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Need Help with Scott Sehlin's Lithium Build

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  • Need Help with Scott Sehlin's Lithium Build

    (First: I'm new to speaker building/design)

    I'm in the midst of building Scott's Lithium Speakers.

    It's a small speaker and the way I'm currently building the cabinets, I'll only get one try to build the crossover before its permanently glued/fixed in the cabinet.

    So I built the cabinets, double taped the baffle to the enclosure, fished the speaker wires through the port so I can attempt some measurements and external crossover build/test.

    Generally, my response is similar to Scott's measurements (included below) from say 300hz at up.

    Scott's:

    Click image for larger version  Name:	Scott's Measurements.JPG Views:	2 Size:	87.8 KB ID:	1400224

    However, I have a MASSIVE bump in my response from 300hz down - and I can't for the life of me figured out why.
    Click image for larger version

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    Assuming I had messed up in my measurements, I've tried doing the following:
    1) Tooks ND105-4 measurements from Parts Express Website
    - The measurements show a rising low-end response
    2) I modeled low-end response using Jeff Bagby's Frequency Response Modeler
    - The Response Modeler indicated a 1db bump between 100hz to 200hz (due to a small cabinet)
    3) I merged 1 and 2 above and adjusted for diffraction (again using the spreadsheet above).
    4) I then simulated Scott's crossover with the above measurements and - surprisingly - the simulation exactly matched my measured response.

    My next step was to see what the transfer function looks like - I've included that as well.

    Click image for larger version  Name:	Transfer Function.JPG Views:	1 Size:	128.6 KB ID:	1400222
    Click image for larger version

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    At first, I thought maybe I have a defective woofer - but I compared my measurements with the modeled response using the manufacturer's files and Mr. Bagby's spreadsheet - and the response in reasonably close.

    The comparison included below (blue = mine, red = manu) :
    Click image for larger version  Name:	My Measurements vs Manufracturer's (Diffraction Adjusted).JPG Views:	1 Size:	154.4 KB ID:	1400221

    I know I must be doing something very simple wrong. Any help would be much appreciated.

  • #2
    Happy to upload my measurements files if anyone needs them.

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    • #3
      Is your measurement "gated" (doesn't seem like it)?
      Are you measuring "near field", or from at least 20" - 40" away.
      If you measure "far field", with these on stands, away from walls, the rise should go away (if it's from BSC).
      Your XO is most likely OK.

      Comment


      • #4
        Chris, Measurements were from Far field - gated. I then merged near field response and adjusted for baffle step. When playing music - I can hear the bump (maybe because it’s in my head now from looking at the measurements - but also because it’s 6 dB bump). Per the transfer function - it appears there is nearly 12db attenuation between 150hz and 1,100Hz. Assuming the driver is nearly flat (based on mine and manufacturer’s measurement) between this range - does this amount of attenuation seem reasonable? Sent from my iPhone using Tapatalk

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        • #5
          Did you (box) model YOUR woofers' (actual T/S parms) in the Lith cab? You could probably make the vent longer to reduce a hump.
          You need a "few" dB just to flatten the "rise" (from 600 / 1200 or so), then 6dB more (if full BSC), then another -6down at the 2.5k Fc.
          So... doesn't really seem TOO out of line. Like you say, your ears are now possibly over-sensitive to the sim your eyes have seen !

          I'm not real great at doing the 300Hz near-far "merge" myself (when a vent is involved), but I was thinking... plug the port and measure the woofer near, then again w/the port open and compare the box model of closed and vented (with your actual box volume) to help guide you in your port output "splice" at 300 (or whatnot).

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