I've been casually looking for more scientific studies regarding the efficacy of "bear bells" as a bear deterrent and a haven't come across any yet.
However, the top two bear experts (Smith and Herrero) are NOT giving bear bells a thumbs up! The consensus, so far, seems to be that bells - don't - work!
Apparently, bears seem to regard bells as ambient noise, like the wind or birds. Instead, there is nothing more alarming and Un-forest like than a human voice. So, the best advice is to to make a lot of noise (walking on gravel, stepping on twigs, etc.), but above all, TALK, SING, YELL, any sound with your voice is great.
What about, "Yo, bear!"? Maybe if you're alone, but if you're with others, it's been suggested that you reserve the word "bear" for when there actually Is a bear sighting. That way, others will know if they hear the word "bear" there is a BEAR!!
And bear bells? It's an easy buck or two for REI, and the rest, out of the pocket from the uninformed and inexperienced. They SHOULD be told the protection bells offer is all in their head, but somehow, I doubt it will be mentioned as part of REI sales training.
As I said before, if I find any substantial indication that bells ATTRACT bears, I will revisit this subject!!!