just a bit of background: when humans are youung, we can pick up on higher pitched noises, but as the ear ages, we lose that ability. some people have been testing this device in the UK called the 'Mosquito", which uses these high pitched noises to trive away teenagers (younger folks).
yes it works, but now some crafty people have taken the technology to a new level, and turned it into a ringtone for a cell phone, and if used to alert when you get a text message, students can hear it in class, but the teacher cant.
read about it here
I thought it was interesting how something ment to drive kids away is now being used for them to comunicate in almost a whole different language.