DIY Horn Speakers - Step 3: The Plan

Most endeavors start with a plan (some of the best started with a mistake, which is content for a different blog).  In this case, I started with a plan, and Erik (from volvotreter) was invaluable because he has plans on his website.  Imitation is the sincerest form of flattery, so I decided to copy his plans verbatim (with a few tweaks).  I decided to test my lathe out and make the 320 Hz tractrix horns first.  I downloaded this tractrix horn calculator spreadsheet from volvotreter, plugged in a few numbers based on the driver I planned to use (JBL 2446j), and wholla!  Out shot a stream of data points with AutoCAD measurements and dimensions.  AutoCAD would have saved me a lot of time, but since I didn't have a copy, I decided to use the tool that I did have--Adobe Illustrator.  I tried using the automatic graphing feature in Illustrator, but just couldn't make it happen.  It looks to be pretty limited to very basic graphing, but it could have just been operator error.  Luckily, I carpool to work with my brother, so I had some downtime to burn.  Several commutes later, I had a very accurate graphical representation of the inside profile of my to-be-built 320 Hz tractrix contour.  If anyone would like this file (or a PDF of it), let me know, and I'll email it to you.

Update: I located a DIY Audio post appropriately titled "Plotting a curve for a Tractrix Horn in Adobe Illustrator with Javascript."  If you're plotting in Illustrator, this could save you some time, assuming it will work for you.

Tip: I plotted every third data point, which was still overkill when hand-turning.  If you do as I did and plot it in Illustrator, or god help you if you are planning to plot by hand, plot every 5-7 points and round your numbers to something reasonable.  The spreadsheet kicks out mm to the hundredths place.  This level of accuracy will be lost anyway when you hand-turn.  

Next step--cut this illustration into several slices  that are the thickness of my baltic birch sheets (about 15 mm).  It turned out to be just over 19 slices.  In Illustrator, I measured each radius so I could roughly approximate the outside dimensions of the slices to cut out on the router (next step).  Since I don't have a 24" wide printer at home, I printed out two copies at work.  If you don't have access to a wide printer, you could just take the illustration to Kinkos.

I blue taped one of my printouts onto a piece of baltic birch and cut out the profile on my bandsaw. Considering the thickness of the template, I then had to round the corners with a round-over router bit so the round edge would fit into the center of the round horn.