The NRR (Noise Reduction Rating) value is a United States standard that allows you to compare the degree of noise reduction of different hearing protectors. Independent laboratories test earplugs and earmuffs to determine their rating. In Europe, the SNR (Single Number Rating) is used instead of the NRR. This rating is slightly less strict than the US standard. The higher these ratings are, the more the hearing protection attenuates.
Often times, both NRR and SNR ratings are communicated on Alpine product packages.