two things:
just make sure they are
compression models, not cut off. compression types make the louder sounds instantly quieter, the cut-off types clip the sound entirely, leaving you without hearing for a second or two.
and be sure it has
two microphone pick-ups so you get stereo sound. this allows you to tell direction of sounds, very precisely.
i bought Gentex Corporation Wolf Ears Model 1030A years ago which were the only muffs to meet those specs at the time (they are no longer in production).
yet, that said, this review says there are even better models available today:
http://www.gun-tests.com/performance/dec96earmuff.htmlmy suggestion: buy the best protection you can, with the highest dB rating, regardless of price. they will protect your hearing (mine is screwed up) and allow you to defend yourself or hunt without hearing loss.