There’s a device called the Mosquito that emits an annoying sound at a very high frequency—so high that only young people can hear it. It’s marketed by its manufacturers as a means of discouraging kids from loitering in streets and other public spaces. (Shopkeepers have blasted Barry Manilow, to similar effect.) The UK’s first children’s commissioner, Sir Al Aynsley Green, called it “an ultrasonic weapon designed to stop kids gathering, ” and cited its use as proof that the nation’s attitude tow…