Among drivers, cyclists, pedestrians, I think it's fair to say drivers and pedestrians are the most likely to be using cellphones or have earplugs in their ears while driving/walking. Much has been said about the danger caused by such drivers. But I'm finding it increasingly annoying that many pedestrians belong to this category, too. There have been so many times (more than half) that when I rang bell behind pedestrians, they were as deaf as a beetle, even when you are already closely passing them. In the majority cases I found out they were either on cellphone or listening to something with earplugs.
I don't use Air Zound because it doesn't sound like a car horn anyway and I don't want to worry about theft when parking the bike outside. (Mind you, some folks may not even react when hearing Air Zound.)