For many amateur radio repeaters a 1750 Hz tone call is needed to access them. As a fun project I designed a pipe that produces that tone.
I printed it on an Anet A8 3D-printer at 0.1mm layer thickness and it works fine. Depending on the pressure it generates tones between 1600 and 1850 Hz before it overblows. I was able to access multiple repeaters with this pipe, but some require a pretty steady tone.