An inventor by the name of Howard Stapleton has developed a high pitched audio device called The Mosquito designed to deter teens from loitering outside shops.

From the Times Online article, "Can a shriek in a box see off troublesome teenagers?":

It emits a piercing, high frequency sound that is audible only, in 90 per cent of cases, to people under the age of 20. While teenagers are forced to run for cover, most adults remain oblivious.

Stapleton said he came up with the idea after reading how hearing levels changed with age.

I wonder if the device will have the unintended consequence of making the stores that use it a target for vandalism or theft. Whereas before the stores only had trouble with loitering teens but after installing these devices, it seems to me anyway, they are only asking for real trouble.

Update: The New York Times has an article about The Mosquito.