London, Reuters--What could annoy teenagers enough to make them stop hanging out with friends and go home? No, it's not a visit from their mothers, and not a threat to take away their cellphones or pocket money. It's high-frequency noise. The UK police recently agreed to use a device   called the Sonic Teenager Deterrent. It sends out a sound that makes teenagers become so impatient and angry that they have to cover their ears tightly and walk away. The sound is at extreme high-pitch that can be heard by those under 20. The body's natural ability to detect some wave bands decreases almost entirely after 20, so few adults can hear the sounds. The black-box device, nicknamed the Mosquito because of its sound, can be fixed to the outside walls of shops, offices and homes. It sounds to youngsters like a crazy insect or a badly played violin. But it causes no physical damage. A number of police forces and councils have given permission to use the system and want to install   it at trouble spots. Staffordshire Police Inspector Amanda Davies, who has given the device to shopkeepers in the Moorlands area, said," It is controlled by the shopkeepers--if they can see through their window that there is a problem, they turn the device on for a while until the group has run away." From the passage we can know that   _  .
young people often suffer from pains in ears shopkeepers are troubled by noisy insects high-frequency noise is beyond the listening ability of people over 20 the police invented a new device to deal with teenagers