Can you hear this?

This sound is generally only heard by people under 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.

Listen here

NYTimes article here on other uses for such a technology - ex. "signaling the arrival of a text message without being detected by an elder of the species" in class.

Thanks Fiona and Matt!