The device, called the Mosquito ("It's small and annoying," Stapleton said), emits a high-frequency pulsing sound that, he said, 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.The whole point is to keep teenagers from hanging out in front of your shop and annoying customers who will actually buy something.
My mother bought me an ultrasonic mosquito repeller a while back. It is a little box you can strap to your arm or leg and it drives off the little bloodsuckers. No word whether it works on teenagers too. According to the advertising, the frequency was high enough to be out of the human audio range. My brother and I could hear it just fine, although my parents and my aunts could not.
I play ultimate frisbee after work and the field we were using was quite buggy. So I brought it to work with me. Coworkers my age could hear it, those closer to retirement could not. I found that a little background noise would also keep me from hearing the tone. I was fine when I wore it on my ankle outside, but I could hear it clear as a bell in the office.