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  • #1

    Coverage Pattern of an 8" MTM

    I am thinking about building a "proper" (slightly more ideal than a 15" 2-way) mids and highs box. I am wondering how to determine what the coverage pattern of 2 8-inch divers will be at a given crossover point with a center-to-center distance of about 12-16 inches, depending on the size of the waveguide that goes between them. My assumption would be that the horizontal coverage (with the drivers oriented vertically) would be the same as if there were a single driver, while the vertical coverage would be narrower. The end goal is to find a lens for the compression drivers I already have ( D250T-8) such that the coverage pattern of the mid drivers matches the tweeter at the crossover point.


    Thanks!
    Last edited by PunkSweeper; 05-06-2015, 10:48 PM. Reason: grammr
    18hz is scary.
    Tags: None
  • #2
    Re: Coverage Pattern of an 8" MTM

    As a comparison:
    My in-wall MTM use (2) 215mm ( or 8.5" ) frames.
    CTC spacing is 13.5" ( 6.75" to center of H.F. ).
    4th order active @1.8Khz
    "Not a Speaker Designer - Not even on the Internet"
    “Pride is your greatest enemy, humility is your greatest friend.”
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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    • #3
      Re: Coverage Pattern of an 8" MTM

      I built a pair of 8" TMMs and used these horns: http://www.parts-express.com/dayton-...-bolt--270-304

      The horizontal polar matched up quite nicely around 2500 Hz for me.

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      • #4
        Re: Coverage Pattern of an 8" MTM

        Are D'Appolito 3/2 arrangements used in pro audio? A ctc driver spacing of one wavelength at the crossover frequency should offer hemi-circular dispersion.

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        • #5
          Re: Coverage Pattern of an 8" MTM

          Would it be fair to speculate that since my CTC will be larger with the MTM design, the vertical coverage would be narrower? Around 40 degrees perhaps?
          18hz is scary.

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          • #6
            Re: Coverage Pattern of an 8" MTM

            As MTM are not constant directivity; it would vary with frequency.
            "Not a Speaker Designer - Not even on the Internet"
            “Pride is your greatest enemy, humility is your greatest friend.”
            "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

            Comment

            • #7
              Re: Coverage Pattern of an 8" MTM

              I only need to know the directivity at a given crossover frequency, so I can choose a horn.
              18hz is scary.

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              • #8
                Re: Coverage Pattern of an 8" MTM

                Best is to measure it, but it can be calculated. Trigonometry and 1/2 wavelength. There's also some software out there I'm sure. Once you add the third source (the tweeter) the directivity will widen actually.
                https://www.youtube.com/channel/UCm2...oSKdB448TTVEnQ

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                • #9
                  Re: Coverage Pattern of an 8" MTM

                  Cool! Great point about the tweeter as a source being part of the equation, I didn't think of that. Is there a formula you could point me towards that would give me vertical directivety given number of sources, wavelength, and ctc? Also would the overall horizontal coverage angle of two woofers be the same as just one?
                  18hz is scary.

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                  • #10
                    Re: Coverage Pattern of an 8" MTM

                    Originally posted by PunkSweeper View Post
                    ...Also would the overall horizontal coverage angle of two woofers be the same as just one?
                    Yes
                    "Not a Speaker Designer - Not even on the Internet"
                    “Pride is your greatest enemy, humility is your greatest friend.”
                    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

                    Comment

                    • #11
                      Re: Coverage Pattern of an 8" MTM

                      Nifty. That's what I thought, but I didn't wanna assume.
                      18hz is scary.

                      Comment

                      • #12
                        Re: Coverage Pattern of an 8" MTM

                        Originally posted by PunkSweeper View Post
                        ...Is there a formula you could point me towards that would give me vertical directivety given number of sources, wavelength, and ctc? ...
                        Formulas:
                        404 Not Found
                        https://engineering.purdue.edu/ece40020/LectureNotes/Chapter_2.pdf
                        "Not a Speaker Designer - Not even on the Internet"
                        “Pride is your greatest enemy, humility is your greatest friend.”
                        "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

                        Comment

                        • #13
                          Re: Coverage Pattern of an 8" MTM

                          Tolvan Data
                          http://www.tolvan.com/index.php?page=/xdir/xdir.php


                          Not completely accurate for horns though.
                          "He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you." Friedrich Nietzsche

                          http://www.diy-ny.com/

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                          • #14
                            Re: Coverage Pattern of an 8" MTM

                            Originally posted by 1100xxben View Post
                            I built a pair of 8" TMMs and used these horns: http://www.parts-express.com/dayton-...-bolt--270-304

                            The horizontal polar matched up quite nicely around 2500 Hz for me.
                            That jives with what I've done, with double 10" and one of the Dayton H6512 horns (i.e, Econowave). done TMM. The two 10" narrowed to something very close to 40 degrees vertical dispersion, at the ~1500-2000 Hz effective acoustic crossover point. It gave a pretty subjectively "even" power balance.

                            For more info on how this arrangement was done by some of the "old masters", look up some of Tomlinson Holman's original THX papers. In that, he describes using double-stacked JBL 15" (the ubiquitous JBL 4648 theater midbass cabinet) with a horn. The two 15", with a 500 Hz crossover point, gave a good 50-60 degree or so vertical dispersion- which matched the big JBL theater horn mid/high drivers of that time. He explains a lot of the math there, too...

                            Regards,
                            Gordon.
                            Speakerfixer.

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                            • #15
                              Re: Coverage Pattern of an 8" MTM

                              Originally posted by 1100xxben View Post
                              I built a pair of 8" TMMs and used these horns: http://www.parts-express.com/dayton-...-bolt--270-304

                              The horizontal polar matched up quite nicely around 2500 Hz for me.
                              I wonder if anyone's tried building a 2x10 using this horn in-between them? - http://www.parts-express.com/eminenc...8-tpi--290-549
                              Brian Steele
                              www.diysubwoofers.org

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