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From the BBC:

Police have fitted a device which emits a high-pitched tone only audible to people under 20 to tackle anti-social behaviour at a health centre.

The "mosquito" unit has been installed at the Selston Health Centre near Kirkby in Ashfield, Notts, to combat vandalism and cases of attempted arson.

The noise the unit produces can only be picked up in the hearing range of young people, typically aged under 20.

Supporters said it should help combat some of the serious criminal offences.