Bells work. They are not silly, useless appendages on your bars and will not result in speed loss.
They cut through idle chatter between twalkers, general traffic noise and most other white noise associated with human activity. Humans attenuate noises in this category very well.
The voice can be associated with other talkers nearby, or with general background murmering by people some distance away. Human voices blend with the cacaphony around them and so can be mis-understood or missed entirely.
That's why it can be so aggravating to try to communicate your approach to people. They're not stupid, lame, or deaf - they just can't pick you out.
At a hundred feet I sound off. If, at about fifty feet they haven't a clue, I hit it again. Bingo every time. Bells are a good idea. Even an important idea. Mine is about 1-1/4" in diameter and cost $5.95 at Target.
True Story:
Last summer I was riding the Burke-Gilman Trail which is very popular with everyone - including morons who feel it's their God-given right to go as fast as possible around baby strollers. While manouvering thru some twalkers, I came upon two gals who were chatting it up on their power-walk. I politely gave the ubiquitous "on yer left" and they responded by crashing into each other as one of them went right and the other went left. A lady coming the other direction on a Huffy tanker went of the trail who-knows-where. I nudged them both harmlessly but that was enough for me. I got a bell and there have been no more dust-ups.