This is a diy train horn that anyone can make at home if they have a 3d printer.

THINGS YOU'LL NEED:
*1 inch inside diameter 1.3 outside diameter pvc
*painters tape or something similar like duct tape
*1/2 inch flexible tubing
*air compressor
*#6-32 x 1/2 inch screws and nuts or something that is about 3.4 mm in diameter and 12 mm or longer with matching nuts
*(optional) glue that bonds to your plastic and pvc
*(optional) T fitting for more than one horn *(optional) mount for horns http://www.thingiverse.com/thing:1484130

1. PRINTING:
   Print back plate and pipe connection with supports I found that the horn didn't need supports on my printer but it may for yours. A higher fill is best but anything above 20% is good.

2. MAKING THE DIAPHRAGM:
   After all is printed take blue painters tape, or something similar like duct tape, and make a sheet out of it by sticking three strips together and then going perpendicular to it with another three or more creating a sheet of tape with no sticky side (as seem in the pictures above). Now that you have the painters tape sheet use the back plate as a stencil to cut a circle out of it with an exacto knife, box cutter, or scissors whatever you have really. Now that you have it cut out use the back plate as a stencil once again to cut the holes out for the screws.

3. ASSEMBLING THE BACK PLATE AND PIPE CONNECTION:
   For this you will need to take 8 screws thread them through the 8 holes on the back plate so that they go towards the side without the hump in it. Once you have them all part way threw take the diaphragm and put it on so that the screws hold it onto the back plate. You're now ready to put the pipe connection and back plate together with the diaphragm in between by putting the two together and screwing the screws through the 8 holes on the pipe connection and finally tightening them on with the nuts on the other side of the pipe connection. Note that this needs to form a relatively air tight seal so tighten them down securely and evenly the diaphragm will act as a seal between the two.

4. CUTTING THE TUBES:
   To get a true train horn sound you need to play a chord (B natural minor 7 but i used F natural minor 7 out of ease for what i had already made before tuning) so you need more than one horn playing together i used one with about an 8 inch pipe and another with about a 13 1/2 inch pipe so you will need to cut two pipes WITH STRAIGHT EDGES to those lengths and two back assemblies for more of a train horn sound but if you're only looking for a loud horn then cut it however you'd like longer for a deeper sound and shorter for a higher sound.

5. FINAL ASSEMBLY:
   Now that you have everything printed, assembled, and cut all that you need to do is fit it all together. To do this simply fit the pipe into the back assembly first, it will be a tight fit but we need that for a good seal without glue for tuning purposes. Slowly ease it in, be careful not to break your diaphragm, blow into the air hose connection and when air no longer blows through you have fit the pipe in against the diaphragm which is good if you are using glue for this you can now mark how far to put it in and glue it in that exact place. IT MUST TOUCH THE DIAPHRAGM TO WORK! Now that the back assembly is on put the funnel on the ground and push the pipe into it until it is securely in again if you are satisfied with the sound and using glue glue it in place when it fits properly and now your air horn is complete.

6. SETUP AND TESTING:
   Use the 1/2 inch tubing to connect your air horn to your compressed air, i normally use between 40 and 120 psi depending on how loud I want it so don't worry about blowing it too hard mine take 120 psi no problem, and now finally blow the horn. If you're using more than one use Paxy's t joint http://www.thingiverse.com/thing:952009 or one from a hardware store to connect your horns together.