You may be familiar with this sound. It can allegedly be heard only by people under 20. The sound is very high pitched with a sine wave of 18,000 Hz. If you can hear it, your hearing is like that of a teenager. Use it as a ring tone on your iPhone, if you don't want to get those embarrassing calls in the middle of talks when you forget to turn off your phone. If you can't hear it, don't worry, you can still make good use of it. Play it really loud in your front yard to keep those annoying teen thieves away from your house. Here is the link.