I keep looking for better ways of protecting my hearing when shooting, and it looks like doubling up (where you wear both plugs and muffs) is recommended for indoor shooting (centerfire gunshots range from 160-170 decibels).
However, if you look at the OSHA recommendations, one will notice that they say that doing so only adds,
Note: 5 dB can be added to the NRR of the most protective device when using both earplugs and muffs.
Given that earplugs often have a NRR of 32 decibels and muffs 22-28 decibels, why is doubling up not more effective?
Wouldn't the noise be reduced by 22 decibels by the muffs and then 32 by the plugs instead of just 5 more decibels?