With age, the higher-frequency sounds, above 11–14 kilohertz, go undetected by the human ear. This knowledge prompted the inspired design of a product with the teenager’s umwelt in mind. The device emits a 17 kilohertz tone—out of the range of most adults’ hearing, but unpleasantly audible to youngsters. Shop owners have used it as a teenager repellent, to discourage loitering around their businesses.
