The "mosquito tone" is simply a 17KHz sine wave that geek teens have been known to use on their cell phone to alert them when they've got a text message so the teachers can't hear it.

In addition, the "mosquito tone" is also being used by adults as a solution to the problem of unwanted gatherings of youths and teens in shopping malls, around shops, railway companies, and even in high school hallways. These devices are being used all over the world.

If you are under the age of 25 you can hear the mosquito sound, but over 25 probably not.
Use this Mosquito Sound Player to see how well you can hear the "mosquito tone" (17KHz). The higher the number the better your hearing. Each MP3 tone is 5 seconds long.