In most of the schools in the United States, as in most of the schools everywhere, the mobile phones are prohibited. And that's quite normal, finally. But on another hand, the students can be smart enough. And the big trick is to have a ring tone that you can hear, because you're young but your teacher can't, because he's old.

The story starts with Compound, a security company, and their main product: the mosquito. An alarm which outputs an ultrasonic sound that only the young people can hear it. The idea is to prevent the gatherings of teenagers. An idea of Nicolas Sarkozy?

Here, it looks like adults won the game. But the idea has evolved and the mosquito was adapted into a ring tone for mobile phones! In United States of course, in UK and I guess soon everywhere in the world... 'Cause even Compund proposes its own, like the official one, called Mosquitone.

So, the fashion trick: receiving an SMS during the class, the teacher does not hear it, everyone laughs. You could use the vibrator but it's less funny anyway.

How does it work?
The drawing here above represents the spectrum of the audio frequencies a human being can hear. When he's young, this spectrum goes from 2000 to 18000 Hz, maybe more. But he gets old, the spectrum narrows. Not for everyone of course, but for most of people, it decreases as indicated above.

A test?
All right, but be sure to use good speakers and a correct PC sound-card, and first turn the volume down, then turn it up slowly. It can be surprising!
The sounds are not compressed (wav, not mp3). Otherwise, it's sure you could not here it...

4000 Hz, like piano :

12000 Hz, more than 60 you can't hear...

15000 Hz, more than 40...

17000 Hz, the Mosquitone's frequency:

20000 Hz, personally I can't hear:


Source : New-York Times, middle of June.
For those who do not know: One hertz (Hz) simply means "one per second" (1 / s); 100 Hz means "one hundred per second", and so on. The unit may be applied to any periodic event – for example, a clock might be said to tick at 1 Hz, or a human heart might be said to beat at 1.2 Hz. Frequency of aperiodic events, such as radioactive decays, is expressed in becquerels.