http://trainhorns.net/sound/

Clicking the play button below will produce a tone that 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.


I can hear it and I am well over 25. For me its most discernible at first then starts to fade away into the background.