Howard Stapleton says his device, the “Mosquito,” emits an uncomfortable high-pitched ultrasonic sound that can be heard by children and teenagers but almost no one over 30.

It has successfully driven away noisy teens from a grocery store in the Welsh town of Barry and a shop in Stapleton’s home town Merthyr Tydfil, making smoking, lounging and foul-mouthed youths a thing of the past.

The ability to hear high frequencies deteriorates with age, but some adults might still be able to hear the Mosquito. No one except young troublemakers appears annoyed, however. ...