As long as we're talking about double source interference for sound, I
thought I'd share something I noticed at our last fire drill. There's a
fire alarm buzzer right outside my room. As I was leaving the room last, I
noticed that the buzzer got louder and softer and louder ... as I walked
past it.
I took a look around to actually find the buzzer, and found it in the wall,
covered by a metal grate whose holes were about 1 cm square, separated by
about .5 cm. Haven't done any math yet to determine if my observation
matches theory.
The grate is cross-hatched, so presumably one could also get a variation in
volume by changing altitude.