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  • Stupid Question on Transmission Line Speakers

    Hello from Melbourne

    I'm curious as to why most traditional (i.e. with a folded division(s) inside the cabinet) 'transmission line' speakers usually have one woofer, and "Mass Loaded Transmission Lines" often have two?

    The Tritrix and Thor MTMTLs have two woofers, but they're the only ones I can think of. I've read various Internet materials on the subject of transmission line speaker design and thoroughly confused myself: the maths and physics is rather over my head and this subject doesn't seem to be covered.

    Is this because the line design is easier when two woofers aren't both pumping air into the transmission line, or because two woofers mean the line has to be longer than for one woofer, so the box has to be bigger?

    Any advice or ideas appreciated, thank you!

    Geoff


  • #2
    The primary reason for using dual woofers in a TL is the same as in any alignment, for the wider dispersion of two smaller drivers compared to one larger driver with the same Vd. If the Thor you're referring to is Joe D'Appolito's that's a pure TL, not mass loaded. His design article in Audio xPress May 2002 is very thorough, reprinted here:
    https://www.audioxpress.com/article/...nsmission-line
    www.billfitzmaurice.com
    www.billfitzmaurice.info/forum

    Comment


    • #3
      Originally posted by billfitzmaurice View Post
      The primary reason for using dual woofers in a TL is the same as in any alignment, for the wider dispersion of two smaller drivers compared to one larger driver with the same Vd. If the Thor you're referring to is Joe D'Appolito's that's a pure TL, not mass loaded. His design article in Audio xPress May 2002 is very thorough, reprinted here:
      https://www.audioxpress.com/article/...nsmission-line
      Thanks Bill, I'll have an optic

      Geoff

      Comment


      • #4
        You can build a high-performance "pure" TL with two woofers, meaning the line's length, either actual or effective if tapered, is the sole tuning mechanism from its 1/4-wavelength resonant frequency. A mass-loaded TL is simply a line whose length is not long enough and resulting 1/4-wavelength resonant frequency not low enough to properly tune the system, thus requiring a port to complete the tuning.
        Paul

        Comment


        • #5
          Originally posted by Paul K. View Post
          You can build a high-performance "pure" TL with two woofers, meaning the line's length, either actual or effective if tapered, is the sole tuning mechanism from its 1/4-wavelength resonant frequency. A mass-loaded TL is simply a line whose length is not long enough and resulting 1/4-wavelength resonant frequency not low enough to properly tune the system, thus requiring a port to complete the tuning.
          Paul
          Thanks Paul

          One of my DIY speakers is a MLTL (Curt Campbell's Slapshots), they sound marvellous, with very natural bass. One day I'd like to muck around and do a 'pure' TL for some spare drivers I have - but not for quite a while as I have another project on the go.

          I was looking at a commercial DIY TMM transmission line which uses the same woofers (SB Acoustics SB16PFCs) in MTM configuration. I only have two drivers, so I was wondering if I could 'cheat' by adapting its cabinet for one woofer.

          Fascinating but complex stuff

          Cheers

          Geoff

          Comment


          • #6
            If you want to "cheat" the TMM design to make it a TM, you'd keep the line length the same and reduce the line's volume by half. If the line is tapered (not an ML-TL), you'd mostly be done. If it's an ML-TL, you'd also have to change the port's length and/or diameter in order to re-establish the original system tuning frequency. Whether tapered or mass-loaded, you'd also have to stuff it at the same density and length.
            Paul

            Originally posted by Geoff Millar View Post

            Thanks Paul

            One of my DIY speakers is a MLTL (Curt Campbell's Slapshots), they sound marvellous, with very natural bass. One day I'd like to muck around and do a 'pure' TL for some spare drivers I have - but not for quite a while as I have another project on the go.

            I was looking at a commercial DIY TMM transmission line which uses the same woofers (SB Acoustics SB16PFCs) in MTM configuration. I only have two drivers, so I was wondering if I could 'cheat' by adapting its cabinet for one woofer.

            Fascinating but complex stuff

            Cheers

            Geoff

            Comment


            • #7
              Originally posted by Paul K. View Post
              If you want to "cheat" the TMM design to make it a TM, you'd keep the line length the same and reduce the line's volume by half. If the line is tapered (not an ML-TL), you'd mostly be done. If it's an ML-TL, you'd also have to change the port's length and/or diameter in order to re-establish the original system tuning frequency. Whether tapered or mass-loaded, you'd also have to stuff it at the same density and length.
              Paul


              Thanks Paul, that's very helpful indeed

              As it's a commercial design I can't post the measurements but have attached a rough diagram:

              The design has a volume of about 70 litres and the line seems to be about 175 cm, it's straight and not tapered .

              The SB woofer specs are Fs: 38Hz, Sd: 124cm sq Qts: 0.4 VAS: 27 litres

              When I 'plugged in' the driver's measurements to a TL calculator, the line length for a cabinet of the same internal width came out at 179cm, which seems pretty close. Volume is about 70 litres, so I'll halve that.

              The opening works out to twice the Sd (I assume because it has two woofers), so I'll halve that.

              Thank you again!

