Just to be clear, air horns are NOT allowed in IG Field.
Ya, I know. But the problem with MTL is no one is defining "air horns". They used to be horns attached to compressed air cans. MTL fans are getting around this with hand-pumped air into the same type of horns. The result is the same but requires a little work by the fan; albeit with an unlimited air supply.
Step 1 here is the CFL and the stadium clearly defining "air horns" to include the hand-pumped variety. Then just enforce the rules.
BUT... will mouth-blown horns get sucked into this as well... how do you define "horns" carefully enough to exclude blown horns. Technically they are all "air horns". I don't have any horns, but many Bomber fans I've sat beside like their blue horns, and the low noise they make is not offensive, nor does it carry over all the other din.