The controversial anti-teenager tool ‘mosquito’ has been turned against the generation that invented it. Using the principle of the ultrasone tone, kids have made a ringtone that is only audible for those under 30. A ringtone that can be used to set of mobile phones in classrooms without the teacher hearing it (assuming the teacher is over 30)

The Mosquito is a device that transmits an ultrahigh sound aimed to keep youngsters out of places where they are not desired. Humans between 20 / 30 lose the capability to hear this ultrasone frequency and do by consequence not hear the sound.
In the UK the Mosquito is widely used by banks, police, council halls, in Belgium it is a new phenomenum. The comune of Elsene has unanimously banned the device for moral reasons: it targets an age group which is a discriminatory act, installing the device gives the signal that youth is not desired in public places. Below is an article from le Soir of 27/04/08.

What is the Mosquito Ringtone?
” A tone outside the audible range of hearing for most people over the age of 30. This means that you can get phone calls and receive text messages in class or school without teachers hearing it. For more in-depth information on how the mosquito ringtone works, please see the Mosquito RingTone FAQ. For information on how to get the ringtone on to your phone, see our Mosquito Ringtone how-to Guide.”
The homepage of Freemosquitoringtones contains ogg / mp3 soundfiles in different frequencies to test if your hearing capabilities matches your age.