http://www.cnn.com/2008/TECH/04/23/teen.be.gone.ap/index.html
The high-frequency sound has been likened to fingernails dragged across a chalkboard or a pesky mosquito buzzing in your ear. It can be heard by most people in their teens and early 20s who still have sensitive hair cells in their inner ears. Whether you can hear the noise depends on how much your hearing has deteriorated: How loud you blast your iPod, for example, could affect your ability to detect it.