I just installed the loudest low frequency air horn (think train) I think I have ever heard. It was so big (long) that a friend gave it to me after he couldn't figure out where to mount it. I ended up welding it underneath to an inside frame rail and while I'm not 100% sure it's completely out of the way of the rocks, I think it is. I already have an on-board air system with an air tank so the horn gets all the air it needs to make it loud.
It is LOUD... and I'm sure my neighbors didn't appreciate the 4-5 short test blasts while getting it all finalized. It is LOUD and I can't wait to use it the next person who really pulls a dangerous bonehead right in front of me.