A bell or 6v car horn is not going to cut it you need something more like this AMAZING-Train Horns On A Bicycle.flv - YouTube
Holy Cats! I just read about it - 115 earsplitting decibels! And no batteries, no CO2 cartridges - you just pump it up once in a while with a bike pump. And it still manages to be kinda small. Hmmm...Okay, I'm tempted. I'm starting to want one.For a loud as heck horn, try one of these:
Airzound Bike Horn -- Delta Cycle