The one draw back of the cannons is that could literally deafen you if you happen to be exposed to too much noise but then again that might not be a bad thing, if European clubs are anything to go by. Every fucking time I go anywhere it's the same songs they have on the radio, and then you get forced to some over expensive bar where a beer is like €8 even though the shop outside sells them for €0.50 because they have a light below a palm tree and that's really fancy.
In some European countries, football violence is actually quite serious so these might be a necessary deterrent or less violent stopping method rather than having riot police charging at drunk people with garden furniture. Alternatively it might make the battle scenes look awesome on the news as Rage Against the Machine blasts over the speaker, or maybe some Shakira to make it more sexy, or the Benny Hill theme tune to make it more hilarious.
The actual noise it makes should be this: