A sonic device originally intended to repel teenagers is being used to create kid-only ringtones.

The Mosquito ring-tone allows teenagers to program their mobile phones to ring at an extremely high frequency which young people can hear, but most adults cannot.

As people grow older their ability to hear higher frequencies lessens. Therefore, sound at higher frequencies which most adults cannot hear are clearly audible to teenagers.

Ironically, the concept was first applied as a way to repel teenagers. British inventor Howard Stapleton introduced a device in 2006 which broadcasts sound at a high frequency designed to annoy teenagers.

The 'Mosquito' Sonic Teenage Deterrent was pitched to store owners and local authorities looking to discourage young people from loitering outside buildings.

But students soon realised that the Mosquito concept could come in very handy for sending text messages or taking calls in the classroom, and the 'silent' ringtone was born.

A number of sites have begun distributing the ringtones for free, offering a number of frequencies designed to be heard by age groups ranging from under 60 to under 24.

One of the distributors, TeenBuzz.org, advises users against playing the tones at high volumes.

The site warned that, even if the user cannot hear the tone, the high-frequency sounds can still damage the eardrum.