My son told me about this one. He saw a story on the news. It seems a number of school students have downloaded ring tones on their mobiles that sound like annoying mosquito drones but the frequency is such that nobody under the age of 30 can hear them. That way they manage to text messages to each other in class without the teachers hearing, although all of the students can.
We had to look this phenomena up on internet to put it to the test. Visit www.freemosquitoringtones.org and check it out for yourselves. They have a range of mosquito drones starting from one that everyone can hear all the way down to one that you can hear only if you are 18 years old or under.
Of course I could hear the one that everyone can hear. I could clearly hear the one that people under 50 can hear but by the time it got down to the one that only people under 39 can hear, it was so extremely faint as to be almost inaudible. And Logan expressed his amazement that I couldn't hear it because to him it was identical to the one that 50 year olds can hear. Of course by the time it got down to the ones that only people under 24 and teens could hear, I could hear absolutely nothing! Amazing.
Logan tells me a fellow on the news tested them out and when he couldn't hear them, he thought it was all a big hoax, but a young visitor in the next room called out, "Can you stop making that awful racket!" And it seems most school students choose the tones for under 30s for their classtime text-messaging.