Well, some kids have figured it out and have started employing that reality in cell phone ring tones, creating secret alerts that only young people can hear. The New York Times wrote today that kids in New York were caught receiving text messages in class when their teacher, a 28-year-old woman, managed to still hear text message alerts.

The students, using information they obtained on the Internet, made their own ring tone that allowed only them and other children to hear when they received a call or text message. What sounded like a high-pitched buzzer went unnoticed by most adults.

The technology exploits the fact that most people's ability to hear noises above 8 kiloherz fades after you hit 40. That reality was first employed in a device called the Mosquito which was sold to store owners in England to keep youths from congregating outside. No one gave much thought to the idea that the technology could be employed for covert ring tones. Until now. Now students in class can get in trouble for both making too much noise and trying to keep things quiet.

By Ryan Kim
http://www.sfgate.com/cgi-bin/blogs/sfgate/detail?blogid=19&entry_id=6035