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Anyone have the tools or inclination to model an TL for me?

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  • Anyone have the tools or inclination to model an TL for me?

    I'm sitting on 4 of the SB15NRXC30, and I'm thinking of doing a TMM or MTM with them. I've got a pair of the double magnet XT25 in boxes, some of the little SB26SCTNs, or I might pick up some Neo3s before THOSE disappear again. So I've got fun options.

    I like the looks of 36L ported, for an F3 around 40hz. I know the power handling wouldn't be great with a significantly larger box of an MLTL or quarter wave (the ported maxes out at 50W), but the sensitivity should be 87-sh and the room is fairly small anyway. I'm leaning towards TMM, because I've never done a 2.5 way crossover, but I've never done a transmission line, either, so I'm open to input.

    Let me know! Thanks all!
    nothing can stop me now

  • #2
    Using the published T/S values, in a simple ML-TL, MTM floorstander having a volume of 34 liters with the SB15s wired in parallel, I got an f3 of 40 Hz with a 2.83v/1m sensitivity of 94 dB SPL and a max output, based on driver excursion, of 107 dB SPL for an input of 30 watts (both of these SPL output are before BSC losses). The internal cabinet dimensions to build with are 6.5"W x 8"D x 40"H, which includes an "extra" volume of 130 in3 to compensate at least partially for volumes consumed by drivers, braces, port and crossover assembly. I modeled with the tweeter's center located 8" below the internal top, and the mass-loading port's center located 3" above the internal bottom and the port has dimensions of 2.5"D x 4"L. The system tuning frequency is 40 Hz. The top half of the enclosure is stuffed with polyester fiber at a density of 0.75 lb/ft3, needing 6-7 ounces total of fiber.
    Paul

    Originally posted by Dirk View Post
    I'm sitting on 4 of the SB15NRXC30, and I'm thinking of doing a TMM or MTM with them. I've got a pair of the double magnet XT25 in boxes, some of the little SB26SCTNs, or I might pick up some Neo3s before THOSE disappear again. So I've got fun options.

    I like the looks of 36L ported, for an F3 around 40hz. I know the power handling wouldn't be great with a significantly larger box of an MLTL or quarter wave (the ported maxes out at 50W), but the sensitivity should be 87-sh and the room is fairly small anyway. I'm leaning towards TMM, because I've never done a 2.5 way crossover, but I've never done a transmission line, either, so I'm open to input.

    Let me know! Thanks all!

    Comment


    • #3
      Paul to the rescue! :-)

      Interesting that your numbers are so close to the box volume & tuning for a normal SBB4 alignment. Did you include any DCR in the model? I added .7 ohms to account for the lowpass coil(s). When I take that out of the model, I see more "droop" before tuning (as I would expect when modeling an MLTL-sized box in a standard box modeler).
      nothing can stop me now

      Comment


      • #4
        It's not all that surprising of the similarity, but I'd expect different port dimensions if the modeling software does not take into account the effects on tuning from the 1/4-wavelength resonant frequency of a taller but not as deep enclosure. I modeled with 0.3 ohms added in series with the paralleled woofers in this case. When I don't know the actual resistance of crossover inductors in series with a driver, my arbitrary default added resistance is usually 0.25 ohms for a 4-ohm impedance and 0.5 ohms for an 8-ohm impedance.
        Paul

        Originally posted by Dirk View Post
        Paul to the rescue! :-)

        Interesting that your numbers are so close to the box volume & tuning for a normal SBB4 alignment. Did you include any DCR in the model? I added .7 ohms to account for the lowpass coil(s). When I take that out of the model, I see more "droop" before tuning (as I would expect when modeling an MLTL-sized box in a standard box modeler).

        Comment


        • #5
          Originally posted by Paul K. View Post
          Using the published T/S values, in a simple ML-TL, MTM floorstander having a volume of 34 liters with the SB15s wired in parallel, I got an f3 of 40 Hz with a 2.83v/1m sensitivity of 94 dB SPL and a max output, based on driver excursion, of 107 dB SPL for an input of 30 watts (both of these SPL output are before BSC losses). The internal cabinet dimensions to build with are 6.5"W x 8"D x 40"H, which includes an "extra" volume of 130 in3 to compensate at least partially for volumes consumed by drivers, braces, port and crossover assembly. I modeled with the tweeter's center located 8" below the internal top, and the mass-loading port's center located 3" above the internal bottom and the port has dimensions of 2.5"D x 4"L. The system tuning frequency is 40 Hz. The top half of the enclosure is stuffed with polyester fiber at a density of 0.75 lb/ft3, needing 6-7 ounces total of fiber.
          Paul
          I'm curious, what does the predicted FR of that MLTL look like?
          Brian Steele
          www.diysubwoofers.org

          Comment


          • #6
            Brian,
            I didn't save a copy of the predicted system bass response but it looks pretty much like the following one except what I modeled for Dirk has a bit lower f3 and a different SPL output:
            Click image for larger version

Name:	830875 MLTL.gif
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            Paul

            Originally posted by Brian Steele View Post

            I'm curious, what does the predicted FR of that MLTL look like?

            Comment


            • #7
              Originally posted by Paul K. View Post
              ... 107 dB SPL for an input of 30 watts (both of these SPL output are before BSC losses).
              Paul


              I have a 2.1 boombox and the center woofer (Scanspeak Discovery 8") models at 107 dB SPL before BSC. That single woofer will rupture ear drums in my 400ft2 room and you'll have a pair speakers at that output.

              Comment


              • #8
                After BSC, they should be 87-88. I'm not worried about the 30W power handling. If I needed more low end output, I'd build something different. :-D

                I'm currently noodling around in CAD with the box design. Tweeter height from floor will be about 34-35", so that's close enough to on-axis but I'm still thinking about doing this as a 2.5-way crossover, with the upper woofer being the .5. We'll see when it's PCD time.
                nothing can stop me now

                Comment

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