I keep looking for better ways of protecting my hearing when shooting, and it looks like wearing 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?