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TL vs Ported Tower

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  • TL vs Ported Tower

    I was reading an old post on here from 2015 about TL's. It was mentioned that if you have a tall ported tower, and the port was positioned properly, the port would reinforce the 1/4 wave. My question is, how tall would this tower have to be and how low would the port have to be? I assume the port would have to be a 1/4 wavelength from the driver. In context, I am building tall ported towers for my HT system using DA115 driver in a MTM. The box is 46 " tall internally. I have these boxes built. Very next step is to cut the hole for the port.

  • #2
    A transmission line works with the compressible waves travelling down a pipe while a ported speaker works with with the whole of the air in the pipe moving as an incompressilble slug while bouncing on the spring of a trapped volume of air. Normally a speaker will use one or the other not both. My first DIY speaker in the 70s (Daline from HFN&RR) attached the line to a trapped volume behind the driver in order to combine to some extent sealed and transmission line behaviour. The tuning of the incompressible slug of air in the line was well below 20 Hz though and irrelevant to the performance of the speaker.

    If you attach a short port at the end of a long line it is not clear what forms the spring to work with the mass of the incompressible slug of air in the port to create a resonance. Not saying it can't be done just that it is not immediately obvious.

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    • #3
      What I don't know about speakers, and to an even greater extent transmission line speakers, would fill a lot of space.

      However, very kind and generous people on this Forum have designed Mass Loaded Transmission Line (MLTL) speakers for me, and their advice is that cabinet dimensions, design and port placement have to be calculated for individual drivers' parameters. The physics and maths which underpin the calculations and design are way over my head!

      One pair is MTM, the other TM. They both sound great: really nice without 'boominess'. I tried the TM in a small-ish vented cabinet which I had already and was of the right size, and then built the test cabinet for the MLTL according to the specs provided by the designer.

      The MLTL sounds better to my ears, with more extended bass than the 'normal' vented design.


      Geoff

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      • #4
        With a MLTL the internal length will reinforce the the quarter wave frequency. The port position is used to control the harmonics. It's prefered to keep the port away from the ends, 1/3 from the end would be a rough guess.
        John H

        Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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        • #5
          Adding to John's comments, the tall cabinet's internal height will have a 1/4-wave resonance simply due to its length, but it won't likely be low enough to tune the system optimally, so the port is necessary to complete the tuning job. Both the woofer's location and the port's location in the "line" effect both the tuning and the smoothness of the overall response, and with a good modeling program that takes into account these locations and the actual internal dimensions of the cabinet, not just the volume in the cabinet while disregarding those dimensions and locations. Usually in cabinets with internal heights of 36" to 45" or so, it's best to locate the woofer's center around 1/5 of the internal height from the top, and locate the port 3-4" from the bottom. In a longer line, but still as an ML-TL, locating the woofer at ~1/3 of the internal height from the top and the port at ~1/6 of the internal height from the bottom is better. In an MTM, the tweeter's center becomes the TL design center, not either of the woofers' center. To restate, however, you really need to model with an appropriate software program to know what you'll get.
          Paul

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          • #6
            Thanks for the replies. Great info here.
            Paul, thank you for explaining this. You were one of the posters in that thread. Can one still get the software for this? I remember trying to get it at one point but could not find it.

            I believe my tweeter center is about 6 inches from the top, so doing the math in my head doesnt seem to work out for this kind of tuning.

            Edit: after some thought, I guess this won't work at all for me. Although the cabs are 46" tall it is blocked off from the top, down to 27 inches. The bottom part was to be used for the crossover. I suppose I could cut an opening to the bottom part, but I am still not sure if the overall design will work for a MLTL.

            BTW, the full internal dimensions are 44" x 4" x 7". Also I am using the NHP25 Ti tweeter crossed over at 3000 hz 4th order.
            Last edited by analogkid442; 10-24-2019, 12:40 PM.

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            • #7
              Martin King's software is no longer available from him, but I have it. In your cabinet if the upper part of its height, above where the crossover is supposed to be located, is 27" high, by itself that height would create a 1/4-wavelength resonance of ~125 Hz, but based on the specified fS and Qts of the DA115, that resonance would have to be ~56 Hz in order to by itself tune the system optimally. By adding a port, though, it could theoretically lower the tuning frequency appropriately. But, with this woofer and these cabinet cavity dimensions, little would be gained other than optimizing the port's location to optimize the overall response's smoothness. If you did manage to open up all of the cabinet's height, the larger volume may simply be too much for the DA115s to work comfortably. BTW, if only the top 27" of the cabinet is used as an ML-TL, and if the tweeter's center is 6" below the top of that height, the TL design center would be located at ~22% of the total height, which would be very close to 1/5.
              Paul

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              • #8
                Thanks Paul, great info. I have some things to think about now.

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                • #9
                  I have to apologize here as I got some numbers somewhat mixed up. The 1/4-wave resonant frequency that would likely be optimum for the DA115 would be ~60 Hz, not 56 Hz, and that would require a line length of 56 inches.
                  Paul

                  Originally posted by analogkid442 View Post
                  Thanks Paul, great info. I have some things to think about now.

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