IT'S been described as like the sound of nails being dragged down a blackboard .

The irritating "Mosquito" alarm is set at such a specific frequency it can only be detected by young ears.

Now it is being used as the latest weapon in deterring gangs of youths from outside a shop.

Thought to be the first alarm of its kind in the county - store bosses have started playing the dispersal alarm to break-up groups of teenagers.

Frequency is between 18-20kHz - the top end of the human hearing spectrum.

Because of age-related hearing loss, better known as presbycusis, from the age of 20 onwards humans gradually lose their sensitivity to noises within these frequencies leaving them audible only to teenagers and children.

Success Scientists say they have now harnessed this knowledge to create the teenage alarms, which are being installed in places where youngsters hang out and cause trouble.

It is already proving a success at one Hampshire store.

Staff at the Co-op in Sandy Lane, Fair Oak, say they are starting to see real differences in the way youngsters behave outside their shop since they installed the system.

They made the move after becoming increasingly annoyed at the behaviour of some groups of youths, particularly over incidents of abuse aimed at their customers and the setting off of fireworks. Police also admitted they had been forced to deploy extra officers to the area.

Sergeant Tim Adams, said: "Most of these incidents involve young people, many are alcohol fuelled."

Co-op manager Ian Stephens, added: "It does seem to do some good by sending them away and preventing trouble before it can start."