TIG Welded Acoustical / Exponential Horns

by GordonKirkwood

29K21928

Intro: TIG Welded Acoustical / Exponential Horns

Exponential Horns are amazing acoustical machines with no moving parts. They also look gorgeous. Here I'll show you how I made several thin-walled aluminum and steel exponential horns. They work great as loudspeakers, and (my original intent) as smoke-ring cannons aka vortex-ring cannons. Whether you've got a 1920's Victrola, or a 2014 High fidelity sound system, exponential horns like this are unbeaten as the most efficient way to spread sound from a limited source.

Acoustical Horns are Like Gears on a Bicycle

Horns are like gears for sound pressure waves, coupling forceful motions of a small quantity of air into small motions of a large quantity of air, or vis versa. They can make a loudspeaker broadcast sound more efficiently, and more directionally, and they can also make a receiving sensor like your ear or a microphone more sensitive and directional. I experimented with using an exponential horn to couple the motions of air from an 18" subwoofer to an aperture, from which swirling toroidal (or 'poloidal' if you're a nit picker) vortexes could propagate long distances as an action-at-a-distance effector for various art and educational projects I'm working on. Nothing like blasting someone's hair back at 20 feet!! You'll also see some photos of that project, including a stainless steel iris, and heavy duty servo-motor controlled pan-tilt gimbal capable of aiming all the above.

You can read more about acoustical horns here: http://en.wikipedia.org/wiki/Horn_(acoustic)

What you'll need

Sheetmetal to make your horn out of. I've used 0.10" thick aluminum for the 4' long horn shown, and 0.065" (16 gauge) mild steel for the smaller horns. (Steel is definitely easier to TIG weld, if you're starting out.)
TIG welder and accessories (filler metal rod, tungstens, etc).
MATLAB software (only if you want to define arbitrary horns with the code I've written, instead of using the files for the particular horns I've included here). You can probably make this code work in the open source free alternative programs to matlab, Scilab and Octave, too.
A CNC waterjet cutter (or a laser cutter powerful enough to cut your thickness of metal, >400W) reaaaaally helps to make the precisely shaped parts that fit together in precisely the right 3D curve. I'm lucky enough to have access to one in Techshop Pittsburgh. You could also make the necessary segments with a bandsaw and a file, if you are up for detail work, by printing the diagrams on paper, scribing over the printed lines with an X-acto knife, then cutting down to the line with a bandsaw followed by grinding. Or farm out the water jet cutting to a shop. But the panels need to fit together SUPER accurately, otherwise your welding job will be much harder.
A Slip Roller to smoothly bend the sheet metal after cutting it to shape.

STEP 1: Size the Horn and Generate (or Borrow) the CAD File

Generate_Exponential_Horn.m is a matlab script I wrote to generate the flattened patterns of the facets of a faceted exponential horn.

The following variables should be modified:
numfacets %default 5, the number of facets of the horn. the number of sides of the polygonal cross section
dia1 %default 43 inches, the outer diameter of the horn at the widest end

dia0 %default 3 inches

If you don't want to run or modify the code to generate your own .DXF files, you can use the ones I've attached here. (Also Vcarve .CRV and flow jet .ORD files). These files should be able to be scaled in either axis and still join together nicely, too. If it's not overwhelming, you could also message me and I might be able to run off a series of files for you.

NOTE: some post-processing of the .DXF file was helpful to make sure that all the lines are in a closed, connected, curve. If I make another horn in the near future I'll update the details here as I refresh my own recollection.

An additional method of specifying your geometry would be to use Pepakura with a CAD model of the horn. But in my innocence of CAD at the point I attempted this the first time, I found it easier to write my own geometry code in Matlab than to figure out how to get Autodesk Inventor to specify a faceted exponential horn and export the flattened facets as 2D cut patterns.

STEP 2: Cut the Sheet Metal Out

A water jet really helps here. I sandblasted my steel horn pieces after cutting them out.

