Do you hear that mosquito? Of course you don’t – you’re over 30 years old.
Someone on the cutting edge of radio – probably John Batchelor, maybe Rush or Kim Komando had mentioned this thing I was just reminded of again.
Some bright kid somewhere realized that adults universally lose high frequency hearing – the cells for that are the first in the cochlea and take the brunt of loud sounds. The actual mechanism is the hairs in the cochlea convert the mechanical vibrations into neuro electrical energy, which requires energy and oxygen from blood flow. When a hair is over stimulated, it starts to deplete the oxygen supply. That results in constricting the blood flow in a protective reaction, but that ultimately kills the nerve cell.
What this means is the sounds in the range made by mosquitoes cannot be heard by most adults, but can be by children. Clever children then set the ringer and text alert ringtones to the mosquito tones, knowing that their teachers and parents will be unable to hear them. If children in your custody seem to communicate behind your back, maybe you know how they do it now.