Dual Protection NRR

For dual protection (ear muffs and plugs are used simultaneously) use the following:

• Determine the laboratory-based NRR for the higher rated protector (NRRh).
• Subtract 7 dB from NRRh if using A-weighted sound level data.
• Add 5 dB to the field-adjusted NRR to account for the use of the second hearing protector.
• Subtract the remainder from the TWA as follows:

Estimated Exposure (dBA) = TWA (dBC) – (NRRh + 5)

Estimated Exposure (dBA) = TWA (dBA) – [(NRRh- 7) + 5] 

Example:TWA=110 dBA, plug NRR=29, and muff NRR=25 dB Estimated Exposure = 110 – [(29 – 7) + 5] = 83 dBA