NPR reports of the various parks and recreation centers in North America that are using sonic devices to repel teens from the premises . Philadelphia, for example, has 30 parks and recreation centers that are outfitted with a small speaker called the Mosquito. "It blares a constant, high-pitched ringing noise all night long -- but one that only teenagers and young adults can hear," reports NPR. "Anyone over age 25 is supposed to be immune because, basically, their ear cells have started to die off ." From the report: