Officials in Philadelphia started using a sonic device called the Mosquito, in an attempt to keep teens from loitering and vandalizing parks. The Mosquito is a beige speaker, emitting a piercing, high-frequency buzzing sound.
They've been using these for 2 years to keep kids out of the parks, so they are turned on at 10pm each night, and make a high frequency sound somewhere between 17.5-18.5 kilohertz. Apparently this frequency is audible to ages 13-25 years old (as our cells in our ears age, we lose the ability to hear higher frequencies). But there are some people outside of this age range who can hear it, and some younger too.
Check out the clip from YouTube below and let me know if you can hear it or not!