They’re using LRAD on the protesters.
http://gizmodo.com/what-is-the-lrad-sound-cannon-5860592
http://www.thenewscommenter.com/news/communist-group-from-chicago-bussing-members-in-to-st-louis-to-hijack-the-ferguson-protests/163802#.U_KpM9vU38Q.twitter