Most humans are born with the ability to hear frequencies from about 20 Hz (low) up to 20,000 Hz (high) but that range shrinks as we get older. A guy in England figured that meant kids can hear things adults can't - specifically, very high frequencies. He created a device that pulses an annoying tone at about 15,000 Hz with the idea that when the device is used, it will repel kids while being unnoticed by adult. Perfect for solving that youth-loitering problem you've been having in front of your shop.