Another Homemade Exponential Horn Project
Posted: Sun Jul 23, 2023 10:20 am
My first paper appliqué horn has always seemed rather bass-weak, so I decided to try my hand at making something a little bigger: an EMG Xb Oversize copycat (sort of). Based on length, mouth, and throat measurements of Xa’s, Xb’s, and Xb Oversizes elsewhere on the forum, I believe this design should be about as big as an Oversize; it has a ~34” diameter mouth and is ~86.5” long from the throat to the mouth, with an 80 Hz flare. The “neck” isn’t bent quite the same as a Xb’s, mostly to reduce the number of different sections and make the construction less complicated.
I originally planned to use heavy-duty paper and Elmer’s Glue-All to make the horn, but instead switched to the same packing paper I used for the last one (more about that below); for ease of construction, it's a multi-piece design, like the last horn, except with the bell divided roughly in half in order to get the 34” mouth through my 29” door.
The bell portion of the former uses the same basic construction method as for the last horn, but it is split into two pieces, with wood discs on the ends, to provide a hard surface to cut against while trimming excess paper.
I had to dissolve the curved Styrofoam “neck” former out of the last horn, so for this one, the former has a wood skeleton filled with expansion foam and paper mâché—hopefully, it will release with some sharp taps from a hammer, instead of just crushing at the hammered end as the last one did. I also built a handle into the large of end of the former for the second (longer) bend, to help pull it out—we shall see how well it all works in practice; hopefully, the former does actually release, as it can’t be dissolved out like the last one.
I had initially planned to sand the expansion foam to the exponential flare and finish it with shellac, but within a few days after sanding, the foam had collapsed by up to half an inch, and after a few more days, it had begun to expand again, beyond the intended surface, so I sanded it all down to at least half an inch below the wood ribs, filled in with paper mâché paste, and gave it several coats of shellac to give a smooth surface and ensure that water from the horn couldn’t rehydrate the paper in the former. (I considered using general-purpose Bondo instead of paper mâché, but paper mâché is much less potentially hazardous.)
Photos below—
Here is the main design page from my notebook—I’m making the lower of the two horns (the B-43a-2); the only difference is that it has a 60° bend around a ~10.5” radius followed by a 150° bend around a 12” radius, instead of two 60° bends around ~11.25” radii (in opposite directions), followed by a 90° bend around a 12” radius, meaning that it can be made in two pieces instead of three.
The "Neck" Former--
The wood skeleton cut out and sanded to a circular cross-section:
The ribs glued to the two halves of the skeleton of the first curve:
The halves of the skeleton glued together:
The first side of the former filled with expansion foam, after it started to collapse:
Detail of collapsed foam on the short curve:
Detail of collapsed foam on the large curve. The expansion foam pushed the end half-disc out, so I had to cut out some foam and reglue the wood, hence the gap:
The second side of the former filled with expansion foam:
The first side after a few more days—more collapsing of the foam:
As the expansion foam wasn’t working quite as anticipated, I sanded it down and filled in with paper mâché. The first application of paper mâché (paste) on the first half of the former wasn’t quite thick enough, so I had to do another after it dried; this is the first layer:
The first side after the second layer of paper mâché dried, before sanding (apparently, I didn’t get a picture right after sanding):
The finished former for the first curve. I sanded the paper mâché with 220 grit sandpaper after using an assortment of files and rasps to remove the excess material, then gave it two coats of shellac, sanded with 220 grit paper, and repeated the shellac-and-sanding process with 400, 600, and 800 grit paper.
The finished former for the second curve. The surface isn’t perfectly even, but it should be good enough--the orange areas are where I sanded the paper mâché too far and filled in with wood filler. Unfortunately, the paper mâché pulled in one of the wood half-discs on the end that the expansion foam had pushed out, but it is only about an ⅛” off at the most, so I left it alone, as I should be able to fill in the missing length on the horn with paper mâché paste:
The Bell Former--
The Styrofoam rings for the bell former, before gluing and sanding:
The formers for the two halves of the bell (prior to sanding) with the wood end discs glued and clamped in place:
The latch to keep the two halves of the bell together while sanding (to ensure a smooth flare):
The formers for the two halves of the bell after sanding:
The formers for the two halves of the bell after three coats of shellac (latched together):
The Horn--
The horn spigot—1.5” outer diameter DOM steel tubing (1 ⅜” inner diameter):
I bought a large roll of thick paper to make the horn, and finished two layers (overlapping paper strips about 50%) on the first section of former, but the layers didn’t laminate, I think because the paper was too thick and stiff for me to press it in place properly; so I had to tear it all off and start again with the same thinner paper I used for the last horn—so far, I have three layers done (overlapping about 75%), and the paper is laminating properly now.
The first section of the horn with two layers of the thick paper:
Detail of delamination, after cutting some paper away—I could feel that the overlapped paper was delaminated for most of the section’s length, which was confirmed by cutting some of the outer layers away:
The first section of the horn after tearing off the thick paper and applying a layer of thinner paper:
The same section after two subsequent applications of thinner paper (three applications total)—it took about three hours to do the third application, although I think I was overlapping somewhat more than 75%:
Unfortunately, the paper shrank a bit as it dried, so I need to add another partial layer at the large end to overlap with the next section of former by about ½", in order to align the the first section of the horn with the second section of the former:
More to come over the next few weeks!--I'm hoping to be able to test the horn before I go back to college in about three weeks, although I doubt I will be able to get the full ¼" thickness for the neck by then.
I originally planned to use heavy-duty paper and Elmer’s Glue-All to make the horn, but instead switched to the same packing paper I used for the last one (more about that below); for ease of construction, it's a multi-piece design, like the last horn, except with the bell divided roughly in half in order to get the 34” mouth through my 29” door.
The bell portion of the former uses the same basic construction method as for the last horn, but it is split into two pieces, with wood discs on the ends, to provide a hard surface to cut against while trimming excess paper.
I had to dissolve the curved Styrofoam “neck” former out of the last horn, so for this one, the former has a wood skeleton filled with expansion foam and paper mâché—hopefully, it will release with some sharp taps from a hammer, instead of just crushing at the hammered end as the last one did. I also built a handle into the large of end of the former for the second (longer) bend, to help pull it out—we shall see how well it all works in practice; hopefully, the former does actually release, as it can’t be dissolved out like the last one.
I had initially planned to sand the expansion foam to the exponential flare and finish it with shellac, but within a few days after sanding, the foam had collapsed by up to half an inch, and after a few more days, it had begun to expand again, beyond the intended surface, so I sanded it all down to at least half an inch below the wood ribs, filled in with paper mâché paste, and gave it several coats of shellac to give a smooth surface and ensure that water from the horn couldn’t rehydrate the paper in the former. (I considered using general-purpose Bondo instead of paper mâché, but paper mâché is much less potentially hazardous.)
Photos below—
Here is the main design page from my notebook—I’m making the lower of the two horns (the B-43a-2); the only difference is that it has a 60° bend around a ~10.5” radius followed by a 150° bend around a 12” radius, instead of two 60° bends around ~11.25” radii (in opposite directions), followed by a 90° bend around a 12” radius, meaning that it can be made in two pieces instead of three.
The wood skeleton cut out and sanded to a circular cross-section:
The Styrofoam rings for the bell former, before gluing and sanding:
The horn spigot—1.5” outer diameter DOM steel tubing (1 ⅜” inner diameter):
The first section of the horn with two layers of the thick paper:
