This Place Is Dead linked to this incredible-sounding story:

A yob-busting alarm using ultra high sound to drive teenage thugs from shopping centres has been hijacked to create inaudible classroom ringtones.Techno-savvy school kids are getting away with using mobile phones in class through the creation of ringtones adults cannot hear.

British firm Compound Security has been praised by the police for its Mosquito device which was developed to stop teenage gangs hanging around outside shops.

Once installed on a building, it emits a high-pitched sound, like a constant insect buzzing, to drive the problem away.

It is highly effective because it cannot be heard by most people over the age of 20 but is deeply irritating to teenagers.

Now the youngsters have turned the tables by making high-pitched recordings which allow them to exchange text messages in class unheard.

Known as “Teen Buzz”, it is spread from phone to phone via text message and blue tooth technology.

I call shenanigans. It’s easy to create a tone of a specified frequency in any audio editor, and I can happily hear 15Khz, 17.5KHz and 20KHz tones (20KHz is the usual upper limit of human hearing) despite my advanced age (33), several years of iPod abuse and 16 years of playing extremely loud guitars in various rock bands. I can also hear Teen Buzz fine, despite its frequency of 18 and a bit KHz. That puts it well within most people’s hearing range – but well outside the typical frequency range attainable with mobile phone loudspeakers, which struggle with frequencies above 10KHz.