A while ago a company released a device which claims to discourage teenagers by emitting a high pitched noise that is amazingly annoying (which sounds like this: "You're beautiful..." Nah, only kidding). Due to the frequency drop-off in hearing after most people are older than about 20, non-teenagers can't hear the sound, at least not easily and not from a distance.
The teenagers have put this to use though, using samples of the alarm and using it as ringtones. When their phone goes off in class, the teachers can't hear it. That's got to say something about the suberting nature of the human condition; one the other hand it may just be a funny story.