the mosquito device

The Mosquito device functions as a high frequency buzz (17,4KHz) is employed to keep away teenagers from gathering in publicly accessible spaces like shopping malls, street corners, courtyards, etc. Mosquito is supposed to target specifically the population under 25. Unlike their older cohabitants, the young population should be able to hear the repelling sound buzz at 5 dB above background noise levels. In practice this age border does not function exactly as intended.
The Mosquito device patented in 2005 by Howard Stapleton, was installed in the last few years in numerous spots in European and American cities, where young people would gather and exhibit the so-called ‘anti social behaviour’.

It turned out some older (more than 25 years old) people can hear the tone but at the same time not all teenagers can hear it. It became popular amongst teenagers as a ring tone for cell phones during classes, especially the ones given by elderly teachers.

Mosquito devices mounted on a street light pole in downtown Chicago; courtesy of Dave Hoffman

http://en.wikipedia.org/wiki/The_mosquito

http://news.bbc.co.uk/2/hi/uk_news/wales/south_east/4415318.stm