My bell usually causes them to start looking around for the source of the sound. They often think they have kicked something on the path. Saying "passing left" seems to work best and is less ambiguous than "on your left". However, many people are seemingly lost in thought or plugged into their personal sound systems and either startle randomly or totally miss any warning unless it approaches the threshold of pain in volume. Avoiding a collision and/or injury to all concerned is my first priority followed by a reasonable level of consideration for fellow beings.