I have a few that are really quiet, i don't know nothing about and DB ratings but one ruger precision you can only hear the firing pin hit. now this is using sub sonic ammo too. even with regular ammo it's really quiet. using a dead air mask. Different ammo has a lot to do with it too. i also have some 9mm that are really quiet. also i have noticed different guns are quieter than others using same ammo and suppressor. blowback will make some sound, some allow more than others.
But far as suppressing 5.56 and up, no they are not whisper quiet but they are noticeably reduced sound and Some AR's are quieter than others. The more slack between the halves the more blowback and sound.