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I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

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  • I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

    Being the fan of exotic mid-engine ineligibility as I am, and the exotic nature of the types of drivers this future build uses, I decided to choose a name from the Italian arena. It's an old name from DeTomaso, the precursor to the Pantera, popular in the late 60's to early 70's. You see, these cars used commonplace engines from the American market, and were relatively easy to get parts for. The materials made them exotic. The Mangusta had a split wing-door arrangement covering the engine bay with very large windows, allowing an 'open view rear-end'.




    Since PE is stocking the units (I think they stock the Dipolar unit, as I got a pair as a door prize at MWAF 2013. If not, another forum poster purchased a set direct from Beston for about $90 shipped, IIRC. I got the CF120-4 as a door prize at MWAF 2015.), that makes them easily accessible. Being the CF120-4 has a large motor, and the Beston RT002A-Dipolar is an open-back tweeter, the namesake fits.

    I also want to point out a rendering of an updated Mangusta, called the Legacy by design student Maxime de Keiser. To me- this is absolutely stunning!




    The tentative blueprint looks like it just might work.... CF120-4 will overlap the Beston's faceplate for close CTC, and I will not be flushing the difficult pincushion frame.



    Since I want to retain the open-back nature of the Beston, I used the DBS from Jeff's Excel package to emulate what it will be doing, and used the manufacturer FR plot at the onset and adapted it for use. The CF120-4 looks great to 52Hz in 5 ltrs, with 2x 1.0" diameter ports, 6.1" long.







    NOTE: I am not using the stock faceplate that comes with the Beston, as it has issues. There is/are resonance(s) that results from the way these are constructed since it is not glued and sealed to the steel plate. There is just no easy way to accomplish this seal myself. To top it off, there are 2 through-holes that vent from front to rear of the magnet structure into the cavities behind the stock faceplate when attached. If you don't seal these holes (even on the non-dipolar units), the faceplate *WILL* buzz loudly with any kind of woofer cabinet pressure. I recommend sealed cavities for this or the sealed Beston is you are not going dipole. Since I'm going open- the holes are less likely to leak and cause issues, but the faceplate is still not going to be used. The outer most 4 screws come out to reveal this result. Besides- that black steel plate looks awesome!!!



    Stock unit with faceplate:


    Same unit without faceplate:


    So, on to the nuts and bolts......

    The cabinet is going to be an equal sided hexagon, and volume was easy to find using the 30/60/90 geometric triangle laws as seen on the blueprint above. I hope to use the opposed lateral edges for triangular ports. That should be pretty simple. Biscuits for assembly will be a must.

    Here is the initial sim. Mind you this is very preliminary, as dipolar stuff is all new to me in application. I made some assumptions, and modeled with both 6dB and 12dB additional slopes to extend the tails from 1k on the manufacturer's website. I do know that summation will be strongly affected by the room, and adjustment may be required:



    I had to go third order to eliminate the dipolar peak on the rolloff of the tweeter, as well as tilt down the strongly rising response. This involved a split 10 ohm resistance by a pair of 5 ohms, as well as a shunt cap to reduce the topend. 6 parts:



    The woofer wasn't really that hard to use at all. 3rd order electrical with damping resistor made easy work of the arrangement:



    Sensitivity comes out to about 83dB, and an easy LR4 acoustic at 2.8k. Kinda funny how both of these units have bit of a plateau 'amidships' in rolloff, and that they align well. Reverse null tickles ~-40dB from reference for those that want to know. The next picture is that of a sim at 30* off axis overlaid with the on-axis plot:



    No- it's not super flat, but it is tonally flat, and smoothing would just reduce resolution of the picture overall.

    I'll be updating this as I go...
    Wolf
    "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
    "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
    "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
    "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

    *InDIYana event website*

    Photobucket pages:
    https://app.photobucket.com/u/wolf_teeth_speaker

    My blog/writeups/thoughts here at PE:
    http://techtalk.parts-express.com/blog.php?u=4102

  • #2
    Re: I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

    That is probably not a good model for the dipole diffraction since the tweeter is a line source and not a round half inch radiator. You might try a line source dipole with this.

    http://www.tolvan.com/index.php?page=/edge/edge.php

    Comment


    • #3
      Re: I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

      And yet- it likely does in the horizontal dispersion. Vertical- no, I agree.

