Symmetro: a proof-of-concept

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  • augerpro
    Super Senior Member
    • Aug 2006
    • 1866

    Symmetro: a proof-of-concept

    Thought I'd share a project Derek (deschu) and myself are working on. This started with my experience that despite the technical shortcomings of the MTM, I often really like their sound, better than an identical TM at times. I'm not the only one and I think the assumption is that there is reduced sound power - especially vertically - around the crossover, which is typically a region sensitive to the ears. I also know that I hate low ceilings for good sound. After the wall behind the speaker, the ceiling seems - to me - the next worst offender. So I started thinking about how I could control vertical dispersion, while maintaining - or even increasing - horizontal dispersion compared to a typical speaker. I'd already had an idea for the layout, either MMTMM or WMTMW, with the proper driver spacing vs crossover to constrain the vertical dispersion. But the mids must be tiny to get close enough so they're vertical dispersion is constrained, but not so much that it was very different from the rest of the speaker response. When I started working with the Tectonic Elements 46mm BMR in a soundbar project and an AMT in a dipole, the final concept clicked.

    So an appropriate tweeter with narrowed vertical dispersion and wide horizontal dispersion was needed. A small faceplate was also required. After a search of a very short list, the Dayton AMT2-4 was selected.

    The woofer had only to cover up to about 500hz, but I wanted very low distortion there, and not too sensitive to match the BMR's which must be run in series. This was not as easy as I'd hoped, many drivers have very low distortion performance in the bass, but not so much in the midrange, while other were great in the midrange while 3rd and 5th harmonic really increased in the bass. I thought a great compromise would be the Usher 8945A or 8948A, and I love the pulpy cone look of the 8945A. BUT just about the same time the Dayton Esoteric woofers came back so I threw both in the basket for testing. In the end the Dayton ES180TiA was a perfect fit, not too sensitive, fantastic distortion measurement through the whole design's passband, and being a Dayton will go on sale much more often then the Ushers. No it doesn't have the pulpy cone, but the carbon fiber is pretty dang cool too.

    So now that all that's out of the way: here is an *almost* final mockup drawing

    Click image for larger version

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    For the conceptual modeling you will notice that the vertical dispersion is quite constrained in the window where ceiling/floor bounce matter. Constrained and even. The mid/tweet crossover region- expected to be the worst - is constrained but also bigger than say a typical MTM. In addition above and below that is also constrained, so that what does get reflected will at least be a smoother response than say, a lot midrange/bass energy bouncing up or down, then suddenly a laser beam in the mid/treble area, and then back to treble spraying everywhere. Here is the polar response from 500hz-4500hz of a typical 2-way TM:
    Click image for larger version

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    And a typical 3-way TMW:
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    And Symmetro:
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    Now the aim here was not supposed to be the "best ever speaker". It's a proof of concept. A solution to a problem that has bugged me. But I like what I see so far. Will it do everything I hope when I get it in-room? We'll see
    ~Brandon 8O
    Please donate to my Waveguides for CNC and 3D Printing Project!!
    Please donate to my Monster Box Construction Methods Project!!
    DriverVault
    Soma Sonus
  • augerpro
    Super Senior Member
    • Aug 2006
    • 1866

    #2
    And actually when I did those plots, I had forgot to add realistic acoustic centers in the typical examples, so they would actually be a bit worse through the crossover (the smallest lobes) than depicted.
    ~Brandon 8O
    Please donate to my Waveguides for CNC and 3D Printing Project!!
    Please donate to my Monster Box Construction Methods Project!!
    DriverVault
    Soma Sonus

    Comment

    • JonMarsh
      Mad Max Moderator
      • Aug 2000
      • 15259

      #3
      You know me, I was just going to ask a bit about acoustic centers and specific transfer functions... but this looks like an interesting project, and like fun, too!

      So when do you start making sawdust? :W

      I'll be keeping an eye on this. I might wonder if it wouldn't benefit from a waveguide loaded tweeter with controlled vertical response, too, given the direction you want to go.


      My only simple remedy for this issue is a listening room with high ceiling... not always feasible to obtain!
      the AudioWorx
      Natalie P
      M8ta
      Modula Neo DCC
      Modula MT XE
      Modula Xtreme
      Isiris
      Wavecor Ardent

      SMJ
      Minerva Monitor
      Calliope
      Ardent D

      In Development...
      Isiris Mk II updates- in final test stage!
      Obi-Wan
      Saint-Saëns Symphonique/AKA SMJ-40
      Modula PWB
      Calliope CC Supreme
      Natalie P Ultra
      Natalie P Supreme
      Janus BP1 Sub


      Resistance is not futile, it is Volts divided by Amperes...
      Just ask Mr. Ohm....

