Mosquito is a high-pitched sound "audible only to teenagers" sold by Britain's Compound Security. It is sold to shopkeepers to use as a teenager repellent -- the idea is to play it loudly in and around shops and "chase away those annoying teenagers!!!"
The kids have reportedly converted the high-pitched noise and turned it into a ringtone, which, being inaudible to grownups, can then be used to receive texts and calls in class without alerting teachers.
This is either a magnificent hoax or just plain magnificent -- either way, I love this Little Brother Watches Back parable.
Schoolchildren have recorded the sound, which they named Teen Buzz, and spread it from phone to phone via text messages and Bluetooth technology.Link (Thanks, Seth and WIll!)
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.
Update: JS sez, "Considering that such high tones are virtually unattainable for the cell-phone loudspeakers I find the story highly suspect. Besides, the sound used as a ringtone would be compressed in some way (maybe not in the newer models, but would all kids have them?), further reducing the possibility that such high frequency content is preserved. I did little research and found this link where cell-phone audio capabilities are presented in detail. According to them the cell-phone's piezoelectric speaker caps its frequency response about at 10khz, while the Teen Buzz plays at 18khz to 20khz."
I had similar doubts -- which suggests that these kids have done something even more subversive than creating an adult-proof ringtone: they've convinced adults that there's an inaudible sound that they can all hear.
Update 2: James sez, "I found this article about the mosquito system. It includes a link to an MP3 of the sound. I'm 18 and I can hear it, but neither my mom nor my step dad (both in their 50's) could distinguish the sound. It's worth noting that my step dad is a country music singer who has a very well trained ear. Since the sound carries over to MP3, and most new phones can play MP3s as ringtones, it would seem likely that students could use the mosquito sound as an adult proof ringer."
Update 3: Gregory sez, "Here's a data sheet for a piezoelectric speaker for cell phones, and shows frequency response measured out to 20kHz. The link that JS found did not say that frequencies above 10kHz were unattainable, but said "The frequency response of piezoelectric speakers is similar to small geometry moving coil speakers up to ~10 KHz bandwidth." As you can see by the data sheet at the URL listed above, small piezoelectric speakers are quite capable of being driven at frequencies above 20kHz. In fact, piezoelectric speakers are commonly used as tweeters in some sound systems; high frequencies are easy, it's the lows that give small speakers problems. A far more important question is the frequency response of the amplifiers that are driving the cell phone speakers. Amplifiers are typically band-limited to reduce noise and increase stability. What is the band limit for the phones in question?"
Update 3: Tony sez, "I've just had a look at 'Mosquito'. It's recorded at a low level, a sort of 'European siren', switching between two high tones at 2Hz. There are some giggles & rumble present (cells would probably not pass these audibly), but the high tones measure around 15,000 to 17,000 Hz. Interested geezers should pitch-shift the sound down an octave. That's exactly the same range as old TV flybacks used to emit ... which I *used* to be able to hear walking by someone's house."