Somewhere in 2001, I bought some surplus drivers from stryke audio. Among these, were a couple of vifa P17WG woofers. I had planned to build basic bass-reflex speakers, which would be powered by the alesis modules I bought from Apexjr. But my plans changed. I have a fine amplifier for my speaker, so there was no direct need for active speakers. And in the meantime, I had heard some transmission line speakers and fell a little in love with the sound. And when Martin King released his TL papers and computer-model, I decided that I wanted to build TL's.

I measured the parameters of my woofers and compared them with the factory specs. Just as with my Scan Speak woofers, there was a big difference. However, I did several measurements, at different times, and got consistent results, so I decided to trust my own measurements over the factory specs. I also added a compensation magnet (before determining the parameters) to the woofer to cancel out the stray magnetic field, because these speakers will be placed close to my pc.

The choice for the tweeter followed after some emailing with Steve Sedmak. I had limited my choice to Peerless and Vifa tweeters, because they're affordable and their proven quality. At that time, Steve just had purchased the same tweeters. He measured them and had a preference for the Peerless tweeters. They have lower distortion, and a better frequency response. And on top of that, they somewhat cheaper too! So I settled on the Peerless 812978 (WA10/TV)

After studying all the papers that Martin King had written, and checking out the worksheets, I fired up Mathcad and entered the parameters into the worksheet. And after a few hours of simulating and messing around with the sheets, I hadn't gotten anywhere! This was proving to be a lot more difficult to get working than the usual box-design software. I searched the web, and found a website on which several designs were featured, which were based on Martin's worksheets. I asked the writer of the website -Bob Brines- for help, and thanks to Bob, I'm now the proud owner of a pair of properly designed TL's! Bob had some rules of thumb, that usually give a very good staring point for optimizing a TL design. In my case -I wanted to have a relatively small enclosure, with an F3 around 35-40 hz- a conventional TL would work best, with a starting width of about 4 times the surface of the woofer(Sd), and a terminus of about 0.5Sd. Length would be around 60-70 inch, with about 0,5lb/ft³ of stuffing. When I entered these figures into the worksheets, I already had a better design than in the hours I was trying to get results myself!. Now that I had a good starting point, it was a matter of tweaking. I increased and decreased line lenght, tried different stuffing methods, different sizes for TL top area and terminus and different positions for the woofer. In the meantime, Bob has written a page about this on his website, which is very helpfull information for anyone trying to design his first TL. Just visit his website and go to the "design a TL" page.

I first planned for a design that uses two woofers per box, because of the rather low efficiency of the woofers, but that makes for one very large box, so I quickly gave up that idea. After several emails and some evenings of simulating, I decided on the following geometry: Line length of 62", tapered from 3Sd at the start to 0.5Sd at the end. Some heavy stuffing (1lb/ft³) in the start of the line, some lighter (0,5lb/ft³) in the middle and no stuffing at the end of the line. TL's are very sensitive to the position of the woofer. By careful positioning of the woofer, it is possble to shape the response above roughly 100Hz. In my TL, I positioned the woofer 12 inch from the start of the line to cancel out the first glitch in the response.

This geometry gives the following response (Blue is infinite baffle response, red is TL response):