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

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  • #16
    This is exactly why I intend to go coaxial for my home theater. I'll be going for a WCW design (C for coax), identical for the front 3 speakers with the center just lying on it's side. A crossover point of about 300 for woofers to coax should put the comb filtering low enough to keep the center speech recognition strong.

    The all angle uniform dispersion of a coax in this application outweighs the typical coax downsides IMO.
    Electronics engineer, woofer enthusiast, and musician.
    Wogg Music
    Published projects: PPA100 Bass Guitar Amp, ISO El-Cheapo Sub, Indy 8 2.1 powered sub, MicroSat

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    • #17
      Originally posted by wogg View Post
      This is exactly why I intend to go coaxial for my home theater. I'll be going for a WCW design (C for coax), identical for the front 3 speakers with the center just lying on it's side. A crossover point of about 300 for woofers to coax should put the comb filtering low enough to keep the center speech recognition strong.

      The all angle uniform dispersion of a coax in this application outweighs the typical coax downsides IMO.
      Based on my own modelling, 300hz is an ideal low pass for the midrange. What coax are you looking at? The only ones that would seem to work are the dayton cx120/150 and the various sb acoustics ones. I haven't seen distortion for either, but I suspect they're both workable.

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      • #18
        I was playing with a 3-way design a few weeks ago, and listened to the tweeter, and mid without the woofer. I was listening to mostly female vocalists. I was crossed at 300hz, and when I disconnected the woofer, it was like a large part of the vocal range was gone. This was with the ladies. I continued to lower the mid cutoff until I thought nothing in the vocal range was missing. I think it turned out to be about 250hz, or lower. Oddly, the mid didn't really seem very taxed at normal spl. A while back, I helped Ani_101 with a x-over in a 3-way. The mid was tiny, and I think we crossed it below 200hz. I think it covered 200hz to about 8k. The tweeter was very small as well. It might be that a 3" mid can't handle HT duty crossed that low for a loud action movie, but might be OK otherwise.

        Tonight I was playing my new center, which is a re-purposed 5.25" TM. I have plenty of room for a larger center. This one was an odd box that wasn't being used, so I figured it would do for a temporary center. I played some rock concert DVDs, and was checking the voicing, when I heard what sounded like a driver voice coil coming loose, or a voice coil rub. I thought, well that's what I get for pushing it. I turned off all the speakers except the center. This was a dts Eagles concert. It's really interesting to see how the sound is mixed. Many tracks had just Henley's voice, and the bass in the center. I played woofer only, and then tweeter only, but couldn't tell which sounded the worse. I wiggled wires, and connections too. Anyway, I was convinced the speaker was blown, until I skipped ahead a few tracks, and everything seemed fine. After about an hour of center only without issue, I went back to the the track that I heard the problem. Turns out that it's in the recording!

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        • #19
          Originally posted by civit View Post

          Based on my own modelling, 300hz is an ideal low pass for the midrange. What coax are you looking at? The only ones that would seem to work are the dayton cx120/150 and the various sb acoustics ones. I haven't seen distortion for either, but I suspect they're both workable.
          I'm also cheap... so I was going to go with the Dayton CX120 flanked by a pair of TCP115. Should be able to get the cabinet down to only 6" tall / wide. I've heard the Daytons in a couple of the coaxial projects from MWAF last year, sounded plenty good for my purposes.
          Electronics engineer, woofer enthusiast, and musician.
          Wogg Music
          Published projects: PPA100 Bass Guitar Amp, ISO El-Cheapo Sub, Indy 8 2.1 powered sub, MicroSat

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          • #20
            250 to 300 hz seems to be the optimal XO point for a three way center. First, middle C on the piano is 261 hz. 300hz is only about E above middle C. Second, most female vocals are above middle C. This makes the piano left hand vs. piano right hand plus female vocals play in different drivers. That seems like a good logical division.

            Now also look at a good typical mtm on its side as a center. Here is the dispersion plot for the excellent (for an mtm) Outlaw LCR on its side:
            https://www.audioholics.com/bookshel...sv2/conclusion

            You can see that the destructive interference for a two way starts above 300 hz, so once again, the choice to cross a three way center below 300hz looks like the best choice if you want to avoid off axis issues.
            -Bob

            The PEDS 2.1 mini system
            My A7 Project - another small desktop speaker
            The B3 Hybrid Dipole - thread incomplete and outdated

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            • #21
              Originally posted by BOBinGA View Post
              250 to 300 hz seems to be the optimal XO point for a three way center. First, middle C on the piano is 261 hz. 300hz is only about E above middle C. Second, most female vocals are above middle C. This makes the piano left hand vs. piano right hand plus female vocals play in different drivers. That seems like a good logical division.

              Now also look at a good typical mtm on its side as a center. Here is the dispersion plot for the excellent (for an mtm) Outlaw LCR on its side:
              https://www.audioholics.com/bookshel...sv2/conclusion

              You can see that the destructive interference for a two way starts above 300 hz, so once again, the choice to cross a three way center below 300hz looks like the best choice if you want to avoid off axis issues.
              That's a really great example of the downfalls of a MTM on its side. Funny how the cancellation patterns are scattered between 300 and 2k where the tweeter crosses, that's just how those 2 woofers interact based on their spacing and the angle. With a cross at 300, the wavelengths are long enough to avoid the majority of that, but will still depend on the woofer spacing to determine how low the comb filtering starts.
              Electronics engineer, woofer enthusiast, and musician.
              Wogg Music
              Published projects: PPA100 Bass Guitar Amp, ISO El-Cheapo Sub, Indy 8 2.1 powered sub, MicroSat

              Comment


              • #22
                This atmos diagram does not show a loveseat up against a sidewall. Any HT setup will have a limited sweet spot for listening. An MTM will suffice for most CC applications, IMHO. If you sit way off to the side you're compromising more than sound. The Audioholic article excoriating MTM CCs also marveled at the clear sound of the sideways MTM.

