Compact Transmission Line Speakers Electronics & Audio
My idea was to replace my unscientifically designed (but reasonably good looking, and reasonably good sounding) rear surround speakers with new, carefully designed ones. The old ones were a guess work mishmash that I had concocted about a year before.
First, I’ll show what I was replacing. The drivers are Vifa P13WH0008 and Seas 19TAF. A pretty good combination for this size speaker. Unfortunately, to the best of my knowledge, these drivers are no longer in production:
What I had in mind was something that would be about the same size box, but with a tapered “transmission line” inside:
The line is about 60 inches long, with roughly a 3:1 taper. This line length and taper closely matches the quarter wavelength of the driver atFs.Martin King’s mathcad modelsfor modeling transmission lines gave me the approximate frequency response:
Box volume is approximately 14 litres when the internal divider, centre brace and driver volumes are accounted for.
Before going further, I have to say that if I had this project to do over, I’m fairly certain I wouldn’t do it the way I did it here. I would go with an alignment that is better suited to this driver, such as a ported box: about 10 litres volume tuned to 60Hz. Back then, I was all about doing it the hard way, and I have become more practical since then.
To get started, I cut out all of the pieces that make up the sides, front and back of each box. The material I used was 1/2″ Baltic birch plywood:
The corners are mitered on all sides, to avoid having the end grain show. This makes the cuts a bit more difficult to do, but worth the effort if you want a clean look.
The panels are placed, face up, next to each other and the seems are taped together. This is then flipped over so that it can be glued. The tape acts as a clamp for each corner:
Only the two front corners are glued for now. The back panel is taped in place to hold the shape, but will be removed after the glue has dried:
The top and bottom are glued on in the same way, and left to dry overnight:
The vent at the end of the transmission line is at the bottom of the box, and is about 1/2″ x 6″. The panel that divides the vent from the start of the line is glued in place, with strips of plywood as temporary spacers:
After the glue has dried, the boxes are flipped over and the recesses for the drivers are routed. I need to do this now so that I’ll know where to notch the brace that will go in next:
The braces are cut to size and notched for the drivers. A series of holes are drilled through the brace to make it acoustically transparent. These holes were made with hole saws:
The brace glued in. This is left to dry for a few hours:
The next step is to glue in the centre panel and the brace that goes between it and the back panel:
Before the back panel can be glued on, I routed the wires through on either side of the brace:
And mounted the binding posts in the back panel:
The back panel is then glued in place and left to dry overnight. The next day they are done, ready for finishing:
For the finish, I used stain and clear polyurethane:
In the photo above, the drivers are installed and the speakers are mounted. The crossover is a single 6.2uF capacitor on the tweeter, and a 2nd order Linkwitz-Rileyfilter(I’ve forgotten the frequency by now) on the woofer.
The speakers were then in service for about ten months as the rear surrounds in my home theater setup. They sounded very good, and I was (almost) satisfied on that front, but I’ve never been thrilled with the way the finish on the Baltic birch plywood turned out. I found there were hundreds of tiny cracks in the face veneer that really stood out when I applied the stain. The grain pattern was not the best either and certainly not a match for the maple of themain speakers. For all of its great qualities, Baltic birch is not a finishing grade of plywood.
So, I decided that I would take the time to veneer these. This would fix the mediocre appearance and allow me to make another improvement: round the edges, at least along the sides.
I started by sanding the sides, top, bottom and front with a 100 grit disk on my random orbit sander. The front corners were rounded with a 1/2″ radius round over bit in the router. With this prep done, I could apply the first pieces of veneer on the top and bottom, then an oversize piece is cut to wrap the sides and front continuously. I glued the front first, then did the sides.:
Here’s how it turned out:
Although the low quality photos don’t show it well, this veneer is a huge improvement. The grain is more uniform and takes stain more evenly:
While I had the drivers out for the veneer / refinish, I thought I’d take another crack at designing a crossover. Working with the Passive Crossover Designer, I came up with what I hoped would be a good design. In order to us this utility, I needed to have the frequency response plots in the .frd format and the impedance response in the .zma format. I downloaded the SPL tracer to convert the graphic image of the spec sheets into usable plots.
This was a little tedious, but it does the job well.
With this data in the designer spreadsheet, I played around until I had what looks like a good alignment. Here is the projected overall response:
The crossover is a series type with a Zobel on the woofer. It uses a 2nd order slope on the woofer and a 3rd order on the tweeter, crossed at 4500Hz.
The Present Day… (2012)
Five years later and, unfortunately, these speakers are still not finished. Shortly after finishing the crossover design, I got distracted with other long term projects, and never did get back to them. I still have them and I may finish them, eventually.
One problem is that the “home theater” idea has lost its appeal, for me. I’m very happy with just my two main speakers, but would like to get my centre channel speaker finished, to help my fading hearing distinguish dialogue more clearly. But that’s another story.