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TL theory question: Exit point

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  • TL theory question: Exit point

    I'm trying to get a slightly better understanding of transmission line theory. Most sources talk about the length of the line as the line and mention it can be folded or unfolded, but often don't make mention of where the end of the line is located relative to the driver. If we ignore tapers and pipe resonances and all that in convenient stuff, the fundamental role of the TL is to phase shift the rear wave relative to the front wave, at the point they both reach the listener's ear. If that is the case, aren't these two configurations I've drawn effectively tuned to the same frequency, or am I missing something?
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  • #2
    I suspect that either 'A' or 'B' needs a smile added to their faces depending upon the channel cross-sectional-area dimensions of the transmission lines.

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    • #3
      Dude, from a unschooled armchair analysis, you are mostly correct, and yes, both of your diagrams are equivalent.
      But the fundamental aspect of most modern TLs is the tuning of a quarter-wave resonance.
      Most very experienced TL guys will say that it doesn't matter whether you have an exit on the front or back of a TL box,
      even though that means the back wave will shift 180 degrees in reference to the direct driver sound from the front.
      The wavelength frequencies that come out of a port or TL exit are simply too long to matter in that regard.
      Hornresp will show you much of this, although it's a bit tough to figure out at first.


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      • #4
        Originally posted by zx82net View Post
        the fundamental role of the TL is to phase shift the rear wave relative to the front wave
        The fundamental role of the TL is to augment the front wave with the rear wave, just like a bass reflex. Where it differs is the method of tuning, relying on the quarter wave resonant frequency of the pipe rather than the resonant frequency of the rear chamber and port air mass. As is the case with bass reflex ports the location of a TL terminus doesn't matter, as its output is in frequencies low enough to radiate omni-directionally. The requisite phase shift of the terminus output relative to the front wave is provided by the pipe length.

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        • #5
          The Premier source of technical information for T-lInes is Martin J. King, he hangs out on Facebook. Start here---> Quarter Wavelength Loudspeaker Design (quarter-wave.com)

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          • #6
            I don't think that your two diagrams will have the same outcome.

            One factor is the mass of the air that is constrained behind the woofer. It is roughly double in the second diagram and could lower the resonant frequency of the system. The extra mass and friction will likely affect the damping also.

            A second factor is when the pressure wave reaches the end of the line, the sudden impedance change may cause a reflection back towards the cone. The pipe length can be tuned so that the pressure wave returns to the woofer cone at the right time to damp the motion at its resonant frequency. I believe that this will lower the Qms of the system. The second diagram will damp the motion at roughly half the frequency of the first diagram.

            In full disclosure, all of this is only theory to me. I have not actually build a t line to test these theories so I do not know for sure how much effect these factors actually have in the real world.

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            • #7
              The difference between the two diagrams is in the length of the lines. The second being roughly twice as long would have an Fp roughly an octave lower. I don't think the OP was considering that when he made the drawing, as his question was about the effect of where the end of the line is located relative to the driver. The OP also said that his drawings ignored all of the finer points of TL design. Your other concerns relate to bass reflex enclosures, not transmission lines, unless they're mass loaded, which is a TL-Reflex hybrid.
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              • #8
                Originally posted by billfitzmaurice View Post
                The fundamental role of the TL is to augment the front wave with the rear wave, just like a bass reflex. Where it differs is the method of tuning, relying on the quarter wave resonant frequency of the pipe rather than the resonant frequency of the rear chamber and port air mass. As is the case with bass reflex ports the location of a TL terminus doesn't matter, as its output is in frequencies low enough to radiate omni-directionally. The requisite phase shift of the terminus output relative to the front wave is provided by the pipe length.
                Yes, agreed, the radiation should be considered omnidirectional, if it isn't, Case A is broken. However, the fact that two sources radiate omni-directionally doesn't prevent them causing an interference pattern. Clearly, the comparison I have illustrated is pathological, the two cases are only equivalent on the axis to the listener. Once to you move away from that axis, the "tuning" frequency of case A changes. (By "tuning frequency" I mean the fundamental wavelength of constructive interference between the front and back waves from the driver, which is a fixed property of the line, regardless of driver Fs.)

                So ,when you say:
                Originally posted by billfitzmaurice View Post
                As is the case with bass reflex ports the location of a TL terminus doesn't matter, as its output is in frequencies low enough to radiate omni-directionally. The requisite phase shift of the terminus output relative to the front wave is provided by the pipe length.
                This is only true if the distance between the driver and the terminus is small compared to the tuning wavelength, which is saying, it only applies the a folded transmission line, not a straight one.

                I'm not trying to be difficult, just trying to understand this properly.

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                • #9
                  If you ignore pipe resonances, then it might model nicely as an open baffle? It looks like a U frame...

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                  • #10
                    The rear wave time delay created by the length of the pipe results in sufficient phase shift to prevent the front and rear waves from cancelling each other. You'd only have a problem with frequencies well above Fp mixing with the front wave. This was true with the original Voigt pipe, which didn't employ damping. Bailey fixed that shortcoming by stuffing the line, which damped passage of frequencies above Fp and suppressed the pipe harmonics. A similar effect is seen with rear loaded folded horns, where there is always a cancellation mode where the front and rear waves meet 180 degrees apart, typically in the midbass, an octave or more above the Fc. This can't be eliminated by stuffing the horn, as that would render the horn inoperative. It's the main reason why rear loaded folded horns disappeared from the scene. This JBL 4530 response is typical of rear loaders:
                    Click image for larger version

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                    • #11
                      Originally posted by Billet View Post
                      If you ignore pipe resonances, then it might model nicely as an open baffle? It looks like a U frame...
                      Thanks for the suggestion, but I'm not trying to model a particular design, just to understand the physics of it. So many of the articles you find aimed at the layman leave you guessing, because there are unstated assumptions. Such as; a TL is always folded.

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                      • #12
                        Originally posted by billfitzmaurice View Post
                        The rear wave time delay created by the length of the pipe results in sufficient phase shift to prevent the front and rear waves from cancelling each other. You'd only have a problem with frequencies well above Fp mixing with the front wave. This was true with the original Voigt pipe, which didn't employ damping. Bailey fixed that shortcoming by stuffing the line, which damped passage of frequencies above Fp and suppressed the pipe harmonics.
                        Ah, good comparison with the Voight pipe, since it is somewhat like "Case A" rotated through 90 degrees. In the far field, I'd expect the un-stuffed Voigt pipe to have the first null where L = Lambda, where L is the length from the driver to the open end. For Case A, the null would occur at L = Lambda/2 because you need to add the "return path" from the open end to the driver.


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