This is my first instructable, and I decided to document how i made a small horn for a speaker. The measurements for the horn are based on a air pressure horn i found on the internet, as i had no better reference.
The tools i used are quite basic, only some pliers, a knife and some scissors, a saw, and a soldering iron. The materials were similarly basic, i used some copper and brass sheet metal i had lying around, a bit of wood, some solder and glue. For testing i used a stereo version of osgeld's pocket protest.
Step 1: Getting Measurements
To avoid building someting that looks like a horn but is kind of out of shape in the end, i found a picture of an air pressure horn on the internet. it was a nice side view, so it was fairly easy to get some measurements of it.
To get these measurements i printed out the picture, marked the different positions at which i wanted to know the diameter of the horn and took measurements. Based on these measurements i calculated the radii of the cones, their lengths and their circumferences (the formula for the angle is alpha = (circumference from the horn picture * 180) / (radius * pi) ). The result of this whole thing is the horn.dxf, which i also exported as a .png so don't have to get LibreCAD or something else to open it.
I won't explain how to use a CAD program, even if it's only 2D, as it's pretty self-explanatory.
Step 2: Cutting Out Parts
To cut out all the parts, i first printed out the drawing from last step. I roughly cut out the parts, so i could stick them onto the sheet metal with some adhesive tape. I made sure to tape over the lines, otherwise the paper would rip while cutting. To cut out the metal parts, i simply used a knife and for the parts that i didn't cut through i just bent it until it broke out. This works fine with the copper i have as it's only 0.1 mm thick. If it's too thick to break it out, i used scissors to cut the parts. This was the case for the brass ring, as it's 0.3 mm.
Step 3: Soldering Everything Together
To solder everything together, i first coated the edges i wanted to solder, then i bent the rings roughly into shape and soldered them together. After that i used my scissors to get everything nice and round. Then i soldered the rings together. I tried to use as little solder as possible, but i didn't always succeed. It would have made everything look nicer, but this way it holds together. To fix everything to a speaker I made another cone which went onto the thin end of the horn. I have no idea if it's better to use a cone or simply a flat piece of metal, but the cone looks better and is a bit more stable. This cone has approximately the same diameter as the speaker, which i superglued to it. At this point, i also soldered wires to the speaker.
Step 4: Making a Housing for the Speaker
The next step was to make a small housing for the speaker so it looks a bit nicer an the speaker doesn't have to stand on the table. I used some bits of wood which i cut from 9 mm by 9 mm square wood strip. My speaker has a diameter of 45 mm, and i decided to make it in the shape of a hexagon. The inside edge is about 23 mm long, the outer side about 34 mm. This gives it an angle of about 60 °, which results in a hexagon. I made two of these rings, and the bottom is cut from some cherry wood veneer i found in some corner. It's all stuck together with wood glue, and the speaker is stuck to this with superglue. It didn't turn out beautiful, but it works.
Step 5: Testing and Finishing It
To test this thing i used a stereo version of the Pocket Protest, which is basically a LM 386 amp. The sound of the horn is a bit odd, because if you point it at your ear it sounds better than the speaker without the horn, but as soon as you leave this area it sounds a bit tinny. I have no idea why, but that's what it's like.
I like shiny things, so i polished it. Especially the brass ring looks much better this way.