Depending on which bike that I am on, I have one, three, or four signals that I use.
With a clear DING DONG, I get 60% reaction
Followed by BRIIING BRIING, I get 85% reaction
A teenie ding, ding by itself doesn't add anything
"On your left" might bring it to 90%
For those who've kept track I have one left. I have never run across a pedestrian who is doing something so stupid, something that I can't avoid, that I need it. My conclusion is ten percent will not hear you using conventional signalling methods.
The fourth is an automotive horn,
Loud Bicycle | Car horn for bikes
But I run across golf cart drivers all the time that do something so stupid that it is necessary to talk car to them. It has saved me more than once, for this reason it is necessary for me. Now I average 4 - 5,000 miles annually, so your needs and bell budget are different.
I am certain that I have done more analysis of bells, the tone of bells, the reaction of pedestrians to signals than most posters here and I am convinced there is no way to get much more than 90 percent acknowledgement of a bikes presence. But I do get quite a few thanks for warning people..