      Comment

      • augerpro
        Super Senior Member
        • Aug 2006
        • 1866

        #4
        My partner is excited to start cutting, so soon! Kind of nice partnering with someone to do all the tedious stuff, I just do the XO work when the speaker gets shipped to me.

        The AMT does narrow vertically actually, even though it's a shorter AMT.
        ~Brandon 8O
        Please donate to my Waveguides for CNC and 3D Printing Project!!
        Please donate to my Monster Box Construction Methods Project!!
        DriverVault
        Soma Sonus

        Comment

        • Paul K.
          Senior Member
          • Jul 2008
          • 180

          #5
          I'm just the opposite as I like to do the cabinet design and all of the woodworking but farm out the crossover design!
          Paul

          Originally posted by augerpro
          My partner is excited to start cutting, so soon! Kind of nice partnering with someone to do all the tedious stuff, I just do the XO work when the speaker gets shipped to me.

          The AMT does narrow vertically actually, even though it's a shorter AMT.

          Comment

          • augerpro
            Super Senior Member
            • Aug 2006
            • 1866

            #6
            I'll take that as a future offer Paul haha!
            ~Brandon 8O
            Please donate to my Waveguides for CNC and 3D Printing Project!!
            Please donate to my Monster Box Construction Methods Project!!
            DriverVault
            Soma Sonus

            Comment

            • JonMarsh
              Mad Max Moderator
              • Aug 2000
              • 15259

              #7
              Originally posted by augerpro
              My partner is excited to start cutting, so soon! Kind of nice partnering with someone to do all the tedious stuff, I just do the XO work when the speaker gets shipped to me.

              The AMT does narrow vertically actually, even though it's a shorter AMT.
              I have one on the way to test - an open box special. Gotta quit chasing butterflies... :W
              the AudioWorx
              Natalie P
              M8ta
              Modula Neo DCC
              Modula MT XE
              Modula Xtreme
              Isiris
              Wavecor Ardent

              SMJ
              Minerva Monitor
              Calliope
              Ardent D

              In Development...
              Isiris Mk II updates- in final test stage!
              Obi-Wan
              Saint-Saëns Symphonique/AKA SMJ-40
              Modula PWB
              Calliope CC Supreme
              Natalie P Ultra
              Natalie P Supreme
              Janus BP1 Sub


              Resistance is not futile, it is Volts divided by Amperes...
              Just ask Mr. Ohm....

              Comment

              • augerpro
                Super Senior Member
                • Aug 2006
                • 1866

                #8
                I think for your standards the Aurum Cantus AMT's are the ones you want, best distortion performance I've seen in any AMT, except maybe the uber-spendy Mundorf or Beyma. These AMT2-4 should not be crossed very low.
                ~Brandon 8O
                Please donate to my Waveguides for CNC and 3D Printing Project!!
                Please donate to my Monster Box Construction Methods Project!!
                DriverVault
                Soma Sonus

                Comment

                • 5th element
                  Supreme Being Moderator
                  • Sep 2009
                  • 1671

                  #9
                  I've always been intrigued by speakers that attempt to address vertical directivity as you've done here. It will be interesting to see how these turn out. Floor bounce is always a problem with trade-offs whatever you do, but this looks like it could definitely help with that.

                  I too much prefer to do the design work, rather than the building!
                  What you screamin' for, every five minutes there's a bomb or something. I'm leavin' Bzzzzzzz!
                  5th Element, otherwise known as Matt.
                  Now with website. www.5een.co.uk Still under construction.

                  Comment

                  • augerpro
                    Super Senior Member
                    • Aug 2006
                    • 1866

                    #10
                    Yeah I'm hoping the result is meaningful. Really any symmetric WMTMW type of layout will have reduced vertical dispersion at some frequencies, just not many plan for and design around that fact. This was attempt to optimize that dispersion and make it more uniform through the whole speaker passband (or near as possible).
                    ~Brandon 8O
                    Please donate to my Waveguides for CNC and 3D Printing Project!!
                    Please donate to my Monster Box Construction Methods Project!!
                    DriverVault
                    Soma Sonus

                    Comment

                    • ergo
                      Senior Member
                      • Mar 2005
                      • 675

                      #11
                      This Symmetro will be an interesting project.

