100 Hz Multi-sided Conical Midbass Horns

I set out to create a midbass / upper bass horn that could fill the space between my 320 Tractrix horn (loaded with JBL 2446J drivers) and my 38 Hz tapped horns.  I originally intended to build Volvotreter’s midbass horn, then I intended to lengthen that design get closer to full sized horns (100 Hz round horns should have a mouth diameter of 109 cm at full size which makes it difficult to integrate with other drivers), then I posted a question on Audio Asylum asking about midbass (see the thread).  I finally ended up designing my own with Hornresp.  I already had a pair of EV 15L drivers to use, so I entered that information in using Volvotreter's EV 15L specs.  I didn't quite understand the Hornresp parameters, but then I found the Hornresp guide.  This helped me make sense of the abbreviations and coding in the program.

In Hornresp, I experimented with throat, mouth, and length parameters.  I tried to keep the length short and the mouth as narrow as possible while preserving a decent response curve.  Here's where I ended up:

The length is 90 cm.
The mouth is 80 cm (diameter).
The throat is 21 cm (diameter).

I liked the idea of a multi-sided conical horn instead of a rectangular horn.  I decided on a 16-sided horn because it would be the easiest to draw (a circle bisected four times). They should look nice with my round Tractrix midrange horns.  

I drew a 2d sketch with dimensions.

Then I sketched a 3d image

For the angles, I found a nice online miter calculator that automatically calculates all of the cut angles.  You can either use the "N-sided pyramid" or "N-sided box."  This is what I came up with:

 The first step was to cut the 16 "petals" out.  I chose 3/4" baltic birch for strength and because I don't like working with MDF.  I brought a few sheets home and measured the first petal.  I adjusted my Festool saw to 10.71 degree blade tilt.  The Festool tracksaw is a great tool for speaker-building.  It cuts straight and splinter-free with very little dust when hooked up to a vacuum.  

Next I cut the narrow ends at an 18 degree angle with my mitre saw so the throat would rest flat against the driver enclosure.  I used one of the cutoffs from the petal to lined the petals up perpendicular with the blade.

For previous mitered projects, I used blue tape to assemble.  I tried 2" blue tape the length of each piece and even duct tape to test the angles before gluing.  The tape wasn't nearly strong enough to hold the piece together while I pulled the ends up to form a cone.

Plan B.  I created an angled platform at the same angle that I used for the petals and borrowed a Festool Domino jointer.  

I marked each petal with three pencil marks where I was to place the domino and cut the loose tendon joints. 

The assembly was difficult.  Exact angles are critical here since there are so many sides.  If I'm off by even .1 degree on each joint, when I end up with two halves, they won't fit together properly.  I needed a template to glue on that was the right angle, so I glued two of my offcut pieces together (each cut at the 10.71 degree angle) making sure that the angle was as close to 158.58 degrees as possible.  Then, I screwed on a narrow "ledge on the bottom of the angled template that I could use to clamp the pieces together.  Then I glued up the dominos and the connecting board edges, pushed the two petals together and then clamped one petal securely to the angled template (really tight so it wouldn't shift when clamped the other petal to it).  Then I used bar clamps to create tension between the joints.  My goal here was to get four quarters that I could then clamp together with rope.

One thing I would have changed about the glue-up...if I could do it over again, I would have stuck blue tape to the edges of the inside pieces before gluing so I wouldn't have had to spend so much time sanding the glue out of the corners.  I used this technique on the stands that the mid-horns will sit on and it saved a lot of time.  Just stick blue tape along the edge along the entire side that you plan to glue (where the squeeze out will happen).  After clamping, wipe the excess glue away and peel away the blue tape (taking the squeeze out with it).

Here's the final glue-up of the four quarters. I used ropes and twisted them like a tourniquet.  The clamps you see are rope guides to prevent the rope from sliding down to the narrow end of the cone when tightening. 

Once I finished the cone, I needed a good way to mount the compression chamber to the horn throat.  Both the horn and the compression chamber are really heavy, so I needed a lot of strength.  I chose to make "wings" that would create a flat surface that I could mount the compression chamber box to.  I used the Festool Domino tool, which adds a lot more strength than I could get from biscuits.  

This is what the horn looks like with four "wings" installed.

Next, I made a compression chamber using two layers of 5/8" baltic birch to create over 1" walls.  

Here is the compression chamber mounted to the horn.  Two sets of screws connect the box to the horn.  One set is also the set that will fasten the driver.  I used a flush trim bit to route the compression chamber exit to match the horn.

Assembled...ready to spend five hours sanding and finish them off with some General glossy finish...