A British security company originally marketed The Mosquito as an "ultrasonic teenager repellant." Retailers in England broadcast the sound outside their stores to prevent teenagers from loitering. Older customers couldn't hear it. But after just a few minutes, the sound became so grating to teenagers, they'd leave the area.
The technology behind the teen repellent relies on the fact that, as we age, we gradually lose our ability to hear high-pitched sounds. Typically, humans can only hear pitches that fall between 20 hertz and 20,000 hertz. The Mosquito emits a buzz that comes in at 17,000 hertz, at the very top of the human hearing range. By age 25, most people can no longer detect such high pitches.
It wasn't long before teenagers co-opted The Mosquito and made it their own. Students from Europe to the United States are downloading The Mosquito (or one of its many knock-off versions) onto their cell phones. They say they can now send text messages in class without getting in trouble. As long as their teachers are old enough, they can't hear the telltale ringtones.
The idea of an inaudible ringtone got reporter Nikki Tundel thinking about all the other types of communication we can't hear.