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Center Channel - MTM, Coaxial or ????

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  • Center Channel - MTM, Coaxial or ????

    Looking for pros/cons/suggestions for a center channel and maybe extending the basic design to surrounds.

    Current fronts are FaitalPro 8PR200 and Morel Cat 378, actively crossed by my AVR.

    I've got some 5FE120s and a Cat378 so that's a pretty straight forward MTM design and easily changed for a surround build.

    I picked up a 8HX150 for 50 bucks so that's available to use. Originally bought it to use as a rear channel for a 6.1 setup but that's not set in stone. Never worked with a pro sound coaxial before. Also wondering about placement, that's a big box below the TV so it would probably have to go above it, which is a bunch of weight to support. And I'd still need to design and build something for the surrounds.

    Also been contemplating an array of small full range speakers. I'm pleased with how the 8PR200 and 5FE120 sound so I'd probably lean towards using 4FE32 or 4FE35s. I've never done an array and don't have any clue as to how many drivers I would need to use and how to do shading or if it's really needed. The 4FE32/35s are available in 4,8 and 16 ohm variants so I think a combination could be arrived at to get close to the SPL and power handling of the fronts. Each flavor has it's pros and cons but I seem to be leaning towards the 4FE35. 25 bucks vs 40 adds up to some money if I end up buying a dozen or so of them. The 8K peak of the 35 should be able to be handled by the AVR's room correction software, Audysey MultiEQ XT32.

    Sorry for the rambling post, I'm stuck in vicious analysis paralysis loop and would appreciate some input.

  • #2
    Every center channel I've done the drivers were driven by where I wanted to mount the center channel. I'd lean toward the traditional MTM with predictable off axis issues. The horizontal array would be a challenge.
    John H

    Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower


    • #3
      A horizontal array works if you make it concave.


      • #4
        D'Apolitto tells you how to do an MTM. But really why not just a MT like the mains? The advantage of multiple mids is power handling, but if you do the sim, you will find a single 5 or 6 will exceed instant hearing damage levels and still be within x-max. Of course, less displacement does mean lower distortion.

        In theory, coaxial may have advantages. In practice, not as easy. At least I have not heard a good one yet. Pro-sound drivers are optimized for PA use, not what you want in your living room. Good reasons for different trade-offs. A center above the screen may give an odd perception for localization depending on where your mains are. I much prefer it right below. Try it. Your results may vary.

        Probably offending someone, but have you played with "full range"? I have. What I found is some good mid-ranges. Terrible for "full range" even assuming 80 Hz cutoff. Never found one that did not sound a lot better with a tweeter. EQ does not overcome physics.

        There was a writeup in Speaker Builder on tricks to handle the phase in an array. Both by how they were wired and by curve. Tricky.

        A design I though might be worth looking at is M-TT-M as in a O:O format using small format tweeters scrunched together as tight as possible. More than PSD-Lite can simulate though.


        • #5
          Center channel is mainly used for vocals. If voice intelligibility is what you seek then DML's is the way to go.


          • #6
            Over at DIYSG they developed a coaxial meant for hi-fi. Never heard it, but it's supposed to be a decent sounding driver. I agree that, without lots of EQ, most PA coaxials are best left in stage monitors etc.


            • #7
              Originally posted by tvrgeek View Post
              D'Apolitto tells you how to do an MTM. But really why not just a MT like the mains? The advantage of multiple mids is power handling
              The primary benefit to multiple midbasses is being able to use two smaller drivers that have at least the same, if not more, displacement than one larger driver, giving wider midrange dispersion, which allows a higher crossover. But this only applies with a vertical MTM. With the wide horizontal CTC of the midbasses in a horizontal MTM a single larger midbass would work better where dispersion is concerned, if the enclosure will fit under the TV. That 'if' is the usual reason for using smaller multiple drivers in a horizontal center. Marketing is at the heart of it. A wide and low center looks better under a TV, while matters of dispersion and driver displacement mean absolutely nothing to the average consumer.


              • #8
                I find it beneficial to use identical drivers across the front. The last thing I want is a change in character as the sound moves. I also venture that the forum participants who are building actually do want to understand dispersion and excursion. Not average consumers.


                • #9
                  Thanks for the feedback. So I believe I've formulated a plan of action to at least start moving forwards.

                  I'm not going to explore using the coax, doesn't look like it's a viable solution that would be fairly easy to implement.
                  An array is out, I think it would probably voice too different from my existing fronts and also the learning curve of building one

                  It would be nice for the center to match the fronts. I'm going to have to make some new measurements and see what it would take to build the crossover since the current fronts are actively crossed with 48dB an octave xover or go back to passive with 3 identical xovers.. And decide if having a 8 inch two way center channel is viable, both physically and aesthetically and financially.

                  I'll build up a MTM test box for 5FE120s and cat378 that I have. Measure it in the front, center and surround positions. That should give me enough data to do xovers for fronts, centers or surrounds, depending on my final decision. I through a quick sim together using spec sheet data in VituixCAD with guessed baffle dimensions and it looks like a first order xover on the 5FE120s and a little padding and a 2nd order on the CAT378 should give pretty good results.

                  Again thanks for getting my out my analysis paralysis....


                  • #10
                    Rule two of project management:
                    There comes a time in any project to shoot the engineers and start production.

                    Rule one is of course:
                    The first 90% or a project takes the first 90% of time/money. The last 10% takes the other 90% or time/money.