Noise which can only be heard by teenagers

Hehhee that's too funny:
Dog Whistle to Control Youths -- (BBC -- March 15, 2006)

A high-pitched "dog whistle" device is to be used by police in north Staffordshire to stop groups of nuisance youths hanging around shops. The Mosquito sends out a pulsing 80-decibel frequency noise which can usually only be heard by teenagers and those in their early 20s. The device is fitted outside the shop and can be turned on by shopkeepers to move youths on.