NPR did a great story on this. A park in Philadelphia is using speakers to play a high-pitch tone that only people between the ages of 13 and 25 can hear...mostly. Some older people with better hearing may be able to hear the tone. But it is a fairly annoying high frequency that is meant to deter young people from loitering, vandalizing, etc.
Check out the audio clip below. The tone that you're listening for is at about the 30-second mark: