Created by Train Horns
Clicking the play button below will produce a tone that is generally only heard by people under the age of 25. It has been used as a deterrent device to keep teenagers from loitering in malls and shops, and sounds similar to a buzzing mosquito. Typically the longer you listen to it, the more annoying it gets.
This type of 'technology' is also being used the other way around, i.e. ring tones for teens that, supposedly, 'adults' can't hear, so teens can text message each other/receive calls in class and the teacher will never hear the ring.
Of course, I don't think they've considered the (a) young teachers and (b) fact that all the other kids in class will be reacting to the annoying sound, and this might clue in a stodgy old adult.