In that old battle of the wills between young people and their keepers, the young have found a new weapon that could change the balance of power on the cell phone front: a ring tone that many adults cannot hear.

In settings where cell phone use is forbidden — in class, for example — it is perfect for signaling the arrival of a text message without being detected by an elder of the species.

"When I heard about it, I didn't believe it at first," said Donna Lewis, a technology teacher at the Trinity School in New York City. "But one of the kids gave me a copy, and I sent it to a colleague. She played it for her first-graders. All of them could hear it, and neither she nor I could."

The technology, which relies on the fact that most adults gradually lose the ability to hear high-pitched sounds, was developed in Britain but has only recently spread to America — by Internet, of course.

Recently, in classes at Trinity and elsewhere, some students have begun testing the boundaries of their new technology. One place was Michelle Musorofiti's freshman honors math class at Roslyn High School on Long Island.

At Roslyn, as at most schools, cell phones must be turned off during class. But one recent morning, a high-pitched ring tone went off that set teeth on edge for anyone who could hear it. To the students' surprise, that group included their teacher.

"Whose cell phone is that?" Musorofiti demanded, demonstrating that at 28, her ears had not lost their sensitivity to strangely annoying, high-pitched, though virtually inaudible tones.

"You can hear that?" one of them asked.

"Adults are not supposed to be able to hear that," said another, according to the teacher's account.

She heard that, Musorofiti said. "Now turn it off," she said.

The cell phone ring tone Musorofiti heard was the offshoot of an invention called the Mosquito, developed last year by a Welsh security company to annoy teenagers and gratify adults, not the other way around.

It was marketed as an ultrasonic teenager repellent, an ear-splitting 17-kilohertz buzzer designed to help shopkeepers disperse young people loitering in front of their stores while leaving adults unaffected.

The principle behind it is a biological reality that hearing experts refer to as presbycusis, or aging ear. While Musorofiti is not likely to have it, most adults over 40 or 50 seem to have some symptoms, scientists say.

While most human communication takes place in a frequency range between 200 and 8,000 hertz (a hertz being the scientific unit of frequency equal to one cycle per second), most adults' ability to hear frequencies higher than that begins to deteriorate in early middle age.