Basic Quarter wave transition line Design tutorial

Ok, so you want to design a Transmition line enclosure for your sub. But it all seems a bit to complicated for you. Well guess what, it is actually quite simple. The majority of websites will tell you that a T-line’s in a car is just to large, well there kind of right if your using a 15” driver with a low Fs. But there are a few shortcuts we can use to make a T-line for a car not much bigger than a large ported box.

Ok, so now we need to get to it. To select a driver for your T-line is quite simple. The ideal driver for a T-line has a Qts of .40 and lower, and Qms of 7 and lower. Now for the Hard core math’s involved in designing a T-line. T-lines are based on the speed of sound, and the length of the sound wave. Now, sound travels at 344.424m (1130ft) per second. Now our T-line is “tuned” to a quarter length of the sound wave of our Drivers Fs. To find this all we do is divide 344.424 (1130ft)/Fs (lets say or driver is a Type R so the Fs would be 27Hz) witch equals 12.765m. This is the full length of a 27Hz sound wave. Like I said before we need to find a quarter length of out 27Hz sound wave, so all we need to do is divide 12.765 by 4. So 12.765/4 = 3.189m. Now you might still think that this is pretty long considering that a true T-line is just that a line/port that’s area is equal to Sd (cone area), or fairly close to it.

In our case the Sd of a Type R 12” is 467.59 cm^2. So our T-line is 3.189m long with a port area of xxcm^2. Now cause this is going to be in a car environment we don’t have to be so exact with the measurements, we can round the length and the area, for this example I am going to round the length to 3200mm and the area to 467.59cm^2.

Now we need to get our port dimensions down for this T-line i'm going to use a port height of 140mm and a port width of 330mm and the length as stated above of 3200mm. Now you might be saying that this is still way to big to fit in the boot of your car. But, just like a conventional port in a ported enclosure, we can fold our T-line. This is where just like designing a regular enclosure you need to figure out the maximum dimensions of the area in your boot you are (can) sacrifice for your enclosure. For this particular enclosure I have the maximum dimensions of 1040mm x 400mm x 500mm. for this T’ line we will need to have 3 folds to fit it in out space. (Note I am not using the boot of my car for this, it is actually going in my house). The line will look something like the crappy paint drawing below with the folds.

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Note the thin black line this represents the center of out T’ line. This is where we measure the length of the line from when folding it. I put the 45 degree pieces of MDF in the corners of the port to try and keep it the same width all the way thru, and to also help direct the air flow smoother. Now you also might have come to realize that you might not always be able to make the port deep enough to accommodate the mounting depth of the driver you are using. To make up for this you can either mount the driver inverted (Note: if doing this you need to reverse the phase of the driver (Swap the positive and negative wires around)), or make a mounting baffle up similar to if you were mounting deep speakers in your doors.

From here it is just a case of building the enclosure just like you would with a normal enclosure. At this stage I haven’t built the enclosure as yet. When I do I will post build pics of it in this thread, for now you will just have to settle for some 3D drawings.

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Now for more SPL use you can take a shortcut of sorts with the length of your T-line by tuning it to the Bass boost on your HU or amp. For example say that your amp has a boost at 40Hz you can turn this on and make your line to this instead of your drivers Fs. You might think that the Driver will not play as low, but if you are using a driver with a Q of around .30 or lower you will be surprised with how low the driver will play when taking into account cabin gain etc.

I know that I haven’t covered EVERYTHING involving T-lines like Tapering etc, but this tutorial is aimed at a basic intro to designing Quarter wave transmition line enclosures.

Edit: Also posted on OzA. http://www.ozaudiophiles.com/forums/index.php?showtopic=51

nice - and easy too. Perhaps if you would like to put the 'advanced' T line theory and implementation in the SQ forum, it would be good for those mathochists (sic!) among us.

I would do that, But at the moment what i have written above is about all i know. I dont have any practical knowlege about tappering, or filling or anything like that. Only the tiny bit i have read.

Maby one of the more experianced members like Volenti could post up a more advanced T-line artical.

You've done a very good job of covering the basics, nice

I'll look at adding to this as I can, I'll start by modeling your type-R 12'' TL in my software and show a couple of modifications that will better suit car loading and also smooth the response with tapering, driver offset, fill ect.

so the volume of the box (approx) will be the port area times T-line length?

from reading that, im getting this : the port size is maintained for the entire length of the T line, and you have to build the box around the specs of the T line and port area? instead of working out volumes etc...

also, i dont know much about these, but what are the benifits? SQ? how would it compare to something like a 'wicked one'?

My understanding of the theory in this design is to extend the low frequency response below the Fs of the driver, so a sub can play lower before it rolls off.

being that it is tuned so that the TL output is +3dB at the point where the sub's output is -3dB would then extend the lower frequency.

The area of the TL will determine the bandwidth of the lower end. A wider TL alows the output to extend lower, but sacrificing power handling.

Making the TL tapered will help this lower power problem.
The 'fill' allows the length of the TL to be reduced by calculating a 'velocity factor' of the fill, so the 1/4 wave happens with shorter TL due to the fill 'slowing down' the wave.

If I'm wrong, shoot me (or tell me to edit my post)

sounds facinating i mioght have to do somthing like this for my home theatre subby im gonig to build. was going to go a large ported box. and im not the best with wood working so a horn is out of the Question but maths aside it looks easy enough to build.

so T line length is subs FS devided by speed of soudn devided by 4.

and port area is cone area? i can do that!!!

BS you will! Finish your car first haha

Even though its not a traditional TL did you want me to put up some build pics of my QWEV Lancer?

BTW very good tut, very easy to read and understand

Thanks mate. Info on Tapering etc would be awsome. Espcialy info on how to fold a tapered line. Cause that seems abit to complex for me:(



Yes the aprox volume of a T-line is the port area multiplyed by the Line length. The benefets of this type of box is increased SQ and they are fairly efficiant to a point. Altho i wouldnt suggest running more than your subs rated power to it when it is in a T-line, as the sub will act as if it is free to air (cause it actualy is......kinda) and bottom out.

POI.ZNR your spot on.



Sure go for it! The more info the better, as im still learning about these things to

Thaks for the kind words

As others have said, very nice intro. Simple to understand and the pictures really helped

Just a note on this, powerhandling wise TL's aren't all that much different from a big ported box, you have an impedance peak at the line's resonant frequency just like a ported box, and just like a ported box the sub has massive mechanical powerhandling at this frequency, you'll mainly run into problems when you drop below the line tuning, which is the case for ported boxes as well, nothing a correctly set subsonic filter can't handle.

Ah ok. I was told that the sub pretty much acts as if it was free to air in that aspect of things.

that was a good read cheers

It's possible this came about from some peoples experience with subs that have poor suspension/magnetic linearity, these subs can suffer from "suck in/out" in vented enclosures and a similar thing happens to them in TL's as well but since the mass of air in the line is coupled to the cone it's much more dramatic, using such drivers in clamshell isobaric eliminates this problem in both vented and TL's, It's not a problem I see very often with quality drivers though.

Interesting guys - VERY interesting!

Best write up ive seen on TL designs. I've wanted to experiment with making mains for a HT setup using TL boxes - like LSK have in their flagship range!