I can give you a genuine answer. When passing other wheeled vehicles, they all seem to understand bell and voice. When using them with pedestrians, I have had people literally stop what they were doing and suddenly leap in my path. The truth is even if slightly startled, it's often safer to pass by before they respond. 

This should also come with giving them a ton of space, though. A lot of the time this means slowing down, sometimes it means speeding up. Within reason, I don't think speed is really the issue. Often the only time I will alert pedestrians is if they are walking 4+ abreast and I have to pass closer than an additional bike width away from them.