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Midwest Audio Fest

It’s that time audio enthusiasts! Registration for the 2019 Speaker Design Competition is now open! Visit midwestaudiofest.com for details and to list your speaker project. We are excited to see all returning participants, and look forward to meeting some new designers this year, as well! Be sure your plans include a visit to the Parts Express Tent Sale for the lowest prices of the year, and the Audio Swap Meet where you can buy and trade with other audio fans. We hope to see you this summer! Vivian and Jill
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Horizontal Dispersion of Center Channel MTM - how bad is bad?

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  • Horizontal Dispersion of Center Channel MTM - how bad is bad?

    I'm designing a 3 way center channel right now, with one design goal being low height. Most 3 way center channels are quite big, but mine is about 7 inches high. RS150p x 2, rs100p, some small tweeter, probably a sb26tcn.

    In the interest of exploring the conventional approach, I modeled some horizontal MTMs, using a small format tweeter set above the woofers, so that the woofers can be as close as possible. I used the sb29 neo ring radiator, which would probably be my choice for a small but very robust HF unit.

    Even using this best-case scenario, with a shallow crossover, I was shocked to see how bad the horizontal dispersion was - I don't have the graphs in front of me now, but you're looking at 6db down at 20 degrees off axis in the lower treble region. That's quite narrow dispersion, and sort of defeats the purpose of having a center channel to begin with.

    What am I missing here? The horizontal MTM is a popular approach for obvious reasons, and I've seen some clever implementations (cambridge audio uses a BMR, which has its own problems), but it seems really sub optimal, even when using small woofers.

  • #2
    Not to put too fine a point on it, but lots of commercial speakers are sub-optimal. As long as somebody buys it and likes it though, the manufacturer usually isn't real concerned.
    Francis

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    • #3
      You're not missing anything. Horizontal MTMs are popular for how they look, not how they sound. Your perspective is quite different from that of the average consumers who buy them.
      www.billfitzmaurice.com
      www.billfitzmaurice.info/forum

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      • #4
        I'm working on a center, with a 7 inch woofer, that fits on a 7.5" baffle (although my baffle is 9", the last 1.5" is taken up by a grille that carries the roundover, so it can be done. I had to truncate the tweeter faceplate, but that's easy. It can definitely be done. Check out post 10 of this thread:
        http://techtalk.parts-express.com/fo...s-with-omnimic

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        • #5
          Originally posted by Adam_M View Post
          I'm working on a center, with a 7 inch woofer, that fits on a 7.5" baffle (although my baffle is 9", the last 1.5" is taken up by a grille that carries the roundover, so it can be done. I had to truncate the tweeter faceplate, but that's easy. It can definitely be done. Check out post 10 of this thread:
          http://techtalk.parts-express.com/fo...s-with-omnimic
          That's not dissimilar from my design - I started using a tectonic BMR and a really tiny tweeter, but moved up to a slightly larger mid and tweeter for performance reasons. F3 of around 60hz and more than sufficient SPL for a small room.
          Click image for larger version  Name:	baffle.JPG Views:	1 Size:	44.8 KB ID:	1399162

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          • #6
            I'll note that although most modern MTM designs use LR4, D'Appolito used BW3 for his original design, with the woofers and tweeter in phase quadrature. The advantage of that approach is wide dispersion.
            Francis

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            • #7
              No matter what crossover you use it won't alter the size of the drivers or their spacing, which are the primary factors that limit dispersion.
              www.billfitzmaurice.com
              www.billfitzmaurice.info/forum

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              • #8
                Originally posted by billfitzmaurice View Post
                No matter what crossover you use it won't alter the size of the drivers or their spacing, which are the primary factors that limit dispersion.
                Through the crossover region it does help dispersion, due to the tweeter being in quadrature with the woofers. The polar pattern is kind of wavy, but not terribly so.
                Francis

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                • #9
                  I struggled with the same dilemma when I designed my center channel. I also considered a three-way design. After several simulations and a careful look at my listening room, I decided I was too worried about it. Clearly there is a disadvantage to a horizontal MTM arrangement. The extent to which this will be a noticeable problem depends largely on you room dimensions. Specifically, take a practical look at how far off center you will be sitting. In my room, the furthest off center I could get is around 20 degrees off center. At that point, the frequency response dip from the center channel is not my only problem. I would also be outside the window where my fronts and surrounds would provide the intended surround effect. If I have all four seats of the sofa occupied, the worst case seat is less than 15 degrees off center. The frequency dip at the crossover frequency for my system is not noticeable under these conditions, and a more exotic design was not required. Obviously this is just my situation and yours may be different. In addition to keeping your driver spacing as small as possible, move your crossover frequency as low as you can get it. This will help to minimize the dip.

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                  • #10
                    My BAMTM center with a low xover point minimized comb filtering, always sounded fine to me.

                    It doesn't HAVE to be an MTM.

                    Build Thread: SR71-inspired Center Channel - Techtalk Speaker Building, Audio, Video Discussion Forum

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                    • #11
                      Originally posted by djg View Post
                      My BAMTM center with a low xover point minimized comb filtering, always sounded fine to me.

                      It doesn't HAVE to be an MTM.

                      Build Thread: SR71-inspired Center Channel - Techtalk Speaker Building, Audio, Video Discussion Forum
                      That's an interesting approach, I wanted the output of two woofers though.

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                      • #12
                        Originally posted by pick View Post
                        I struggled with the same dilemma when I designed my center channel. I also considered a three-way design. After several simulations and a careful look at my listening room, I decided I was too worried about it. Clearly there is a disadvantage to a horizontal MTM arrangement. The extent to which this will be a noticeable problem depends largely on you room dimensions. Specifically, take a practical look at how far off center you will be sitting. In my room, the furthest off center I could get is around 20 degrees off center.
                        This is a good point, and I did do some analysis on my room. I have a couch on the left wall and one on the back wall essentially - for someone sitting on the left couch, they would be pretty far off axis. I agree that a very well designed MTM could be fine for a single couch in front of the CC, which is a very typical setup.

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                        • #13
                          When Zaph designed a CC for his ZDT 3.5 towers, he made it a WTMW 3 way, his writeup is worth reading, he discusses other factors in CC design you might find interesting. He uses the BAMTM as a worse case for a MTM CC, which as I stated, sounded fine to me.

                          Zaph|Audio - ZDT3.5

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                          • #14
                            Three identical speakers are the best setup.

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                            • #15
                              My current modest setup is the usual. I would like to hang a really big flat screen on the wall above the stairs and have three identical front speakers.

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