The other day, I was in a bathroom stall doing my business, and I randomly started humming some song (can't remember exactly what it was). Well it just so happened that I hit the resonant frequency of that stall. Talk about a surprise! I could make a very quite sound with my mouth closed, and if I hit just the right frequency the stall practically started throbbing. Just the very quite vibration of my throat would make the stall utter a loud hum. I was just wondering if anyone else has ever noticed this. I have plans for future tests. I'm going to build some kind of audio generator that I can adjust the frequency of until I hit resonance, and see if I can knock the stall down. Anyway the next time you get a chance try this. I'm sure you and the person in the next stall will get a kick out of it.