By Gary (Kevin) Wolf, COHC - Certified Occupational Hearing Conservationist
I think it's important to point out that N.I.O.S.H. has concluded that, for occupational applications, the protection average of stock, in the ear, hearing protection with a stated 30-Db. NRR. is actually closer to 11 Db. NRR. . This conclusion sights a few factors including the way they are worn, ear canal size and shape and their tendency to work themselves loose.
Whereas, N.I.O.S.H. has concluded that custom earplugs, if made well, have an average of only about 2 Db. NRR. variance from the stated rating.
Additionally, I assert, (as is evidenced by the fact that the highest rated hearing protection in the form of an earmuff is 30 Db. NRR.), that due to bone conduction... the absolute highest amount of hearing protection one could ever receive is between 30 to 31 Db. NRR. .