Hearing Protection and NRR (Noise Reduction Rating)
If two hearing protection devices are used, an ear plug inside the ear canal followed by ear muffs over the ear, the ultimate noise exposure would be an additional 5 to 10 decibels greater than the higher rated NRR of the two devices.
How is NRR determined anyways? This single number rating is determined by the Environmental Protection Agency as specified under 40 CFR (Code of Federal Regulations) Part 211, Product Noise Labeling, Subpart B - Hearing Protective Devices. The raw data compiled from test subjects is than plugged into a formula which than produces the NRR:
Also, keep in mind that unless hearing protection devices are correctly fit and used properly, the attenuation suggested by NRR may not over-estimated. Indeed, lay-use of such devices without training typically result in sound attenuation far below what NRR may suggest.
The highest rated NRR disposable ear plugs are around 33:
The highest rated NRR ear muffs are around 30: