Zound Earplugs have a NRR (noise reduction rating) of 21 dB. The APV is the minimum attenuation protection provided by the hearing protector (in each of the octave frequency bands) for a percentage of the population. It is the APV that is used to predict the noise levels ‘in the ear’ with the hearing protector in place.
So, the noise level ‘in the ear’ (dB) = the noise level external to the hearing protector (dB) – APV(dB), in each of the octave frequency bands.