Click here to read an article on the creation of the "Mosquito Noise," a 17 kilohertz sound file that was created to discourage young "troublemakers" from hanging out in front of stores, in construction zones, or in other places that they are creating trouble or putting themselves in danger.
Its genius lies in the fact that, generally speaking, only those under the age of about 25 can hear the noise (which I understand is quite shrill and annoying). Presumably, those over 25 years of age are less likely to make trouble, and can go about their business in a mature fashion. It may seem a bit "age-ist" and discriminatory, but initial reports are that it seems to work better than constantly calling the police.
In an interesting side note, clever youth have begun to download this file as a "silent" ring tone for their cell phones, allowing them to receive calls without the "old folks" knowing it. In the game between Youth and Age, I'd say the score is a 1:1 tie, then.
To download file, click below.