I have looked around a various silencers, and talked to my local LEO (just to make sure they would sign) and have decided to get my first can (maybe two).  I have noticed some suppressors list -30db.  Does this mean that they knock 30db off of the un-suppressed report of the caliber that it's made for?  The reason I ask is when looking a rifle silencers some say 30db (with no "-" in front), does this mean the sound generated w/ the suppressor is 30db?  How many decibels does an un-suppressed .308 round make vs. say a .45 pistol (I know barrel length and environmental conditions come in to play)

Any guidance is appreciated.

Thanks