Homemade Exponential Horn Project

[Ethan]

What glue are you using for this project that you describe as "plastic-like"?

Maybe the glue is adhering to the styrofoam and Vaseline is not a good release agent? Just wondering, since not all glue adheres to styrofoam... This is only a thought, not based on real experience... What if you used a glue that doesn't adhere to styrofoam for the initial layers closest to the form and after it hardens use something else for the outer layers - after the form is removed? Just thinking out loud... what do I know?

I’m using Elmer’s Glue-All (PVA glue), which definitely adheres to styrofoam; I used it to glue the former together.

Vaseline is supposed to be a good release agent for horns, but it could be that I didn’t use enough; there are certainly places where the glue adhered to the former and tore out little chunks when I extracted the piece from the end, although they came off quite easily with a paper towel, so it must not have been a very strong bond. I tried dipping the paper in plain water instead of glue water on the first couple of layers in order to keep the glue off the former, but it would appear that some seeped through anyway.

Using a different glue for the first few layers might work, but I coated the former with shellac varnish to make it smoother, so it would have to be a glue that doesn’t bond to shellac.

What about rubbing the former with wax instead of Vaseline? I was just looking for glues that don’t bond to shellac, and came across a woodworking forum where someone suggested waxing a shellacked jig to keep wood glue from sticking to it; perhaps that would work for horns as well.

Perhaps dissolve the foam with methylene chloride paint stripper, or acetone?

Either acetone or D-limonene should work (possibly paint stripper as well)—they’re both supposed to be able to dissolve styrofoam—but D-limonene might be easier to work with, as it’s just orange oil.

I used kitchen cling wrap on my mold… it separated on the first try.

David

I tried that on a couple of tests and it released quite well, but I couldn’t get it to cling smoothly to the former; it always pulled and wrinkled—How did you get it to lie flat?

I used kitchen cling wrap on my mold… it separated on the first try.

David

Very useful to know, many thanks. That's what I've been planning to use at some point in future when I remake the majority of a broken EMG Mk IX horn.

How are you planning to finish the edge of the horn mouth? I tried lining up the torn paper edges at the end of the curved section, but the edge is very ragged, which seems unsuitable for the mouth.

[dzavracky]

I didn't go much further with my horn after it came off of the mold. The newspaper isn't stiff enough to hold the horn shape... so I scrapped that one. All I can say is that the cling wrap made a good release coating. I am sure with some patience you could carefully wrap the cling wrap around the former so that it lays flat.

David

[Inigo]

This issue of the horn mouth is important. Our colleague emgcr proved that an Emg horn with the mouth circle spoiled, rough and unfinished, interacted with the sound waves protruding from the horn and gave a bad sound. Something related to the rough edge spoiling the wavefront shape when emerging from the horn. The border must be neatly cut and well smooth for producing that 3d sound so special to these machines.
You could try to smooth it out and maybe make a perfect round border piece for it, kind of a ring with a cut in its circular cross section, all along the ring, so you can introduce the horn border into it. There are commercial plastic ropes in rolls with this shape, used to cover the edges of wood boards or similar, so you could cut a piece of it to cover the horn edge all along the circle, making it smoother. The original emg horns had a thickened border, I believe.

[emgcr]

You are quite right Inigo, the subject is extremely complex and, I believe, still not entirely understood but in the case of the remanufactured Oversize EMG horns empirical evidence demonstrated that the shape of the mouth of the bell is of the utmost importance to promote as smooth and easy a passage as possible for the wave-front to emerge or transfer into the new environment (free air) with maximum acoustic effect---matched impedance.

https://acousticstoday.org/7-the-world- ... impedance/

https://www.grc.com/acoustics/an-introd ... theory.pdf

EMG horns did not have any special rings at this point and, in some cases, can become oval but Expert horns did---integrated into the paperwork, strengthening the structure, which was a good idea to hold the shape. The examples I have come across were made of aluminium.

[Ethan]

I am sure with some patience you could carefully wrap the cling wrap around the former so that it lays flat.

Theoretically, there should be a way; I’m just not sure that I have enough patience, at least when it comes to plastic wrap—it usually ends up turning into a jumbled mess when I handle it. Still, it won’t hurt to try—Did you use one piece wrapped all the way around, or smaller pieces overlapping?

Our colleague emgcr proved that an Emg horn with the mouth circle spoiled, rough and unfinished, interacted with the sound waves protruding from the horn and gave a bad sound.

I read about that in emgcr’s thread about the new Xb Oversize horns—in fact, that’s most likely what made me think that the ragged edge would be unsuitable. It does make sense, as the irregularly-shaped edge probably distorts the (ideally) spherical wavefronts.

