TM CC with MTM L/R?

[HareBrained]

So, I've been playing around with designing and building my own HT 5.2 set-up. I have been researching and posting as different ideas occur to me. One thing I've noticed is despite several MTM center channel designs, many people discourage using an MTM as a center channel because of the lobing that can occur in the horizontal plane caused by the interaction of the woofers.

The obvious concern with using a TM with MTM main speakers is level of output. Most receivers have individual level controls for their outputs. But also, many of the drivers I've been considering have both 4- and 8-ohm versions.

For the surrounds, I was considering a low-BSC TM, which could be used for the center as well.

So, what's you opinions about using a TM for the center channel with MTM mains? Are the pitfalls of MTM CC something to be concerned about? Is my assumption valid that a surround design can be used as a center channel? I'm assuming that I would only need to design 2 configurations. If I need to have 3 designs, what would I need to do differently between the TM CC and the TM SS?

-John

[JonW]

Hi John,

I'm no center channel expert. But we had a recent discussion here that I found helpful:

http://www.htguide.com/forum/showthread.php4?t=31365

[HareBrained]

OK, with well over 100 views and only 1 response (thanks JonW) maybe I need to spice things up a little.

Has anyone used a 2.5-way for the L/C/R? What considerations need to made regarding the driver placement, if any? Do the woofers have to be next to each other (TMM vs MTM) considering a ~300Hz LP on the .5 woofer?

[---k---]

Sorry. Your question seemed somewhat theoretical to me. The theory questions seemed to get asked a lot, and I'm just burned out trying to answer them because someone always wants to argue. The truth is that everything is a compromise.

As for driver placement, with a 2.5 you should follow the same rules as any other speaker, and try and keep the center of drivers playing overlapping frequencies within 1 wavelength apart. With a 2.5 and 300hz crossover, the wavelength is pretty big, so you shouldn't have to worry about it.

[HareBrained]

Sorry. Your question seemed somewhat theoretical to me. The theory questions seemed to get asked a lot, and I'm just burned out trying to answer them because someone always wants to argue. The truth is that everything is a compromise.

As for driver placement, with a 2.5 you should follow the same rules as any other speaker, and try and keep the center of drivers playing overlapping frequencies within 1 wavelength apart. With a 2.5 and 300hz crossover, the wavelength is pretty big, so you shouldn't have to worry about it.

Thanks for confirming my thoughts on the 2.5-way configuration. This greatly changes my system design ideas. I want to use some small speakers in small cabinets. This means a sealed cabinet with two woofers could reach my objectives.

Yeah, my questions tend to be more distilled since I'm trying to extract as much information as possible. In this case, so many people are talking about the advantages of MTM main speakers and the disadvantages of MTM center channels, I couldn't help but ask why not an TM center with MTM mains. There seem to be ways to balance their output, whether it's lower impedance versions of the same drivers or increased input signal. And then it occurred to me that the 2.5-way configuration has some advantages that could be used in each position. So, thus the questions there.

I'm still interested in other opinions about each of the combinations so please contribute.

-John

[Dennis H]

You can model all your various combinations of drivers in ARPE.

[JonW]

Has anyone used a 2.5-way for the L/C/R? What considerations need to made regarding the driver placement, if any? Do the woofers have to be next to each other (TMM vs MTM) considering a ~300Hz LP on the .5 woofer?

In this link Zaph talks about issues with 2-way versus 2.5-ways. Not a detailed treatise on the topic but you can learn a few useful things about driver placements and such:

http://www.zaphaudio.com/ZRT.html

[HareBrained]

JonW, again thanks for the link. I didn't realize the ZRT had the 2.5-way option. Zaph does point out that a 2.5-way design is, by nature, a full BSC implementation, and as most center channels are placed in close proximity to other objects or a wall, I would guess that it doesn't make a good CC design basis. And depending on the L/R placement, may not be advisable for use in HT at all.

