Someone in Great Britain developed a high pitch sound beyond the hearing capabilities of most adults. While teens not only hear the sound, they find it annoying. Adults, on the other hand, can barely, if at all, perceive the tone.
The original intent was to create a sound distasteful to teens that could be played where they tend to congregate. Instead, teens have placed the tone on their cell phones so adults, especially teachers, cannot hear their cell phones ring.
At this website you can download a number of different frequencies to play in class. http://www.teenbuzz.org/
Below are two YouTube videos that go through a number of frequencies within a 30 second video clip.
http://www.youtube.com/watch?v=IrewnzQYrPI Lower to higher
http://www.youtube.com/watch?v=9g0yThhJcxY Higher to lower