In my experience bells don't do ****. They certainly are not more effective then yelling "look out!" or "heads up" etc. The same people who are likely to swerve if you yell are likely to swerve/freak out if you ring your bell. Also too many idiots use a bell as an imaginary safety net. They come up on a group of cars or pedestrians or whatever and just ring the **** out of their bell assuming that everything will be fine. Making your presence known is a good thing but being aware, slowing down when things get congested and just generally riding defensively takes you a lot further imo.