Transmision Line ?

Really? I thought that Wikipedia entry was chock full of errors, misunderstandings and bizarre explanations.

Other than that, it was great..... :)

-k
Ken this is a very time consuming subject to explain.

The Wikipedia entry does give one a general idea about the theory of the driver loading behind this type of enclosure.

These links might be of some help to the OP. :)

http://www.audiokarma.org/forums/showthread.php?t=422244

http://www.audiokarma.org/forums/showthread.php?t=189662&page=2

http://www.quarter-wave.com/TLs/TL_Theory.html
 
The DCM Time Frame series implements a hybrid transmission line, which is sort of a mix between a transmission line and a bass reflex. IE - not a true transmission line, but an interesting version of one.
 
I should clarify:

I understand their fans love them, and will forever keep loving them. However, I have yet to meet anyone who can give a clear and consistent definition of what a, "transmission line loudspeaker," actually is. Few of the concoctions that audio folks like to refer to as, "transmission lines," really meet the engineering definition of such. Most are just labyrinthine infinite baffles or completely over-damped reflex enclosures.
 
A TF600 is not an example of a "true" transmission line.

The DCM brochure calls it a "rear ported hybrid staggered transmission line"
 
To cut to the chase, it's a sort of porting method that utilizes a series of chambers inside the speaker enclosure that the air pressure created by the back wave of the bass driver has to travel through before it exits the cabinet....which when done properly embellishes bass production. This enables a small-ish bass or mid-bass driver (say 8 inches or less) to produce much more bass (higher output at lower frequencies) than it would typically be able to produce in a ported or sealed design. The end result is usually pretty impressive as you see a small bass driver but hear the type of bass production you may be accustomed to hearing from much larger bass drivers. As with all things, some designs do it better than others.
 
To cut to the chase, it's a sort of porting method that utilizes a series of chambers inside the speaker enclosure that the air pressure created by the back wave of the bass driver has to travel through before it exits the cabinet....which when done properly embellishes bass production. This enables a small-ish bass or mid-bass driver (say 8 inches or less) to produce much more bass (higher output at lower frequencies) than it would typically be able to produce in a ported or sealed design. The end result is usually pretty impressive as you see a small bass driver but hear the type of bass production you may be accustomed to hearing from much larger bass drivers. As with all things, some designs do it better than others.

So what I getting out of it is that regardless what a transmission line is or isn't, it has a greater bass output than would be predicted by Thiele-Small theory.

As an example I have the EPI MT2, a tower design with two 4 inch woofers in a omni configuration. When tested by Stereo Review they said "The low-frequency output rolled off smoothly at about 8 dB per octave".
This is in contrast to a ported system with 24 dB/octave rolloff or an acoustic suspension with a 12 dB/octave rolloff.

Please correct me if my facts are wrong.
 
So what I getting out of it is that regardless what a transmission line is or isn't, it has a greater bass output than would be predicted by Thiele-Small theory.

A transmission line can be designed using Thiele-Small methods just like any other speaker. The bass output can be designed to produce more SPL (or less) than a bass reflex enclosure. This usually means the TL must increase in area as you go from the driver to the open end resulting in a bigger cabinet volume compared to the bass reflex design tuned to the same frequency. No free lunch.

As an example I have the EPI MT2, a tower design with two 4 inch woofers in a omni configuration. When tested by Stereo Review they said "The low-frequency output rolled off smoothly at about 8 dB per octave".
This is in contrast to a ported system with 24 dB/octave rolloff or an acoustic suspension with a 12 dB/octave rolloff.

A TL will roll-off below the tuning frequency at 24 dB/octave. But because of the way the enclosure works and the addition of fiber damping it typically has a more rounded off transition from flat SPL response to 24 dB/octave roll-off. This gradual transition to 24 dB/octave can fool one into thinking that the roll-off is much less than the 24 dB/octave or even the 12 dB/octave for a closed box design.
 
Pardon my ignorance but what does "transmission Line" mean. I have a set of DCM TF600's.