STEP 3: Bend the Sheet Metal So It Bends AT LEAST As Much As the Horn (more Is OK)

Using a slip roller, put AT LEAST as much curvature into the horn as the most-curved part of the horn. It is not necessary to make the thing fit any particular curvature prior to welding though. This seems counterintuitive to some people. You incrementally tack it together working up the seams, drawing the pieces together for each tack weld by hand. The curvature of the horn will naturally emerge if you simply make the facets join at the edges. The reason it needs to be initially bent more than the final curvature is that this will ensure that the loose (untacked) ends of the horn don't get trapped inside the horn as it is drawn together.

STEP 4: Tack the Widest Corner Points Together

STEP 5: Work Around the Seams, Tacking an Inch or Two Higher Than the Last Tack, in a Helical Pattern

Bring together the segments and tack weld the widest part of the horn together. Using jigs or an assistant or your free hand as necessary to bend each piece back until it is touching the adjacent piece in the spot you are about to tack. If working with steel and it fits very well, as my first steel horn shown here was, you can do it all yourself, one hand on the torch and one hand bending the metal to the right angle. But TIG tacking is chancy without using additional filler rod on steel, and damn near impossible without filler rod on aluminum, and using filler rod while TIG tacking requires two hands. In that case, you might use jigs or an assistant to bend the facets so they kiss in the area you will tack. Proceed in a helical pattern tacking around the horn until the horn is fully tacked every couple of inches on all seams all the way to the top.

STEP 6: Fuse the Seams Entirely

If your experience parallels mine, and you're working with steel, you are about to have one of the most sublime TIG welding experiences of your life. The water-jet cut metal panels are so precise and fit so well together that for most of the welding length you can fuse the edges together without needing to add filler metal. Start welding on one of the tacks, and then just melt the edges together the whole way up. During most of the welding on this horn I was grinning like an idiot under my hood, delighting in the realization that I can now make weld seams that look so smooth as to be confused with merely bent metal. Watch the puddle, move as the puddle allows and with as smooth a motion as possible, and have a filler rod ready to dip into the puddle if you notice a hole opening up. Get into a comfortable hand position, breath deep, and enjoy yourself.

(Conversely, if working with aluminum, you are about to get damned frustrated, cuss a lot, and ask yourself why you're doing this. You'll get through it though, and don't be afraid to take breaks.)

I recommend starting with steel.

STEP 7: Maybe Powdercoat It!

Now that you've got a welded exponential horn, you might want to powder coat it! Here, I've just put a base coat of white powder coating primer on one of my horns. The top coat after this was black so that the 200mW laser I use in conjunction with these would not reflect off of them.

STEP 8: Have Fun With Your Exponential Horn

Fun things you can do with your Exponential horn:

1) listen to things with the narrow end to your ear. You'll hear details and soft sounds than you could otherwise.

2) make a vortex cannon out of it like I did- shoot down some flying drones with smoke rings, impress kids and girlfriends, shine lasers through the smoke.

3) set two vortex cannons facing each other and observe their beautiful symmetric annihilation. Like this:

3) Use it as a loudspeaker, in which use it will make things a lot louder, AND more directional. I found, when singing through the 4' long horn shown, that my vocal chords felt like they were vibrating in a more dense medium than normal air; I could feel the vibrations deeper in my throat. You'll hear echoes of your voice bouncing off of buildings, confirming that you're putting a LOT more sound out than you normally could.

4) your ideas / input on what to do with horns like this?

22 Comments

David the R 9 years ago

Great stuff, very high quality work.

Is there any specific advantage to using these high tech, expensive technologies over using something like doorskin, tape and glue construction, and epoxy coatings? A lot of classic speakers were made of paper.

Thanks again, beautiful work.

GordonKirkwood 9 years ago

Great Question, David.

