T-Line Design rules

Can anybody give a quick run-down on the rules that apply to a T-Line enclosure. The port size in relation to the sub size etc. Minimum port length for 10", 12", 15" subs. Sub up port back work the best.

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 I’m trying to do this from my phone because I can’t login on the computer. I’ll get you more details as soon as I log in on the computer but here’s the basics:

  • The line area (width x height of the t-line) should equal or slightly exceed the cone area of the driver used
  • The line length should be 1/4 wavelength tuned to the resonant frequency of your speaker (I’ll post the formula I use later but it involves speed of sound and is fairly easy)
  • Box volume = T-Line cross sectional area x Line length (make sure to use the same units)  (i.e. cm, mm, or in)
  • different fill materials  affect the speed of sound inside the enclosure, as do the rigidity of the walls. As a beginning point, try for 0.5lbs of filling per ft³ 

I’ll try to include some real world formulas and examples and even some pictures as soon as I can get the forum to let me log back in. I’ve built a few for around my house and almost any driver can be used in one. There is a lot of ‘trial and error’ or should I say ‘adjusting to taste’ that follows the build. It can be fun and rewarding and you’ll learn a lot about acoustics if your really dive into the limited research that is available. 



<sent from my iPhone X>

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I appreciate the help. I want to try a 10" T-line under 1K @ 2 ohm, and use this to practice on. Eventually I want to do the same w/15" =>1.5K @ 1 ohm.

+15 KrazyK

14 Nov 2019, 20:06

Those will be HUGE enclosures! Go big or go home right? I actually have a spreadsheet calculator for Excel based on Martin J King's alignment tables for traditional transmission line enclosures. It will calculate a standard line with no taper as well as a line that tapers down to the port ( useful in taming the FR of an overly excited driver) or vice versa a hybrid horn type that flares out to the port ( Google search for "Voigt Pipe"). It is a reasonably accurate tool that I only recently discovered but as previously stated you will still be tweaking and optimizing your setup/design after construction. Another rule of thumb is Fs as it relates to the Qts of the driver. The midpoint for high/low Qts in a TL is widely considered .35 anything above that number is considered a high Q driver and anything below is considered low Q. Each type will perform differently in a TL. You don't necessarily have to tune to the Fs of the driver being used. If a high Q driver is being used it is actually optimal (at times) to tune below driver Fs. High Q drivers up to a Qts of about .65 are generally very happy about being in this enclosure and perform exceptionally. On the other hand low Q drivers can be quite temperamental about this alignment. Oftentimes it is advantageous to tune slightly above their resonance in order to tame the quite jagged FR produced. This in conjunction with adjusted driver placement along the line ( ⅓ down the line from the closed end is a good starting point) and tapering the line at about a 4 to 1 ratio down to the port. All of these techniques combined with stuffing density and placement help to control lower Q drivers. Check out Martin J King's site and read as much as you can. In the meantime I will see if I can send you the file for the calculator if you can shoot me your email address. Good luck and happy tuning!

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Thank you sir! I've been reading away, and besides occasionally looking up the technical vocabulary, I have been following the path as best I can. Knowing that getting the process dialed in is going to take some trial and error and each build will be unique. I appreciate the spreadsheet calculator. My email is

My cheat sheet thus far

T-Line Math

Important Values:

Fs=Resonance Frequency; __Hz

Sd=Cone Area;

Mounting Depth; __in

Mounting Height;__in

Port Length(PL)

Port Width

Port Area(PA)

Speed of Sound=1130ft/sec

X=1130/Fs of sub ex. 1130/29.6=38.18

X/4 = Y ft. ex. 38.18/4=9.54 ft.x12=114.48" PL

Y ft. X 12=PL in. ex. 9.54*12=114.53(Port Length) Again thank you for the guidance!

+36 LayinLo

15 Nov 2019, 20:04

Looks like you have the basics covered. Join the discussion group we have there. Yahoo is changing them to email only I believe and soon invite only so sign up now or I can send an invite later.

KrazyK gave sound (pun intended) and solid advice. Martin King’s Site will help you tremendously with all the info and links. Another source you may consider is reading through some of the patents in your free time. They can be found on Google.

Ultimately, the most key and basic fundamental with Transmission Line Enclosures is the manipulation of the rear wave and resulting resonances in respect to the timing of arrival (or lack thereof) at the listening position.


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