A frequency selectively audible by age
Here the most universally prevalent disability : selective high-frequency deafness in adulthood, has been put to good use by a student who hijacked an invention designed to scare off teenagers.
The human ear can detect frequencies between 20 and 20,000 hz.. The frequencies of the human voice being between 100 and 8,000 hz. (The highest note on a piano is around 5,000 hz and the lowest around 40 hz).
However, most humans' hearing degrades significantly with age; by the age of 40 we can only perceive between the range of 20 - 15,000 hz and by the age of 50 between 20 - 12,000 hz.
Hence the idea of a security technician in England to emit very high frequency sounds in front of some storefronts to scare away teenagers without bothering adult customers.
And then the student's line about designing a mobile phone ringtone that can only be heard by teenagers where adults forbid communication devices, such as in classrooms....
Various variations of these ringtones exist, all above 17 khertz.
Do you not hear anything? That's because you're not a teenager anymore. In fact, that's why these ringtones have been nicknamed "Teen Buzz". (A mosquito flaps its wings at about 700-1500 hz, but that's another story)
Tested (I'm in my late 40s), I can only guess at a faint crackling sound if I listen carefully with the speakers pushed in, while my kids react strongly calling the sound unpleasant and very audible. In the next room they still hear it distinctly while I perceive strictly nothing...
Bref, the old teachers will have their work cut out for them in detecting the ringing of their students' phones during their classes. Maybe by getting a young substitute teacher!"
To make sure your hearing is up to par, we suggest using a frequency generator like this one. If you can't hear anything above 15,000 hz, you are truly an adult.