This is a simple horn (think: foghorn) that utilizes a balloon and a section of 1/2" schedule 40 PVC. The noise is pretty loud, about the same as a automobile horn, and the frequency is adjustable by using different lengths of PVC, or by using PVC with holes in it like a flute. The sound is created by blowing into the tube through the smallest hole.

After printing, place a section of balloon over the largest hole and hold it in place with a rubber band. After this, insert a section of 1/2" PVC through the middle so that it presses lightly against the balloon. I find that a 16 inch section of PVC creates a nice fog-horn type sound. Blowing into the small hole should create noise.

troubleshooting:
I had to sand the inside hole slightly to get the PVC to have a perfect fit.
Adjusting the tightness of the balloon and the PVC's pressure against the balloon will adjust both the volume and quality of your sound. Experiment!

Teachers:
I print a bunch of these for my students, and then I have many pre-cut sections of pvc in the classroom. Students can experiment with the different lengths and document the frequencies created. It makes for a very noisy and fun way to study sound waves! Soaking the printed plastic in disinfectant between classes eliminates any sharing of germs.