I read a story last month about how someone has created an alarm that only people under the age of 25 can hear. The longer someone is exposed to the sound, the more annoying it becomes.
The system is called 'Mosquito' and is 'designed to prevent loitering by young people' by emitting one seriously evil 'tone' that makes them 'disperse'. Laugh, oh hell I.
Apparently, it's a kinda load pulsating 'schreech'. I just found an MP3, and imagine the fun you could have with this. I tried it at work and, sure as eggs are eggs, the young ones could hear it and us older ones sat there oblivious, unable to hear a damn thing.
I might leave it on over luchtime to see if any of them crack...