      Later,
      Wolf
      "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
      "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
      "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
      "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

      *InDIYana event website*

      Photobucket pages:
      https://app.photobucket.com/u/wolf_teeth_speaker

      My blog/writeups/thoughts here at PE:
      http://techtalk.parts-express.com/blog.php?u=4102

      Comment


      • #4
        Re: I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

        Run both geometries in Edge. Dispersion is not the same thing as dipole cancelation. Also, the tweeter will see the loading from the woofer baffle which is not a dipole baffle. Your actual baffle that is dipole is much smaller than 5.5 by 13. This might be a good time to build a rough mock up and measure the tweeter response.

        Comment


        • #5
          Re: I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

          Originally posted by fdieck View Post
          Run both geometries in Edge. Dispersion is not the same thing as dipole cancelation. Also, the tweeter will see the loading from the woofer baffle which is not a dipole baffle. Your actual baffle that is dipole is much smaller than 5.5 by 13. This might be a good time to build a rough mock up and measure the tweeter response.
          You're right about the driver shape changing the diffraction, but it will actually make his diffraction smoother since the vertical length will cause less illumination of the edge than a point source does. Otherwise, his model should be OK for his purposes. His baffle size for the simulation is correct, as best I can tell. I'm not sure I follow why you think it would be smaller than the 5.5" by 13"
          Click here for Jeff Bagby's Loudspeaker Design Software

          Comment


          • #6
            Re: I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

            Originally posted by Jeff B. View Post
            You're right about the driver shape changing the diffraction, but it will actually make his diffraction smoother since the vertical length will cause less illumination of the edge than a point source does. Otherwise, his model should be OK for his purposes. His baffle size for the simulation is correct, as best I can tell. I'm not sure I follow why you think it would be smaller than the 5.5" by 13"
            Thanks, Jeff! I'm also not following why Fred thinks it's more of a problem than I/you think it is.
            Thanks for the advice earlier, BTW...

            Later,
            Wolf
            "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
            "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
            "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
            "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

            *InDIYana event website*

            Photobucket pages:
            https://app.photobucket.com/u/wolf_teeth_speaker

            My blog/writeups/thoughts here at PE:
            http://techtalk.parts-express.com/blog.php?u=4102

            Comment


            • #7
              Re: I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

              Originally posted by Jeff B. View Post
              You're right about the driver shape changing the diffraction, but it will actually make his diffraction smoother since the vertical length will cause less illumination of the edge than a point source does. Otherwise, his model should be OK for his purposes. His baffle size for the simulation is correct, as best I can tell. I'm not sure I follow why you think it would be smaller than the 5.5" by 13"

              Originally posted by Jeff B. View Post
              You're right about the driver shape changing the diffraction, but it will actually make his diffraction smoother since the vertical length will cause less illumination of the edge than a point source does. Otherwise, his model should be OK for his purposes. His baffle size for the simulation is correct, as best I can tell. I'm not sure I follow why you think it would be smaller than the 5.5" by 13"
              The drawing shows the part of the baffle that might be considered a classic dipole baffle is 5 by 5.5. The depth of the rest of the enclosure does not make it look like a symmetrical dipole. Yes the diffraction should probably be smoother but you also have a significant reflection from the top horizontal surface of the enclosure for the rear radiation from the dipole. John K and Sigfried Linkwitz have covered this topic much better than I can in discussing the NaO Note and LX 521 designs.

              I also wonder if you are using the FR measurements provided by the manufacturer which one would assume were with the waveguide. It would seem to me that removing the wave guide will roll off the bottom even more and that Beston is recommending a 2.8 KHz minimum with a 3rd order crossover.

              I am not pointing out "problems", just the things you might consider to get a more accurate model for your simulation. A reality check or how well
              your raw driver in an actual enclosure is described by your model would seem to save a lot of work down the road.

