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  • MTM driver spacing using XDIR

    I'm using XDir from Tolvan to see what kind of issues I may have with an MTM using a larger ribbon and two smaller midrange drivers. Should I use the center to center distance between one of the mids and tweeter, or should I use the CTC for the two mids?

    The plan was to use DSP to crossover around 1500Hz, and center to center for from mid to tweeter is 158mm, and the distance from mid to mid is 315mm. Obviously, the difference in simulated results is pretty massive.

    You guys got any ideas here?

  • #2
    Can't say I'd trust that calculator, since it doesn't consider the size, and therefore the polar pattern, of the midbasses. The original midbass CTC postulated by Joe D'Appolito was 1 wavelength at the crossover frequency, but he stopped doing that long ago, because of another factor, wavefront integration. Depending on the polar patterns of the midbasses at some point the two wave fronts will become one, as shown by this applet:
    http://www.acoustics.salford.ac.uk/f.../diffract4.php
    This means the further the listening distance the wider the CTC can be, So unless you're using the speakers as near field monitors you can get away with a lot more than 1 wavelength CTC on the midbasses. But if you have doubts put the midbasses adjacent and the ribbon to one side.
    www.billfitzmaurice.com
    www.billfitzmaurice.info/forum

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    • #3
      Bill raises some valid points about that calculator, although it can still give you some idea of what's going on. For MTM I use the mid-to-tweeter CTC spacing as input.
      Francis

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      • #4
        I'm designing these to be used in nearfield on the desktop. Using the wide polar of the ribbon to help with HF roll off that happens with wide placement scenarios(multiple displays).

        The mids should act as a single larger transducer when the frequency is low enough, in the transition to that, the polar should narrow vertically, similar to the large ribbon. Running 4 inch mids at 1500Hz they are below the directivity control of the cone, and should be close to flat way out off axis(as long as there are no diffraction problems). If everything works, crossing steep on the mids, and as slow as possible on the tweeter(hopefully without damage) the drivers should integrate well with wide horizontal and narrow vertical directivity that widens and becomes omni around 250-300Hz. Well that is the dream.

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        • #5
          Originally posted by billfitzmaurice View Post
          Can't say I'd trust that calculator, since it doesn't consider the size, and therefore the polar pattern, of the midbasses. The original midbass CTC postulated by Joe D'Appolito was 1 wavelength at the crossover frequency, but he stopped doing that long ago, because of another factor, wavefront integration. Depending on the polar patterns of the midbasses at some point the two wave fronts will become one, as shown by this applet:
          http://www.acoustics.salford.ac.uk/f.../diffract4.php
          This means the further the listening distance the wider the CTC can be, So unless you're using the speakers as near field monitors you can get away with a lot more than 1 wavelength CTC on the midbasses. But if you have doubts put the midbasses adjacent and the ribbon to one side.
          Really good answer.
          craigk

          " Voicing is often the term used for band aids to cover for initial design/planning errors " - Pallas

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          • #6
            Is there a simulation software that would be a better tool to use?

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            • #7
              Originally posted by noaudiophile View Post
              I'm designing these to be used in nearfield on the desktop.
              I'd reconsider that. You don't see MTMs used as nearfield monitors, the lack of the necessary distance for the midbass wave fronts to integrate is the reason why.

              www.billfitzmaurice.com
              www.billfitzmaurice.info/forum

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              • #8
                Originally posted by billfitzmaurice View Post
                The lack of the necessary distance for the midbass wave fronts to integrate is the reason why.
                What would this lack of wave front integration look like on a measurement?

                I have only heard a handful of well integrated 2-way tweeter and mid combos that deal acceptably with listening closely - I think this has to do with physical time alignment, but more to do with the inverse square law - when sitting 18 inches away from a multiway speaker you are likely to have your ears closer to one driver than the other, making it noticeably louder. The proportion of distance between the drivers in relation to the distance of your head to the drivers is not going to get better by ditching the MTM in favor for a 2 way design. At least in the mtm, as you shift vertically and get further from one mid, you should be getting closer to another at the same rate - hopefully balancing perceived tone.

                Even on the very best noncoaxial nearfield monitors there is usually only a very small vertical window where the sound will be balanced in the very nearfield - I was planning to optimize for that weakness, by making a speaker that is noticeably rolled off outside of a very narrow vertical window. This way the listener naturally knows where to place the speakers and his ears.

