Decibels is abbreviated to dB. "fb" is possibly feedback but that's of no relevance here.
I think you're actually asking about attenuation, ie, the effectiveness of blocking out sound. I only once measured my plugs on a "real ear" probe mic system and from rough memory, they were giving a good 35dB attenuation from 1K up. You will only get that outcome from a good fit and this is the result of high-quality impressions and correct treatment by the shell lab. If either of those isn't good, the fit will be hopeless and the electronics won't save you.
I've only had experience with Peltor muffs so can't comment on any in-the-ear options.
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