So, my room is small (14x12) and the speakers will not be standing alone. I'm back to my original question, TM CC w/ MTM L/R?

Dennis, thanks for the link. I wasn't aware of the ARPE and I will use it in my research.

[---k---]

WMTW!

More serious:
If your not pushing the center really hard and have a sub, a vertical MT will sound excellent, but may not give you the volume level levels to impress us nut jobs here.

If you want to be able to push a bit higher spl levels and have less distortion, a MTM on its side will be acceptable on axis. It could have issue off axis. So, if you're sitting directly in front of it, it will probably be acceptable, but not optimum.

Also note, there is nothing wrong with mixing speakers of different sensitivity levels and level matching using the level controls on your receiver. There is absolutely no harm in this.

[JonW]

I'm back to my original question, TM CC w/ MTM L/R

Well, based on the discussion in the first thread I linked to... My current impression is that there should be no problem at all in terms of matching a TM CC with MTM left and rights. As long as they use the same drivers, roughly the same style cabinet, and they are both designed by the same person. In terms of matching the "timbre," I see no down side at all.

However the one potential problem might be that the TM cannot keep up with the output of the MTM. And if it strains a little, then things will fall apart somewhat. But that's in theory and I can't say if it really matters.

So if you want to overengineer to be sure you're getting max performance, that brings us to a WTMW center channel. A horizontal MTM center is a middle ground in terms of output but it has off-axis issues.

That's probably what I'll do, a WTMW center. I've got a Modula MT as my current center and my Scanspeak MT project for my left and right. They don't match well but they are all MT's. I think it's a driver quality issue. So I may make a WTMW center using similar drivers to the left and right MT's.

[HareBrained]

Thanks for the constructive feedback.

My furniture/seating arrangement has a couple of far off-axis positions which I think would fall into the nulls with a MTM.

Would it be worthwhile to use a 4-ohm version of the same driver (for example RS125-4 in the MT and RS125S-8 in the MTM) to raise the sensitivity? Or are the differences between the versions too great?

[---k---]

Sensitivity is overrated in my opinion. Get an amp that is willing to drive them, and don't worry about sensitivity. I just don't get the current hype surrounding sensitive designs. (although all of my speakers would be considered to have higher than normal sensitivity at 91db.)

Also, note that a lot goes into the final sensitivity and impedance of a speaker and just the drivers. If you take a really sensitive driver, but end up having to pad it down to match the tweeter (or vice versa), you've lost the sensitivity that you were looking for.

[HareBrained]

Sensitivity is overrated in my opinion. Get an amp that is willing to drive them, and don't worry about sensitivity. I just don't get the current hype surrounding sensitive designs. (although all of my speakers would be considered to have higher than normal sensitivity at 91db.)

Also, note that a lot goes into the final sensitivity and impedance of a speaker and just the drivers. If you take a really sensitive driver, but end up having to pad it down to match the tweeter (or vice versa), you've lost the sensitivity that you were looking for.

I'm not worried about the sensitivity per se, but relative sensitivity between the MTM L/R and MT CC. Power and overall speaker load are not a concern either.

The MTM would naturally be a 4 ohm design. If the same woofer is used, the MT would be a 8 ohm (nominal) design. So, not only is there a decrease in radiating area, there is a doubling (nominally) of input power. If I use a 4-ohm driver in the MT, at least the power would be the same.

{and then a miracle occurs...}

... HOLD ON. If I use the M drivers in series, the MTM will have double the area but half the power of the same driver in the MT. Well, it took 4 days and 5 posts to hit upon that ditty. :roll: That answers that. Now I just need to decide on the driver. All that I'm considering will exceed xmax before the power of my receiver runs out.

One of the main reasons I want to use an MTM is to not "over-pad" the tweeter. Most designs I've heard that pair disparate drivers sound like a blanket is between me and the tweeter.

[---k---]

None of CJD's designs have padding. (he gets lucky. ) Maybe something to study.