About a month ago, you may have heard that someone in America had been spreading round a sound clip which is extremely annoying to listen to! Now here's the science: As a youngster, you ears can hear up to 44,000 Hz. But at the end of your teenage years, this slowly drops down to 20,000 Hz roughly. This means you can hear less sounds. Sounds above 20,000 Hz are 'sonic' or 'ultrasonic'.
This means that when you get older, you can't hear the sound.
And teenagers have cleverly been using this sound clip as an advantage over adults. For example, if you had this sound clip on a mobile phone as a ringtone, no adults would be able to hear it! Cool, huh?
However, because the noise is terribly annoying to listen to, some shop owners have also been using it as a deterrent to get evil-looking teenagers away from their shops!
Here's some links:
Google Search - Sonic Teenager Deterrent
Google Search - teen buzz (download links as well)