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.
Instructables / Autodesk folks, I asked the local pittsburgh autodesk employees about generating a faceted exponential horn in Inventor then exporting the flattened sheet metal facets (as well as every Techshop Pittsburgh DC and knowledgeable patron and Inventor Instructor here) and still drew a complete blank. I'd be happy to write an instructable about that aspect if you could get me started. Message me?
- horn model.pdf
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 bent more than the final curvature is that this will ensure that the loose (untacked) ends of the horn don't get in the way, ie., they are all free of the interior as we work upwards. If they were not bent at least this much, you would have problems with the loose ends of the horn becoming trapped inside by the other facets as you weld.
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?