Mosquito Noise

The "mosquito frequency" is a sound that can only be heard by people under 20. It is a sine wave at 18,000 Hz. This sound is used by some teenagers as a ring tone on their mobile phone so that only they can tell when the phone is ringing. It is also used in England to play very loud in areas that authorities don’t want teens to congregate in, as the noise annoys them. As people get older they lose the ability to hear higher pitched sounds.