How would the ring you describe be made to conform to the exponential flare? I’m picturing something like a giant version of the white tubing used for Exhibition soundbox gaskets with a slit made for the horn edge, which would disrupt the expansion of the air-column inside the horn; but if the inner part of the ring could be made flush with the inner surface of the horn, I imagine it would work quite well.

EMG horns did not have any special rings at this point and, in some cases, can become oval but Expert horns did---integrated into the paperwork, strengthening the structure, which was a good idea to hold the shape. The examples I have come across were made of aluminium.

The integrated reinforcing ring sounds like a good idea; do you happen to know how it was incorporated?

I had been thinking of lining up the torn edges as evenly as possible, then, once the horn is off the former, folding a layer over the unfinished rim, possibly after applying a little paper mâché paste to fill in along the ragged edge, but I might be able to make a wood reinforcing ring, leave a couple inches of unglued paper hanging off the end of the former, then fold and glue that over the wood ring.

[emgcr]

Regarding how the ring was made, it is thought that Expert must have employed wooden formers to make their horns although exact details are lost in the mists of time. These would probably have to have been constructed much like shoe-trees (segmented) so that they could be dismantled and withdrawn. It would thus have been easy to have used the bell former as an anvil to panel beat the flared soft aluminium ring. You could probably make such a former for the last two inches of your bell around the mouth to the dimensions of your foam equivalent incorporating and backed by shaped wooden segments cut on a band saw and then covered in thin plywood. A skilled panel-beater might have other suggestions.

The ring would have been incorporated in the bell as the paper was being laid (after a few layers) so that it did not show on the inside or change the internal diameter. It did show on the outside but this would have had no acoustic effect---purely cosmetic---see photos. It would seem that the internal papers were then extended and wrapped around an externally applied bead to form an attractive rolled edge.

EMG tended to fold the last internal papers over the outer edge (no ring) and then cover them with one layer of the final external decorative paper.

[Inigo]

I don't know if the rim reinforcement protruding into the horn would do much harm or not. But I suspect that the real goal is to get a perfect stiff circular rim, very smooth, well centered and straight perpendicular (?) to the horn axis at the mouth. Kind of picturing it as a big nozzle producing soap bubbles... But the wavefronts are much faster bubbles and must be produced as perfect and undistorted as possible. Probably the famous 3D sound effect relays truly at the horn termination. I've read many papers (thanks for that one, Graham, which I already knew, but was interesting to read again) about horns, and the clever and interesting threads written in this forum, about theories and true experimental work developed by colleagues, and reports from experiments done by others, and all the articles about giant gramophones, and Wilson's books several times... And from all this I've got a clear point, that the horn termination is essential for the diffusion of the sound into the room, and for that magical sensation of ethereal sound, as if not coming from the horn.

[Orchorsol]

Fabulous to see all those horn details in the photos - many thanks Graham.

[Inigo]

How would the ring you describe be made to conform to the exponential flare? I’m picturing something like a giant version of the white tubing used for Exhibition soundbox gaskets with a slit made for the horn edge, which would disrupt the expansion of the air-column inside the horn; but if the inner part of the ring could be made flush with the inner surface of the horn, I imagine it would work quite well.

That's exactly what I mean! Yet I think that the small ring narrowing the horn mouth cannot do any harm, as far as you get a perfect smooth circle from which the sound bubbles emerge...
This one is made of foam, but I believe there must be one in plastic, or soft wood...
https://www.amazon.es/dp/B08CZSWP82/ref ... psc=1&vs=1

31F-jVM7TVL._AC_SY780_.jpg

[old country chemist]

Hello Ethan, Your horn project is certainly attracting a big audience-and rightfully so. Good that you are getting good acoustical advice from folks on the forum.
This may not be of any interest to you, but I am mentioning it just in case. When I made the "Neo- Balmain" horn, I thought for some time as to the finishing "ring" for the end of the horn mouth. My wife came up with the answer. We used some flexible rubber tubing about five eights of an inch in diameter,( used in greenhouses as an automatic watering system), with a small internal hole, and with a slightly rough exterior-the reason for that is I wanted to be sure that the several layers of sugar bag paper that I had to apply over the ring would adhere to the surface. It may not be exactly as smooth as Inigo mentions, but I think it adequate, and being flexible rubber it will certainly be as acoustically "dead" as it can be.
I wish you well with your project. Pity you are so far away, it would have been good to have heard it when finished and up and running.
Interestingly, Ginn uses a metal hoop in his finishing ring on horns so I was told.

Alastair Murray in Somerset.