Such cannons can be handled safely, if the crew knows what it's doing. I am (or was) a Civil War reenactor and have been around some big guns. At the 1994 Franklin, Tenn. event (130th anniversary) there were nearly 100 cannons on the field, among 12,000 reenactors and a like number of spectators. No one got hurt during the 3-day event. No telling how many cannon blasts....many thousands in total...there were. The ground shakes from the concussion of the guns...they use 1 pound of black powder each time...car alarms in the spectator parking lot, more than a mile away, were going off!
In a stadium, even an outdoor stadium, like JAS, they would scale back the powder, to a 1/2 pound or less. It would still be plenty loud. But, point is, those reenactors who handle big guns know what they're doing, they train, they follow very regimented procedures all designed to be safe.