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.
While it is a creative solution to a difficult social problem, I doubt its use will become widespread. The social problem I refer to isn't loitering, it's not providing young people with safe environments to grow into adulthood.
What's the Buzz? Rowdy Teenagers Don't Want to Hear It - New York Times