the center channel intelligibility is a solved problem in my system. Here are some more quotes from Toole about a design solution for the MTM design. 
“These designs also show up in vertical arrangements, in which case the acoustical interference is heard after reflection from the floor and ceiling. An intermediate configuration, sometimes called the 2 ½-way, rolls off one of the woofers at a low frequency, allowing the second unit to function as a midrange. The result is a slight improvement in overall performance, but the horizontal-plane interference pattern is then asymmetrical and still not what is needed. The real solution is to add a midrange loudspeaker allowing both woofers to be crossed over at a frequency sufficiently low that the acoustical interference is avoided. The explanation is in the caption.”

I cannot copy the caption but it shows the design with the midrange driver in the middle with the tweeter on top of it. I will find a way to include a picture later.  Hope that helps. 

@erik_squires 
look up Paradigm CC-290. That’s what he is talking about. 

I understand where Toole is going with this, and in absolute arguments, comparing them to ideal, he’s right, they are not optimum. In practice I’ve been quite successful. Maybe I accidentally created the best center channel ever??? << laughs >> Sorry, you all know I don’t believe that. Let me share the design I used. It sounds great. It blends seamlessly with my sides, which are simple 2-ways. With a little EQ, and decent room treatment the sound is seamless and there is no extra distortion caused by using an MTM array and it is perfectly fine to the sides as well.

https://speakermakersjourney.blogspot.com/2016/03/introducing-lm-1c.html
As I’ve alluded to, but not really said, for a counter point to Toole’s arguments, please see Joseph D’Appolito on his eponymous speaker configuration.

P.S. The LM-1C uses 2nd and 3rd order filters.