The simple answer is I used what I had. With the tools that are available to me (at techshop), I could design, cut, bend, and weld a horn like the smaller steel ones shown here in an afternoon, nowadays as little as 4 hours, since I've done it a few times. The water jet time for the smaller steel horns was less than 10 minutes for each horn's parts ($30) and the steel maybe another $10, so this approach is actually not very expensive. I'm not familiar with the paper horn building process (a description would be much appreciated!) but I'd wager that the time to make one out of paper, with multiple epoxy curings and laminations, might be just as time consuming, if not moreso, than doing it this way out of metal. There's also the durability of the result, and the simplificity of not requiring any formwork -- I understand that paper-mache horn building is often made over a meshwork which is first shaped by cutting plywood to the correct contours).

There's a rationale I often give credence to, that if you're going to spend amount of your time on something, it doesn't make sense to skimp on the materials. So $40 in materials / machine costs was acceptable to me, especially as a learning project.

Then there's the added benefit that I really like TIG welding, and making the horns this way afforded me an opportunity to practice that. Most people at TSpgh know that if they need help, I'll usually enjoy any excuse to get under the helmet. This project was no different. I strive towards ultimately building thin walled tubular steel aircraft frames and bicycles, so practice on thin walled materials is something I take any chance to do.

Then, there are the additional benefits of steel construction: high temperature resistance. I'd been talking with Jim Newton about the possibility of launching vortex rings of flammable gasses, which could be ignited, forming legit, aimable, fireballs. As I made these, I had that potential adaptation in mind, with a smile.

While enumerating the benefits of the horn, It seems likely that in some esoteric use, the electrical conductivity of the horn could be helpful. I'm not sure I'm going to try to articulate that one here, though:-)

MU5ICI4N 8 years ago

Thanks for the great 'ible! I've been trying to sort out the math for a faceted exponential horn myself, and finding this instructable was a godsend!

I just had one question; In order to convert the function of my facet from terms of x into terms of the exponential curve, (in other words, 'flatten' it), I am having trouble rewriting the arc length formula ( integrated from 0 to x) to solve for x, so that I can input a known length along a curve from x=0 and receive the x coordinate of the point that given distance along the curve.

It seems you've it figured out with the MATLAB program, so do you know the rewritten arc length formula so that x is isolated?

Thanks :D

Dan

GordonKirkwood 8 years ago

Hi Dan, flattening the sheetmetal is exactly the challenge in doing this. I struggled for a while to get Autodesk Inventor to do it for me, but eventually (after a couple of hours, and also asking the staff at Techshop, and even some Autodesk Engieers), I realized that I could write the code myself faster than I could find whatever buried option in the CAD software, if it even existed. It took me only a couple hours to write the code.

The approach is to define the curve of the panel in 3D, then flatten it by re-parameterizing the panel. You start with panel width as a function of height (or position along horn's axis) and end with panel width as a function of the length of the curved centerline of each panel. I included my Matlab code which you can follow along if you're curious!

MU5ICI4N 8 years ago

Gordon,

Thanks for getting back to me! I eventually gave up on the tricky calculus involved in "flattening" the horn facets... It seemed to be more trouble than it was worth. If Autodesk engineers couldn't figure it out, then I certainly wouldn't be able to!

I used your excellent MATLAB program to create the facet model for my horn, and I recently cut out the 6 facets of my horn using printed out PDF's of the MATLAB figure and my scroll saw. I am using thin, transparent PETG for my segments instead of metal like you used... Now I just have to figure out how to bond the facets together!

Thanks again for the excellent instructable and useful MATLAB code.

lukearnce 9 years ago

I am going to attempt to make this cool contraption. I was wondering what was the size of the sheet metal you used and how many I should buy.

GordonKirkwood 9 years ago

I made this of 16 gauge steel, which is quite coincidentally about 1/16th inch, or about 0.06" thick. You could make it out of anything of course, but your ability to bend it into shape will be significantly more difficult with thicker materials.

Blaise_Gauba 9 years ago

Darn...where's the video? Cool project though.

GordonKirkwood 9 years ago

added video!

audreyobscura 9 years ago

Do you have any video of this piece?

It's super rad!

GordonKirkwood 9 years ago

I posted some video above:-)

kyle brinkerhoff 9 years ago

we demand a video !

also, great work!