No matter what you use, there's always going to be someone who won't notice it.
Bike bell: Once on a path through the park, I rang my bell as I was approaching a pedestrian from behind. She looked up at the sky thinking it was a bird. Many times, elderly people also can't hear it.
A few weeks ago, riding along a separated bike lane, I rang my bell at a guy who was crossing without looking. He didn't even hear the bell until all his friends yelled at him.
Air Zound: Riding along a protected bike lane, I sounded my Airzounds at a guy standing on the bike lane staring up at the building or something on the other side of the street. He just couldn't hear it until I stopped my bike right beside him and yelled "Hey!".
Loud Bike Car Horn: Just yesterday in a protected bike lane, I was almost right-hooked by a commercial van. I sounded the car-horn but I guess he was determined to race through the turn and to hell with the cyclists and pedestrians.
Sometimes I think the best thing is to keep your caliper breaks squeeky.