I agree with 2_i in that people don't recongnize or know what the high pitched, soft 'ding' is from the 'spring & hit once' bells. A conventional cartoon sound effect 'ring ring' bell is most effective. Saying 'on your left' is just about as useless as having a mini bell. I find that an extra loud (and deep-voiced if you're a guy) almost aggrevated sounding "EXCUSE ME!" even gets people on their cell phones to turn around and see who dares to yell at them. Squeaking brakes work well too, but I find them to be embarrassing when i'm around other cyclists. That and you can't sneak around very well either.