Let’s build a T-line subwoofer

So, you want to start your first DIY transmission line subwoofer box. Good. That’s what I was thinking not long ago. As a result, I started designing and building such a box. If you’re not to keen on reading, and prefer watching, you can check out the YouTube video :

If not, then you can continue reading. First of all, I suggest you read this article first so you can get the basics of the transmission line design. After that, you can check out how I done my first T-line project.

T-line basics

Since I know you are not going to read the linked article, I’m gonna take you through my though process in this DIY transmission line subwoofer box project. First, we have to select a woofer. And to keep the size of this box to a minimum, because I know it can get ridiculously large, I went with an 8″ woofer.

Secondly, we have to design the box. And to do so, the best thing to do, is to model the frequency response curve of the box. Problem is that there aren’t many software that can do this. Actually, almost none of them do, even the most expensive software out there doesn’t model the response for transmission lines or horns. There is an app called Hornresp that does this. But the interface looks a bit intimidating (not beginner friendly at all). I’m not in the mood of learning a new program with a steep learning curve.

As a result, I’m going to stick with the basics. Make a line from the back of the speaker to the front. The length is the quarter wavelength of the resonant frequency of the driver. So :

  1. Subwoofer Fs : 31.6 Hz
  2. Wavelength = 343 / 31.6 = 10.85 m
  3. Quarter wavelength = 10.85 / 4 = 2.71 m

The instructions are easy to follow. Make a line, at least the size of the woofer, which is 2.71 meters in length. The idea is to delay the back wave so that it’s no longer out of phase with the front wave.

Transmission line build

We established that we need to make a line which is 2.71 meters in length. If we fold this the normal way, the enclosure will be so wide that it will not have any practical value. As a result, I tried something different. Here is how the enclosure should look like.

DIY transmission line box

Now this is the exterior and you should pretty much get the design. The circular hole is where the driver goes and the square hole is the end of the T-line. Now let me remove the front panel so you understand how the line folds.

folded t-line

So basically the line goes back, up, front, right, back, down, front, right, back, up and front. That is the folding path. The line keeps the same area from start to finish. Mainly it’s a bit bigger than the surface area of the speaker, as it’s basically a square that encompasses the diameter of the woofer.

DIY transmission line subwoofer boxHere’s how the box looks finished, with the speaker mounted onto the box.

Results

Let’s see what happens when we make a box using simple math and no modeling software. Here’s the anechoic response of the woofer :

t-line frequency response

The response graph contains the whole frequency response chart. However, since this is nearfield measurement, only the section below 500 Hz is accurately measured. Since this is a subwoofer we are interested in the 20 – 200 Hz area anyway. So let’s analyze that :

  • Reapeated peaks at 110 Hz, 190 Hz, 270 Hz etc. These are called resonant modes, and are specific to T-lines. Adding dampening material in creative ways will solve this issue.
  • Huge cancellation between 100 and 200 Hz. Making a DIY transmission line subwoofer box will surprise you with impressive dips in the response chart as well. Again, this can be solved with different types of dampening material, placed in different quantities along the line.
  • Somewhat usable frequency range (since this is a subwoofer) from 20 to 80 Hz, presuming that you don’t find offensive that +5 dB peak before roll-off. Also, you would need a very sharp active crossover to filter those unwanted resonant modes in the upper frequency range.

Conclusion

Making a DIY transmission line subwoofer with no aid from a computer app is basically pointless. You can start to add dampening material to fix the issue, but doing so after the box is done is quite a challenge. It’s seems reasonable at the beginning and end of the line, but not somewhere in the middle.

You could argue that you can damp the line in the building phase. But what happens if that doesn’t work out? You can use different density or a different quantity to change things for the better. Again, hard to do when the box is finished. The normal approach is to use a program la Hornresp.

Promise that I will check this app in the future and see how it works and give this T-line anther shot.