# Designing a Transmission Line Cabinet



## PYLTN (May 15, 2013)

Hi folks.

I'm keen to build a pair of transmission line speakers and this is the first time I've seriously looked at the science behind cabinet design - it's pretty mind boggling stuff!

I have got as far as choosing a woofer and tweeter, have used the T/S parameters to come up with the length of the TL to match a quarter of the woofer Fs and am now getting a bit confused with issues like the cross sectional area the TL should be, the ideal volume of the cabinet and how cabinet design and woofer characteristics come together to dictate crossover design.

If anyone can offer me some plain english hints and tips on what I need to do from this point on I'd be very grateful.

Cheers,
Alex


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## Andre (Feb 15, 2010)

This may be of aid:

http://www.t-linespeakers.org/index.html


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## robbo266317 (Sep 22, 2008)

Most designs have an area behind the driver which then opens into the transmission line, which should have a cross-sectional area comparable to the cross-sectional area of the driver or larger.
Typically the line is lined with sound absorbing material, traditionally longhaired wool.
The whole design aproach is to reduce distortion at the drivers resonant frequency and a good explanation of what is happening in a TL is available on wiki. http://en.wikipedia.org/wiki/Acoustic_transmission_line


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## Jazzman53 (Jul 31, 2013)

Hi all, this is my first post here. 
When researching to build my DIY speakers, I reviewed all the literature I could find on transmission lines, including Martin J. King's design criteria, which I never could figure out how to use. I'm a complete dummy with math and I eventually gave up trying to model a TL, lest I end up in a psycho ward.

I finally built my speakers using Roger Sanders' simple generic guidelines for compact transmission lines, which goes something like this: 

* Line length = 1/4 wavelength of woofer FS
* Line section area about 125% of cone area at the front end, tapering to 100% at terminus. Smaller sections are usable with same taper ratio; although at the expense of earlier roll off and reduced efficiency. 
* In folded lines, baffling behind cone should be curved, so back waves can't rebound to the cone.
* Stuff line with polyester fiber fill (pillow stuffing) to a density of 0.5 lbs/ft3. 

For me, the big advantages of the transmission line are the low Q and lack of box resonances rebounding to the cone (coloring the sound). Before mounting the woofers, I stuck my head in the opening and started humming tones while listening for a box resonance... I heard nothing at all, which tells me my TL boxes are pretty much graveyard-dead. And they sound fantastic. 

There are build photos on my DIY speaker page but this is my first post so I can't post a link. My webpage is easy to find... just do a Google search for "Jazzman's DIY ESL Page" Enjoy!


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## AudiocRaver (Jun 6, 2012)

Hey Jazzman53, welcome to Home Theater Shack!


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## Rumble Filldirt (Sep 19, 2012)

Martin Kings basic TL guidelines are:

Tune enclosure 5 - 10 Hz above Fs of speaker if Qts of speaker is lower than 0.35
.
Tune enclosure 5 - 10 Hz lower than Fs of speaker if Qts of speaker is higher than 0.35
This will give a low tuning, good for subwoofers
.
Higher tuning frequencies produce higher terminus SPL's and vice versa
.
The cross section of the line should be 2-3 times of speakers Sd (radiating area)
.
Lines that are straight or taper larger produce excessive upper harmonics
.
Lines that taper smaller towards terminus produce much less upper harmonics of line frequency
.
Harmonics can be reduced more if the speaker is mounted 20% to 33% of the line length
away from the closed end.
.
And, the easiest to design is a TQWT with a 10 to 1 taper 
IE.. cross section of line is 3X Sd at closed end and 0.33 times SD at terminus (open end or port)
.
Of course these are starting points and you need to do some simulations for your exact designs.
.
You can download some Mathcad simulation sheets for free at Martin Kings web site called quarter-wave 

The quarter wave website also has some pretty good TL designs although Martin is heavily biased towards full range speakers, however simply substituting a Woofer with the appropriate T/S parameters and adding a tweeter would work the same way.

Also another good, free, easy to learn simulation tool is Hornresponse.
.
You can find examples of hornresponse simulations at DIY Audio especially in the subwoofer section.

Best of luck.

Rumble

PS Sorry, I can not figure out how to get the paste URL function to work.


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## powerdoc (Sep 8, 2010)

Hello there

Thanks for the info delivered here. 
I plan in the future, to do a transmission line based on a SEAS nextel with a high QTS 0,45 and a Free aire resonance of 49
If I read you well, I can tune it as low as 40 hz ? 
I will stay tuned on this subject, because there is plenty of bass reflex or closed box cabinet , but not so much transmission line design. 

Right now I am working on a B741 project.


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## StanDingwave (Feb 25, 2012)

Welcome! You (OP) seem pretty well informed on what a TL is...or are you? I agree with the standard (?) definition that a "transmission line" usually involves a quarter wave tube (resonator). However, the term was also used originally to mean what I think is called "aperiodic" -- where the intent was to completely absorb the backwave. In contrast, all quarter-wave designs seek to use the backwave to boost output. I personally have "built" what must be the simplest subwoofer (and TL): just stick a subwoofer (driver) in the end of a long cardboard tube. It works well, but looks ugly 

Please share whatever design you come up with.


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## JoeESP9 (Jun 29, 2009)

I've built two TL subs based on the plans/design published by nelson Pass on the Pass DIY site.


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## oceanman (May 30, 2006)

I build only TL-s. 
I can help you in concrete proects.


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