I remember reading a while back about Mosquito, a “youth deterrent system” which is described as a speaker system that emits high-frequency tones that are only heard by teens.
t works by broadcasting unpleasing sounds at a frequency that can be heard by younger people only. It has an effective range of about 20 meters.
Well now techno-savvy pupils have recorded the ultra-high sound – audible only to under-20s- onto their cell phones, and are now receiving calls and text messages in class – without teachers having the faintest idea of what is going on.
This sounds pretty far-fetched but is based on a real condition called Presbycusis, loss of hearing that occurs in individuals as they grow older. Interesting stuff!