I have been working on this project for about a month, and finally got them done this morning. My apologizes to George Roland for not answering his questions via e-mail. Sorry George I accidentally deleted your e-mail before I could answer it. I hope this thread will explain how to do this. A few here have done a 300 Hz version of this horn with excellent results. Jorden (jorjen) for one was inspirational to me to proceed with this project. JWCullison was working on this exact 225 Hz horn that I built, but I don't think he finished due to many irons in the fire, not to mention a new bundle of joy, and building a new home. I don't know how you do what you have done already JC ;) This project was more difficult and time consuming than I anticipated ( aren't they always!) I learned alot of things along the way that I would do differently next time. There are lots of pics to follow and I will try to answer any questions that I can, but be warned, I am only a hobbiest wood worker and only slightly knowledgeable of the science behind the tractrix horn. Enjoy,

Jeremy

 

The 1st pic is of the printout of the tractrix horn calculator that I used to determine the flare. I found this spread sheet program on the volvotretter web site, which has alot of useful information. There were alot of points to layout, I'm sure I could have used alot less points (about 10 fold) and still been plenty accurate enough for this project.

This shows laying out the points on graph paper. You start in a straight line and then from each point on the vertical line, you have to draw a point perpendicular to it a certain distance that is calculated on the spread sheet. This determines the flare.

Here is the completed flare. I laid out a total of about 1500 points, which was extreme overkill. I have been called fastidious, and anal before so that may explain why. If I did it again, I would cut down the points by about 2/3rds.

Here is a shot of the template I made from that pattern.

From there I made the top and bottom plates leaving about 1/4" material so I could sand an angle on the edges to square them up to the sides.

Next is the buck for the middle of the assembly fixture. This is simply made by laying out on plywood, the width of the horn at the mouth, the throat, and the length. In my case those numbers were all metric, but the throat is 2" and the mouth of the horn is about 12" tall by 24" wide. That determines the angle at which the bottom and top sit relative to the mouth and throat. The pieces you cut off to form that angle on the buck, must be saved to use for sanding the angle on the edges of the top and bottom.

Here is my completed fixture with the buck mounted in the middle. The walls around the edge of the fixture are the dimensions of the mouth of the horn plus 3/4" material thickness per side.

Those wedges that were scrap from the buck are now used to hold the top and bottom pieces at the correct angle while sanding the edges square. I used an oscillating spindle sander, but i'm sure it could be done other ways.

Here are the top and bottom mounted in the fixture. I used two screws to screw each side to the buck. Those were the same two holes I used to screw them down to the wedges to sand the angles. These holes will be plugged later.

Here is the material I used for the curved sides. It is bendible MDF about 1/4" thick. I will laminate two layers of this on each side. I was going to use 3 layers, but scraped a couple pieces along the way, so 2 it is. I think it will be plenty dense enough as is.

Here are the two layers glued and brad nailed to the top and bottom plates. Extra material is left on the sides so they can be flush cut later. I though about putting the two slotted sides together so it would look better, but I thought I would get more gluing surface if I put them front to back. This was a mistake, because as I tried to cut the sides flush with my router, the ribs came flying off and it looks like crap.

I ended up using a flush cut hand saw to cut the sides flush, because the router was too aggressive on the ribbed MDF. It was alot more work. This part of the process involved actual blood, sweat, and tears.

Here is the completed horn after pulling it off the fixture. I had no idea how big they were going to be until this point. These are massive.

After adding a plate on the back to mount the driver, it is basically done. The plate is the same height as the mouth, to help the horn sit level, and to support the massive 25 lb. JBL 2482 driver. I will probably add some gussets between the back plate to the top and bottom. A K-600 horn and K-55 are in this pic to give you an idea of how big these are.

Here they are sitting on top of my $25 Cornwalls, with another K-600/K-55 for reference.