                I'm hoping that the speaker will function like a very narrow line source that gets taller as the wave size increases, avoiding combing filtering from the multiple drivers, but gaining the directivity of a vertically larger speaker.


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                • #9
                  Originally posted by noaudiophile View Post
                  What would this lack of wave front integration look like on a measurement?
                  Graphically it would be lobed response, similar to your first picture. Too close to the speaker and the outputs of the midbasses will be strong on each individual axis, with a dip in between the two. The two polar patterns will eventually resolve into a single pattern, but not at the three to six foot listening distance typical with near field monitors. As for 'in the mtm, as you shift vertically and get further from one mid, you should be getting closer to another at the same rate - hopefully balancing perceived tone' what you'll get is comb filtering on the vertical plane. You only avoid that problem with sufficient distance from the source, which can't be realized in near field listening. It's true that any non-coaxial near field monitor has its dispersion issues, but they're worse with an MTM.

                  www.billfitzmaurice.com
                  www.billfitzmaurice.info/forum

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                  • #10
                    Originally posted by noaudiophile View Post
                    315mm. ..
                    I would use this; and have used as a general guide to distance from sources to be beyond what is also called interference field.
                    In this case a little over 8' away as a minimum.

                    "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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                    • #11
                      Have you considered a good, small full range driver.
                      craigk

                      " Voicing is often the term used for band aids to cover for initial design/planning errors " - Pallas

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                      • #12
                        Originally posted by billfitzmaurice View Post
                        what you'll get is comb filtering on the vertical plane.
                        I've seen this before with other speakers which had two identical drivers on the same plane, but the center of the vertical axis was always clean and avoided any artifacts. Also, the combing artifacts were limited to above 1/2 wavelength spacing. I'll see about running a few experiments using two 4 inch drivers I have laying around. Should not be too difficult to set up.

                        Originally posted by craigk View Post
                        Have you considered a good, small full range driver.
                        I've very sensitive to cone breakup, and other treble problems. I'm looking to avoid those things in this build.

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                        • #13
                          Originally posted by noaudiophile View Post

                          I've seen this before with other speakers which had two identical drivers on the same plane, but the center of the vertical axis was always clean and avoided any artifacts. Also, the combing artifacts were limited to above 1/2 wavelength spacing. I'll see about running a few experiments using two 4 inch drivers I have laying around. Should not be too difficult to set up.



                          I've very sensitive to cone breakup, and other treble problems. I'm looking to avoid those things in this build.
                          Cone break up usually occurs at very high volume levels. you stated these were going to be used for nearfield monitoring. How loud do you listen nearfield ? How do you know you are sensitive to cone breakup ? I have been listening to speakers for a long time and have only seen cone breakup a few times, and that was on larger, junk woofers played at extreme levels. Not including loudness competitions.
                          craigk

                          " Voicing is often the term used for band aids to cover for initial design/planning errors " - Pallas

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                          • #14
                            If you can get the midbasses close enough together to fully couple within three feet or less from the baffle you're good as far as combing goes, but that means placing the tweeter to one side. Not that there's anything wrong with that, but then it has to be very tight to the midbasses to prevent combing on the horizontal plane, which is worse than on the vertical.
                            www.billfitzmaurice.com
                            www.billfitzmaurice.info/forum

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                            • #15
                              Originally posted by craigk View Post

                              Cone break up usually occurs at very high volume levels. you stated these were going to be used for nearfield monitoring. How loud do you listen nearfield ? How do you know you are sensitive to cone breakup ? I have been listening to speakers for a long time and have only seen cone breakup a few times, and that was on larger, junk woofers played at extreme levels. Not including loudness competitions.
                              I don't think we are talking about the same thing. I'm referring to the resonance peak in the response of any driver, dependant upon material choice and cone size. Above this point the speaker can not produce a linear sound as the sound waves tend to bounce around and cancel each other before leaving the cone.

                              While you could try and use a 2.5 inch full range driver made of aluminum - both small size and stiffer material push the break up higher in frequency - you would end up with ear bleeding breakup in the 10-15kHz range.

                              I'm looking for the opposite of that sound in this design.

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