A characteristic of sound that is apparently syncronized with onscreen action of secondary importance. 

This technique is often used for the linking of stock sound footage to mobs, parades, battles, or other large-scale scenes where the viewer cannot possible check whether each sound is actually synchronized to an onscreen image. 

It is heavely used by televison news to accompany background action still photographs 

Rick Altman: Sound Theory/Sound Practice page 251 

22/3 1997