If you look at the second short video on the web page:
http://www.guardian.co.uk/world/blog/2009/sep/25/sonic-cannon-g20-pittsburgh
.... you can see that the “peaceful protesters” are all garbed in Global Anarchist Chic, with the black t-shorts, shorts, and mid-calf black stomper boots, not to mention the bandanas and other Global Anarchist regalia.
So in this instance where there no question that they are out to cause mayhem and have violent altercations with the police, that using the sound cannon is the right way to go.
The cannon keeps the police safe while dispersing the Anarchist rats and cockroaches.
But what happens when they start carrying earplugs?
http://www.youtube.com/watch?v=q2PLls02gOU