              Comment


              • #8
                Re: I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

                Originally posted by fdieck View Post
                The drawing shows the part of the baffle that might be considered a classic dipole baffle is 5 by 5.5. The depth of the rest of the enclosure does not make it look like a symmetrical dipole. Yes the diffraction should probably be smoother but you also have a significant reflection from the top horizontal surface of the enclosure for the rear radiation from the dipole. John K and Sigfried Linkwitz have covered this topic much better than I can in discussing the NaO Note and LX 521 designs.
                Unless I apply damping material to the top of the box- which I have thought about.

                Later,
                Wolf
                "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                *InDIYana event website*

                Photobucket pages:
                https://app.photobucket.com/u/wolf_teeth_speaker

                My blog/writeups/thoughts here at PE:
                http://techtalk.parts-express.com/blog.php?u=4102

                Comment


                • #9
                  Re: I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

                  Originally posted by fdieck View Post
                  The drawing shows the part of the baffle that might be considered a classic dipole baffle is 5 by 5.5. The depth of the rest of the enclosure does not make it look like a symmetrical dipole. Yes the diffraction should probably be smoother but you also have a significant reflection from the top horizontal surface of the enclosure for the rear radiation from the dipole. John K and Sigfried Linkwitz have covered this topic much better than I can in discussing the NaO Note and LX 521 designs.
                  There will reflection from the top of the box, but when I designed the Salk Archos it didn't create any issues. You're incorrect about the baffle size though, the planar will see the entire baffle as a boundary between the front and rear wavefronts.
                  Click here for Jeff Bagby's Loudspeaker Design Software

                  Comment


                  • #10
                    Re: I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

                    Originally posted by Jeff B. View Post
                    There will reflection from the top of the box, but when I designed the Salk Archos it didn't create any issues. You're incorrect about the baffle size though, the planar will see the entire baffle as a boundary between the front and rear wavefronts.
                    I didn't mean to imply it wouldn't. It does not see the whole baffle as the same dipole loading/cancelation as top five inches. How much difference will show up in the measurement I don't know. The front and rear radiation sure don't see the same boundary though. It would take less time to measure it than discuss it for several days I would wager. Got to go feed the dog since she is whining at me.

                    Comment


                    • #11
                      Re: I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

                      Originally posted by fdieck View Post
                      I didn't mean to imply it wouldn't. It does not see the whole baffle as the same dipole loading/cancelation as top five inches. How much difference will show up in the measurement I don't know. The front and rear radiation sure don't see the same boundary though. It would take less time to measure it than discuss it for several days I would wager. Got to go feed the dog since she is whining at me.
                      Front and rear radiation is rarely symmetrical, unless you have something like a Magnepan, but I think you are letting the fact that the tweeter is mounted to a smaller baffle of its own fool you regarding the boundary between the two poles. If the cabinet for the woofer was not there and whole baffle was a single panel this size would you perceive the boundary as different? You can reply when you are finished feeding the dog.
                      Click here for Jeff Bagby's Loudspeaker Design Software

                      Comment


                      • #12
                        Re: I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

                        your Mangusta pic made me think of this car I saw down at a museum in Florida, kinda along the same lines...



                        ...and also Italian ta boot!
                        Looks like a neat design you have going.
                        A mains
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                        • #13
                          Re: I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

                          Naming your speakers after sports cars? Now you're talking my language.
                          Isn't it about time we started answering rhetorical questions?

                          Paul Carmody's DIY Audio Projects
                          Twitter: @undefinition1

                          Comment


                          • #14
                            Re: I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

                            Originally posted by Paul Carmody View Post
                            Naming your speakers after sports cars? Now you're talking my language.
                            Like Testarossas?
                            Click here for Jeff Bagby's Loudspeaker Design Software

                            Comment


                            • #15
                              Re: I theorize: "Mangusta", CF120-4/RT002A-Dipolar....

                              Originally posted by Paul Carmody View Post
                              Naming your speakers after sports cars? Now you're talking my language.
                              Until this one- I never found the right combination of name and style. Vector was always on my list as well.....
                              Later,
                              Wolf
                              "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                              "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                              "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                              "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                              *InDIYana event website*

                              Photobucket pages:
                              https://app.photobucket.com/u/wolf_teeth_speaker

                              My blog/writeups/thoughts here at PE:
                              http://techtalk.parts-express.com/blog.php?u=4102

                              Comment

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