                It's nice to know such things, but listening is important too.

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                • #23
                  Originally posted by djg View Post
                  This atmos diagram does not show a loveseat up against a sidewall. Any HT setup will have a limited sweet spot for listening. An MTM will suffice for most CC applications, IMHO. If you sit way off to the side you're compromising more than sound. The Audioholic article excoriating MTM CCs also marveled at the clear sound of the sideways MTM.

                  It's nice to know such things, but listening is important too.
                  In a setup like that, I'd just go with a vertical TM, or TMW

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                  • #24
                    Yes, in post #15 I showed my current system along with a comment about wishing for a better setup. I just pulled that diagram in post #22 off an image search, my apparently poorly expressed point was that there is a preferred seating area for a home theater, and it doesn't include along the sidewalls. Again, IMHO, an MTM is just fine for most setups.

                    In reality, many people have a limited vertical space for a CC, how many threads here from people saying they have so many inches space for a CC?

                    I also linked an old thread for the Blackbird, a sideways TM to match a pair of SR71s. The OP said meh.

                    Before I built my S2000 MTM CC, I used another upright TM of very good quality and lineage, the S2000 sounds better. My ears, my system, my room.

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                    • #25
                      Originally posted by djg View Post
                      Yes, in post #15 I showed my current system along with a comment about wishing for a better setup. I just pulled that diagram in post #22 off an image search, my apparently poorly expressed point was that there is a preferred seating area for a home theater, and it doesn't include along the sidewalls. Again, IMHO, an MTM is just fine for most setups.

                      In reality, many people have a limited vertical space for a CC, how many threads here from people saying they have so many inches space for a CC?

                      I also linked an old thread for the Blackbird, a sideways TM to match a pair of SR71s. The OP said meh.

                      Before I built my S2000 MTM CC, I used another upright TM of very good quality and lineage, the S2000 sounds better. My ears, my system, my room.
                      I was mostly commenting on the picture. I have a projection setup, and lots of room below the screen. With a larger screen, that space could get rather short. I realize some posters ask for a design that fits in a small space. I've built several centers, and it's sometimes hit or miss on how good they sound. There's always a trade-off or two involved. I've had better luck with simple 2-ways. It could be the driver choices, or possibly just a voicing difference. My most recent center has a 5.25" woofer. It's basically a full BSC 2-way, that is very flat on axis. It's probably measures pretty well at 45 degrees too. I messed up the other box that matches it, so it became ideal for a center. Maybe tonight I will try it with an action movie. It may self destruct, as I run it full range so far. It does sound very good with vocals, and that's the highest priority for me. I normally don't use a center, but eventually I will I think. That argument was covered in a recent thread. I think that a real good reason to use a center, is if someone might be sitting off to the side. A friend of mine has a system in his living room, that his wife has filled with more furniture than I would. One seat is about 45 degrees off axis. Another about 30 degrees. He could use a better center, but no-one cares. His is big, and cost a lot. He was at my house one day while I was sweeping some speakers. He can't hear anything above about 4k. He didn't realize it though. His wife probably can, but she likely gets first choice on seating. When I hear his system, what strikes me most is the coloration, or tonal balance being off a bit. That would not be acceptable to me.

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                      • #26
                        The point I was trying to make was that an mtm center is just fine on axis, but it definitely has problems off axis. There are three obvious solutions to the off axis issues - use a three way crossed below 300hz with a vertical tm section between the woofers, use an mtm or tm vertically or use an mt coax.

                        I understand that space constraints often dictate a horizontal mtm center. In fact, that's what I currently use. But having built an mtm center, I can see its shortcomings. It's still vastly better than no center and relying on a stereo phantom center, but there is a lot of room for improvement.
                        -Bob

                        The PEDS 2.1 mini system
                        My A7 Project - another small desktop speaker
                        The B3 Hybrid Dipole - thread incomplete and outdated

                        Comment


                        • #27
                          My first HT system was the D'Appolito designed Audax system, it had a big WMTW CC. Since then I have had MTM CCs. I sit more or less on axis so I'm fine. I have an off to the side loveseat, but people don't visit here to watch movies on my massive system.

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                          • #28
                            I had a d'appolito setup that I loved years ago, all 3 same speaker across the front.

                            The center channel was not laying side to side (like many did 80's and 90's, good luck with 2" thick TV's of today), it was up and down.

                            But I was a solid 10' minimum distance.

                            Ideally the tweet is right up against the bottom of the tv, more of a factor the closer you sit to it.

                            So, yea.



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                            • #29
                              I don't recall anyone ever building one, but a 2.5 way MTM center channel sims better off axis than a 2 way mtm .

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                              • #30
                                Not specifically for CC use, but here's a comparison including a 2.5 design.

                                RS180-RS28 MTM

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