The SNR (Single Number Rating) value is a European standard that allows you to compare the degree of noise reduction of different hearing protectors. Independent laboratories test earplugs and earmuffs to determine the SNR value of an earplug. In the United States, not an SNR, but an NRR (Noise Reduction Rating) is used. That standard is slightly stricter than the European standard. The higher these ratings are, the more the hearing protection attenuates.
Often, both NRR and SNR ratings are listed on Alpine's product packaging.