              Geoff Click image for larger version

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              Comment


              • #8
                After looking at the line's configuration, without any modeling on my part, I will tell you it's very likely a poor design, whether as a TMM or TM. There will likely be large dips in the response due primarily to the line having a more or less constant cross-sectional area. The only way to even begin to smooth out those dips is by using a fairly heavy stuffing density and/or by stuffing the line's whole length, either of which will severely limit, if not obliterate, the output from the terminus, thus removing any bass support that might otherwise be provided. Based on its length of 175 cm, and it having essentially a constant area, the line's 1/4-wavelength resonant frequency will be ~49 Hz, but the driver's fS and Qts indicate it needs to be more like 38 Hz, possibly a bit lower. Also, there is nothing at all "magical" or even necessary or optimum, about using a line area that's 2Sd. It appears this line was "designed" based on some old assumptions/guesses which have long ago been proven to be not very accurate and definitely before accurate TL modeling software became available. I realize my comments may sound rather harsh and possibly discouraging, but I would not want anyone to build a TL that is likely to not be a good performer. I'm not an expert but I'd be happy to design a TL for you and your drivers that would likely perform much better with your drivers.
                Paul

                Originally posted by Geoff Millar View Post

                Thanks Paul, that's very helpful indeed

                As it's a commercial design I can't post the measurements but have attached a rough diagram:

                The design has a volume of about 70 litres and the line seems to be about 175 cm, it's straight and not tapered .

                The SB woofer specs are Fs: 38Hz, Sd: 124cm sq Qts: 0.4 VAS: 27 litres

                When I 'plugged in' the driver's measurements to a TL calculator, the line length for a cabinet of the same internal width came out at 179cm, which seems pretty close. Volume is about 70 litres, so I'll halve that.

                The opening works out to twice the Sd (I assume because it has two woofers), so I'll halve that.

                Thank you again!

                Geoff Click image for larger version  Name:	EPSON001.JPG Views:	1 Size:	214.6 KB ID:	1390492

                Comment


                • #9
                  Originally posted by Paul K. View Post
                  After looking at the line's configuration, without any modeling on my part, I will tell you it's very likely a poor design, whether as a TMM or TM. There will likely be large dips in the response due primarily to the line having a more or less constant cross-sectional area. The only way to even begin to smooth out those dips is by using a fairly heavy stuffing density and/or by stuffing the line's whole length, either of which will severely limit, if not obliterate, the output from the terminus, thus removing any bass support that might otherwise be provided. Based on its length of 175 cm, and it having essentially a constant area, the line's 1/4-wavelength resonant frequency will be ~49 Hz, but the driver's fS and Qts indicate it needs to be more like 38 Hz, possibly a bit lower. Also, there is nothing at all "magical" or even necessary or optimum, about using a line area that's 2Sd. It appears this line was "designed" based on some old assumptions/guesses which have long ago been proven to be not very accurate and definitely before accurate TL modeling software became available. I realize my comments may sound rather harsh and possibly discouraging, but I would not want anyone to build a TL that is likely to not be a good performer. I'm not an expert but I'd be happy to design a TL for you and your drivers that would likely perform much better with your drivers.
                  Paul

                  Thank you Paul, you've just saved me money, time and swear word output! Your comments aren't harsh, they're most welcome.

                  I'd feel guilty accepting your very kind offer to design something, unless you PM me your address and I can send you some music as a thank you!

                  As noted above, I'm not intending to do this for some time but the SB16 sounds very nice and a TL should be an interesting exercise.

                  Unfortunately I don't have measuring equipment, but SB specs are well regarded as being accurate, but I do have 'in box' FR from another web site.

                  Many thanks

                  Geoff

                  Comment


                  • #10
                    There's absolutely no need to feel like you have to "pay" me for helping you out. The correct model number for your driver is SB16PFC25-8, right? I just did a really quick model of it using SB's specs in a tapered TL 55" long and it looked good, and an equivalent ML-TL should also work well. If you want to continue on this route at some point, send me a private message with your email address.
                    Paul

                    Originally posted by Geoff Millar View Post

                    Thank you Paul, you've just saved me money, time and swear word output! Your comments aren't harsh, they're most welcome.

                    I'd feel guilty accepting your very kind offer to design something, unless you PM me your address and I can send you some music as a thank you!

                    As noted above, I'm not intending to do this for some time but the SB16 sounds very nice and a TL should be an interesting exercise.

                    Unfortunately I don't have measuring equipment, but SB specs are well regarded as being accurate, but I do have 'in box' FR from another web site.

                    Many thanks

                    Geoff

                    Comment


                    • #11
                      Originally posted by Paul K. View Post
                      There's absolutely no need to feel like you have to "pay" me for helping you out. The correct model number for your driver is SB16PFC25-8, right? I just did a really quick model of it using SB's specs in a tapered TL 55" long and it looked good, and an equivalent ML-TL should also work well. If you want to continue on this route at some point, send me a private message with your email address.
                      Paul


                      Thanks Paul, that's the correct model number: the woofer has a reinforced plastic frame, but apart from that it's beautifully built.

                      Will be very pleased to accept your kind offer in time and PM you with whatever information I can find. I'll be pairing it with a VIFA BC25TG tweeter and am working on a crossover.

                      Many thanks!

                      Geoff

                      Comment

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