I was unable to hear a tone that is generally only heard by people under the age of 25. Could be because I am over the age of 25.
It has been used as a deterrent device to keep teenagers from loitering in malls and shops, and sounds similar to a buzzing mosquito.
Typically the longer you listen to it, the more annoying it gets.
Hey, I never heard the humming of the light over the kitchen table either.