The device, called the Mosquito ("It's small and annoying," Mr. Stapleton said), emits a high-frequency pulsing sound that, he says, can be heard by most people younger than 20 and almost no one older than 30. The sound is designed to so irritate young people that after several minutes, they cannot stand it and go away.
Field tests at a grocery store in Barry, South Wales, and a shop in Stapleton's home town of Merthyr Tydfil have proved successful, with rowdy yobs driven from the premises.
What's the Buzz? Rowdy Teenagers Don't Want to Hear It - New York Times