A noise designed to drive kids out of shopping malls has been re-engineered as a ringtone which parents and teachers cannot hear.

A Welsh security company markets Mosquito - a loudspeaker which emits an irritating high-pitched sound designed to drive kids away. Because the ability to hear high-pitched sounds declines with age, the noise cannot be heard by older people.

British kids were quick to sample the sound and turn it into a ringtone to be used in class - they can hear it, but teachers can't.

And now the ringtone has crossed the Atlantic. The Welsh company behind Mosquito credited the kids' ingenuity and is now offering an MP3 ringtone from their website.

Kids in Manhattan have been sharing the tone and using it to receive calls during class without teachers hearing.

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