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Testing baffle edge treatments for tweeter diffraction.

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  • Testing baffle edge treatments for tweeter diffraction.

    Since I am starting a new speaker, I wanted to try a couple of different baffle edge treatments to see their effect on tweeter diffraction.

    Tonight I finished the four blanks which are a double layer of 3/4" MDF 8"x15".

    Tonight I did the boring basic stuff, routed the rebate to recess the tweeter, did the through hole and notched for the lead ears.

    I am going to test the following,

    - control which will be left square edged

    - 7/8" roundover on three sides

    - progressively wider 45º chamfer starting a 0" width at the bottom and ending 1" wide going across the top

    - 30º chamfer that starts in the middle of where the woofer will be going to about 1.5" at the top and across

    Tweeter will be an inexpensive SB Acoustics SB26ST-C000-5

    More to follow....

  • #2

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    • #3
      In no particular order, and with much interest in this neglected subject -

      I've noted in my own builds, and seen it in other's measured responses, a tweeter that measures
      ruler flat on an AEC baffle turns out pretty bumpy on a 9 inch wide, 90 degree edge baffle. Yikes.

      "control is left edge squared"
      Hmm, shouldn't that be 3 edges squared? - left, top, right? What will be the change (from square) for the top and right?

      7/8 round over is almost a control in itself, given the prevalence of 3/4 inch roundovers on many builds.
      How about trying a 1 1/4 or 1 1/2 inch roundover?

      I don't think Javad ever published horizontal polars on his kerfed build, but older research in "Speaker Builder" showed
      about a 2 or 3 inch radius (IIRC) removed all diffraction ripples in tweeter response.

      Something I'm thinking about for a solid wood baffle is cutting the baffle sides into a16 sided polygon edge.
      That would give 4 elements of "smoothing" in the 90 degrees between the baffle and side.
      https://en.wikipedia.org/wiki/Hexadecagon

      Looking forward to your results!


      I think I hear a difference - wow, it's amazing!" Ethan Winer: audio myths
      "As God is my witness I'll never be without a good pair of speakers!" Scarlett O'Hara

      High value, high quality RS150/TB28-537SH bookshelf - TARGAS NLA!
      SB13/Vifa BC25SC06 MTM DCR Galeons-SB13-MTM
      My Voxel min sub Yet-another-Voxel-build

      Tangband W6-sub

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      • #4
        Cool. I have to admit in my short life of building speakers I have not yet really tackled edge diffraction in my builds. When I built my continuums, I used felt around the tweeter as used by the BBC in the LS3/5a. I am not sure how effective that is. I had also thought about a texture (or absorbent) on the surface. Obviously, what you are doing is the low hanging fruit and likely what most people will do.

        Can’t wait for your results.

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        • #5
          Don, I have considered this also,. but on a 16 sided polygon, what do you mount the drivers on?
          You can purchase router bits for making polygons. There are also "canoe" bits that make rounded edges, convex and concave, so you can curve slats with them, but the mounting remains an issue in my mind.

          Steve

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          • #6
            Sorry for the misunderstanding. I meant to have a flat baffle as usual, but use a quadrant of a sixteen sided polygon at the left and right vertical edges.
            That way each 90 degree edge would have 4 segments for the transition. I think you might be able to do this with either a router or table saw.
            Click image for larger version

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            Attached Files

            I think I hear a difference - wow, it's amazing!" Ethan Winer: audio myths
            "As God is my witness I'll never be without a good pair of speakers!" Scarlett O'Hara

            High value, high quality RS150/TB28-537SH bookshelf - TARGAS NLA!
            SB13/Vifa BC25SC06 MTM DCR Galeons-SB13-MTM
            My Voxel min sub Yet-another-Voxel-build

            Tangband W6-sub

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            • #7
              You mean like this?
              "She don't love my speakers anymore..."

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              • #8
                The woofer will create peaks and dips too! Maybe more than the edges.

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                • #9
                  I like your empirical test scheme. The comparative results will be interesting. If you can, in addition to on-axis measurements, make some at 15/30/45 degrees off-axis horizontally as well. The baffle edges have as dramatic effect off-axis as they do on-axis. It is time consuming, but well worth it for more complete analysis of the impact of diffraction treatments. Exporting them for overlay makes the differences much easier to compare. If you don't have software for the overlays, you can use my program that is available at my web site.

                  dlr
                  WinPCD - Windows .NET Passive Crossover Designer

                  Dave's Speaker Pages

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                  • #10
                    Originally posted by rpb View Post
                    The woofer will create peaks and dips too! Maybe more than the edges.
                    Agreed, but his test scheme eliminates any non-baffle edge influences, puts them into isolation, highly desirable for the test purposes.

                    dlr
                    WinPCD - Windows .NET Passive Crossover Designer

                    Dave's Speaker Pages

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                    • #11
                      I cut the edge profiles on the three test baffles tonight.

                      - 7/8" roundover on three sides

                      - Progressive 45° chamfer from 0" at the bottom to 1" at the top and across

                      - 30° bevel starting at 0" from mid where the woofer will be to about 2" at the top and across.

                      The roundover was the easy one.

                      For the 45° chamfer, I just made a simple jig to hold the end of the board 7/8" up at the bottom of the baffle.

                      The 30° bevel jig was a little more work, but I have made enough furniture leg tapering jigs before and for this, just made a short length/tall version. It didn't quite land exactly where I wanted it to, but close enough for testing.

                      I hope to take measurements in the next few days.

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