Many dog owners use those high-frequency whistles to help control their furry friends.
Dogs can hear them, humans cannot - or so you think!
Well, it turns out - high-frequency noise can also be used to control another group - young people!
Fox 29's Chris O'Connell traveled to the Chalfont Playground in Northeast Philadelphia where - if you look up - you will see boxes. They are called "mosquito boxes" and are part of a speaker system installed by the city's Parks & Recreation department. Now, listen real close after the park closes - do you hear that noise? If not, you must be an adult.
The goal of this system is to prevent after-hours park loitering and only kids and teens can hear them. One kid told Chris that the sound is annoying.
Since it was installed, there have been no problems with vandalism.
This is the only playground to have the 43-hundred dollar system - but officials say it may be coming to a park near you.