                      Finnish Amphion has also just released a product along these lines. Not a 3way, but has two interesting aspects - D'Applolito with waveguided tweeter and passive radiators. I'd really like to get to listen to these... Might happen as the ferry goes over to Finland often enough.

                      Click image for larger version

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                      Comment

                      • augerpro
                        Super Senior Member
                        • Aug 2006
                        • 1866

                        #12
                        Could use some feedback on port placement. I've never much cared for the idea of a port being far from the woofer. So I specced a placement of 61.8% of the internal height from the top, a good golden ratio for suppressing resonances. But after the builder sent me the CAD the port is REALLY close to the lower woofer and this has me concerned about frequency response ripple due to that. Now the woofer will only run up to around 500hz @ LR4, so maybe it won't show up much? Thoughts?

                        Click image for larger version

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                        ~Brandon 8O
                        Please donate to my Waveguides for CNC and 3D Printing Project!!
                        Please donate to my Monster Box Construction Methods Project!!
                        DriverVault
                        Soma Sonus

                        Comment

                        • wkhanna
                          Grumpy Old Super Moderator Emeritus
                          • Jan 2006
                          • 5673

                          #13
                          you are placing on the front baffle?
                          _


                          Bill

                          Practicing Curmudgeon & Audio Snob
                          ....just an "ON" switch, Please!

                          FinleyAudio

                          Comment

                          • augerpro
                            Super Senior Member
                            • Aug 2006
                            • 1866

                            #14
                            Yeah
                            ~Brandon 8O
                            Please donate to my Waveguides for CNC and 3D Printing Project!!
                            Please donate to my Monster Box Construction Methods Project!!
                            DriverVault
                            Soma Sonus

                            Comment

                            • wkhanna
                              Grumpy Old Super Moderator Emeritus
                              • Jan 2006
                              • 5673

                              #15
                              your skills in speaker design are far beyond my understanding.

                              i do not know what is meant by "frequency response ripple"

                              i have no problem with the front placement, form must follow function for the best possible performance.

                              so rear or bottom placement is not optional?
                              _


                              Bill

                              Practicing Curmudgeon & Audio Snob
                              ....just an "ON" switch, Please!

                              FinleyAudio

                              Comment

                              • augerpro
                                Super Senior Member
                                • Aug 2006
                                • 1866

                                #16
                                Well that large cavity so close to a driver typically causes reflections that will show up in the driver's frequency response. But I'm thinking the driver may operate at low enough frequency where it won't "illuminate" the port because the wavelengths are too large. But I'm not sure, I've never had a large port this close to a driver before.
                                ~Brandon 8O
                                Please donate to my Waveguides for CNC and 3D Printing Project!!
                                Please donate to my Monster Box Construction Methods Project!!
                                DriverVault
                                Soma Sonus

                                Comment

                                • wkhanna
                                  Grumpy Old Super Moderator Emeritus
                                  • Jan 2006
                                  • 5673

                                  #17
                                  ok
                                  now i think i understand what you are concerned about.
                                  it is reflection inside the cabinet from the port tube so close to the back-wave of the woofer.
                                  hence, rear placement would have little to no effect.
                                  _


                                  Bill

                                  Practicing Curmudgeon & Audio Snob
                                  ....just an "ON" switch, Please!

                                  FinleyAudio

                                  Comment

                                  • augerpro
                                    Super Senior Member
                                    • Aug 2006
                                    • 1866

                                    #18
                                    No not inside the cabinet. From the front of the woofer. For example a tweeter will often show a ripple if it is very close to a woofer with a deep cone. In this case it is quite a large hole with a large flare (Powerport).
                                    ~Brandon 8O
                                    Please donate to my Waveguides for CNC and 3D Printing Project!!
                                    Please donate to my Monster Box Construction Methods Project!!
                                    DriverVault
                                    Soma Sonus

                                    Comment

                                    • wkhanna
                                      Grumpy Old Super Moderator Emeritus
                                      • Jan 2006
                                      • 5673

                                      #19
                                      showing my ignorance for the technical details on speaker design, again......ops:

                                      thank you for the explanation!
                                      _


                                      Bill

                                      Practicing Curmudgeon & Audio Snob
                                      ....just an "ON" switch, Please!

                                      FinleyAudio

                                      Comment

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