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Could one accidentally create a Transmission Line?

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  • Could one accidentally create a Transmission Line?

    I plan on building some tall, very slim cabinets soon. I'm a bit concerned that quarter-wave resonances could be an issue. Say one had a cabinet that was 3-4 feet tall, and basically square and hollow like a fencepost. How severe would 1/4-length resonances become? How can I diminish these effects? Really, all I want is a simple bass reflex box that is tunable to a variety of different frequencies--not locked in to one particular driver.
    Isn't it about time we started answering rhetorical questions?

    Paul Carmody's DIY Audio Projects
    Twitter: @undefinition1

  • #2
    Re: Could one accidentally create a Transmission Line?

    Stuff one end of the tower thickly, as well as placing a mass load on the inner ends internally to keep them heavy and dead.

    I did that in the Icthus, and it seemed to work.
    Later,
    Wolf
    "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
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    • #3
      Re: Could one accidentally create a Transmission Line?

      Zaph address this issue a bit in his ZRT build.

      http://www.zaphaudio.com/ZRT.html

      His solution was to densely stuff the bottom of the cabinet with polyfill. It needs to be a fairly thick layer as polyfill attenuates high velocity nodes best.

      Edit: Pretty much what Wolf said.

      -David

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      • #4
        Re: Could one accidentally create a Transmission Line?

        Hi Paul,

        How about you take advantage of the resonance and make it a MLTL?
        Let me know your driver specs and your enclosure size and I can model it up on Martin's MathCAD worksheets for you.

        C
        Curt's Speaker Design Works

        "It is the mark of an educated mind to be able to entertain a thought without accepting it."
        - Aristotle

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        • #5
          Re: Could one accidentally create a Transmission Line?

          Like Curt said! An internal cabinet height of 36 to 48" will have a 1/4-wavelength resonant frequency of ~90 to 70 Hz and that can easily affect the overall tuning frequency. IOW, if you were to base the port's dimensions only on the internal volume of the cabinet, you can easily discover it didn't turn out quite like you expected because the 1/4-wave resonant frequency is contributing to part of the tuning. The longer the line (the taller the cabinet) the more it will affect the tuning. But it's a good thing if you want to use it.
          Paul

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          • #6
            Re: Could one accidentally create a Transmission Line?

            I use Boxnotes for a quick reality check for potentially serious internal resonances

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            • #7
              Re: Could one accidentally create a Transmission Line?

              A few years ago I heard a small transmission line speaker designed by Jim Griffin. IIRC, it had a single full range CSS five inch driver, with a square cross section box about 7 inches by 38 inches.

              Sounded really great - I had to ask him, and he confirmed, yes, it was just a straight tube.

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              • #8
                Re: Could one accidentally create a Transmission Line?

                Originally posted by Paul Carmody View Post
                I plan on building some tall, very slim cabinets soon. I'm a bit concerned that quarter-wave resonances could be an issue. Say one had a cabinet that was 3-4 feet tall, and basically square and hollow like a fencepost. How severe would 1/4-length resonances become? How can I diminish these effects? Really, all I want is a simple bass reflex box that is tunable to a variety of different frequencies--not locked in to one particular driver.
                Paul,

                This definitely could be an issue, but not just because of standing waves: bass tuning of tall narrow enclosures don't match quite like predicted in all the Thiele Small based modeling programs. Thiele-Small explicitly uses lumped paramer analysis which assumes that all dimensions are small compared to the wavelengths of interest, so this can break down for very long enclosures.

                There's an AES paper on exactly this and the author (pretty sure it was Juha Backman, who has written several amazing loudspeaker papers) characterized it using a vector model in one direction, much the same analysis method that Martin used in the TL. He was able to break it down that a tall narrow enclosure can be modeled as a normal enclosure but with an additional inductor added in the theile small model.

                I discussed this with Martin a few years ago, and he decided to simulate it in his TL program. Here's what Martin found:

                "For this one example, the quarter-wave action does have an impact on the SPL response. The bass goes deeper and without the peak predicted by a lumped parameter style of simulation. I conclude that modeling a tall floor standing enclosure using a simple freeware program based on lumped parameter modeling may lead to some inaccuracy in the bass SPL response"

                As Wolf recommended you can reduce the impact with stuffing.

                Dave

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                • #9
                  Re: Could one accidentally create a Transmission Line?

                  Or, just design the enclosure in MJK's ML-TL spreadsheet and simulate it as an ML-TL, even though you don't plan to rely heavily on quarter-wave action to achieve the performance you're designing for, just so you can see its impact.
                  Best Regards,

                  Rory Buszka

                  Taterworks Audio

                  "The work of the individual still remains the spark which moves mankind ahead, even more than teamwork." - Igor I. Sikorsky

                  If it works, but you don't know why it works, then you haven't done any engineering.

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