It's one of those "I heard a news story about this thing ages ago but can't find it on the internet"

Story went:

Number of teenage ne'erdowells assembling in wotcouldonlybedescribedas a threatening-looking group on part of estate, much to annoyance of older residents.

Boffin comes up with the idea of blaring a high frequency sound, audible only to those of more tender years (your range of audible frequencies decreases with age)

Tykes steer clear

very sneaky, if very duffer.