Some savvy teenagers are using a high-frequency ringtone outside of the adult hearing range so they can get away with using their phones in class.

Schoolchildren have recorded the sound, which they named Teen Buzz, and spread it from phone to phone via text messages and Bluetooth technology.

Now they can receive calls and texts during lessons without teachers having the faintest idea what is going on.

A secondary school teacher in Cardiff said: "All the kids were laughing about something, but I didn't know what. They know phones must be turned off during school. They could all hear somebody's phone ringing but I couldn't hear a thing."

Here's an MP3 of the "Mosquito" sound and the 18kHz tone. I'm afraid to admit that I can't hear either sound! (Via Waxy Links.)