:thmbsp::thmbsp::thmbsp:

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Quoting I.M. Fried [ A transmission line is defined, generally, as that system that adds mass to the diaphragm of a driver placed into it, therefore lowering the resonance of the driver below that of the driver in free air. Bass reflex and/ or air suspension systems almost invariably raise the resonance of the driver above it's free air resonance.]

Page 14-16 of the link gives the author's reflections on transmission lines.

http://www.friedproducts.com/Bud Fried/Further Reflections on Loudspeakers.pdf
 
I was curious about Transmission line too and the info on the net can be complicated/not to clear and kinda seem the same as a very low tuned ported box...

i found this guy explains it very well, in normal words.. lol!! check it out

https://www.youtube.com/watch?v=mq247-OCQg0

He was doing OK, but in my opinion made two fundamental mistakes.

1. He recommended that a TL have a cross-sectional area approximately equal to Sd or maybe slightly larger. My experience is that this is too small and will limit the bass output from the TL, it will choke the line's SPL output. I typically start with a cross-sectional area that is about 3 times Sd and adjust from there using computer simulations.

2. He claimed that by adding fiber damping that you significantly slow the speed of sound in the TL resulting in a shorter line and a smaller enclosure. This was the accepted theory for a long time but has recently been debunked, the speed of sound in a TL is about equal to the speed of sound in open air. Fiber damping helps control the SPL produced by higher order standing waves but has very little impact on the tuning frequency SPL output.

Well designed TLs are big compared to similarly tuned bass reflex designs. But they do allow you to use a 6" or 8" woofer and get significant bass SPL output so that the system sounds like you have a bigger woofer. Like everything else in audio it is all about trade-offs.
 
Quoting I.M. Fried [ A transmission line is defined, generally, as that system that adds mass to the diaphragm of a driver placed into it, therefore lowering the resonance of the driver below that of the driver in free air. Bass reflex and/ or air suspension systems almost invariably raise the resonance of the driver above it's free air resonance.]

Page 14-16 of the link gives the author's reflections on transmission lines.

http://www.friedproducts.com/Bud Fried/Further Reflections on Loudspeakers.pdf

If you remove the stuffing from a TL then the impedance curve below 100 Hz will look very much like a bass reflex enclosure, there will be a double humped impedance curve centered about the driver and box resonant frequencies. There will be one impedance peak below the tuning frequency and one above the tuning frequency. In both cases the resonant peak below the tuning frequency is the driver moving in and out of the cabinet and pushing air out and in from the opening (the port in the BR or the open end of the TL). The mass of air combines with the mass of the driver's moving cone and voice coil to yield a lower resonant frequency for the driver/box system. A TL has more in common with a bass reflex design then most people realize or will admit.
 
If you remove the stuffing from a TL then the impedance curve below 100 Hz will look very much like a bass reflex enclosure, there will be a double humped impedance curve centered about the driver and box resonant frequencies. There will be one impedance peak below the tuning frequency and one above the tuning frequency. In both cases the resonant peak below the tuning frequency is the driver moving in and out of the cabinet and pushing air out and in from the opening (the port in the BR or the open end of the TL). The mass of air combines with the mass of the driver's moving cone and voice coil to yield a lower resonant frequency for the driver/box system. A TL has more in common with a bass reflex design then most people realize or will admit.


I've always thought of a transmission line as more of a complicated bass reflex. The math and design involved make it different but I certainly agree they have similarities.


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Yup, definitely similar.

A bass reflex enclosure is a box that acts like a spring and a port that acts like a mass. Mass and spring generate a resonance for the box. Combine a tuned bass reflex box with a driver and you get low frequency SPL reinforcement.

A TL is a "long" line that is distributed mass and stiffness, they are smeared along the length. The distributed mass and stiffness produce a series of resonances at predictable frequencies. Combine a TL, with the fundamental quarter wave tuned to the same frequency as the bass reflex box in the preceding paragraph, with a driver and you get low frequency SPL reinforcement.

You just need to size each to produce the correct amount of low end you require.
 
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