The New York Times recently reported on a ringtone gaining popularity with students, which uses a high-pitched frequency inaudible to most adults as a means of circumventing classroom bans on calls and text messages. Students reportedly borrowed the idea from an invention called the Mosquito, which blares the 17-kilohertz frequency outside of convenience stores in a bid to repel loitering teenagers. The tone capitalizes on the phenomenon that adults lose some of their high-frequency hearing as they enter early middle age.

Links: http://www.nytimes.com/2006/06/12/technology/12ring.html http://graphics.nytimes.com/packages/audio/nyregion/20060610_RINGTONE.mp3