Do you know that some of the ring tones cannot be heard by some older people? Because older people have impaired hearing in the high frequency range, they cannot hear high frequency ring tones while teenagers have no difficulty hearing them. The frequency of the ring tone is said to be around 17 kilohertz.
A British security firm first created this high frequency sound and named it the Mosquito. The purpose of the Mosquito device is to produce sounds at a very high frequency, so as to make those teenagers who hang around in stores go away. The Mosquito sound was reinvented as a ring tone, and it eventually became popular among teenagers. They use it to communicate with each other during school classes. Because teachers cannot hear the ring tone, they can send SMS to each other during classes.