NRR Method (USA)
When hearing protection is worn, your level of exposure is based on the NRR rating of the protection device being used. Keep in mind however that while the NRR is measured in decibels, the hearing protector being used does not reduce the noise exposure to the ear by the same number of decibels associated with the protector. For example if you are at a rock concert and the noise you are exposed to is 100 dB and you put on a pair of NRR 27 muffs the exposure will not be reduced to 73 dB, as you may expect. Rather the reduction is calculated like this, (27-7/2 = 10 dB)
Therefore you are exposed to 100dB and wear a pair of NRR 27 rated muffs 100-10 = 90.0 dB exposure to the ear.
These are Class 1 muffs under the Australian system (the lowest/poorest)
The SLC80 method (Australia)
The workplace noise is measured in C-weighted sound pressure levels as average over the shift, [LCeq,8h]. The SLC80 (Sound Level Conversion valid for 80% of the wearers) of the hearing protector indicates the difference between the measured C-weighted sound pressure level of the workplace noise outside the hearing protector and the A-weighted sound pressure level, attenuated by the hearing protector, under the hearing protector inside the ear canals.
For example the noise level in a workplace was 100 dB(C) (as average over the shift). If you are exposed to a noise level of 100 dB and then wear a Class 5 muff with a rating of 30 dB
100 – 30 = 70 dB(A) exposure to the ear
Australian class system
Class 1. 10 - 13
Class 2. 14 - 17
Class 3. 18 – 21
Class 4. 22 - 25
Class 5. 26 or greater
I hope this further explains the system. Beware of NRR muffs!