An alarm designed to clear teenaged gangs out of shopping centres has been sampled by teenagers so they can hear their phone ringing in class. It works because the noise cannot be heard by anyone over the age of 20.

The Mosquito security product was created by Welsh firm Compound Security and is based on a medical phenomenon known as presbycusis, or age-related hearing loss. 

Anyone turning 20 begins to lose their hearing, starting with the highest frequencies from 18KHz to 20KHz. The company used this information to generate a high-frequency sound that is audible only to teenagers.

Teenagers have now sampled the sound, known as Teen Buzz to those swapping it over Bluetooth and text messages, so they can hear their phone ringing without the teacher knowing.