Decius wrote:

The alarm, which has been praised by
police, is highly effective because its
ultra-high sound can be heard only by
youths but not by most people over 20.

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.

What a cool idea. A few years ago I learned that I can hear ultrasonic rodent repellers. I'll bet I can hear these just fine.

I've read some skepticism about this, primarily from those pointing out that most of the little piezoelectric speakers in phones don't have sufficient frequency response to reproduce the relevant 18kHz - 20kHz sounds. Seems like "vibrate" function might be able to pull this off nearly as well.

RE: Pupils perform 'alarming' feat | Metro.co.uk