NRR - Noise Reduction Rating - Help

All hearing protectors supplied in the USA should be provided with an NRR figure. It can be used to estimate the sound level at the ear when wearing the protectors.
To calculate the level at the ear, fist measure the worker's noise exposure, either in dB(A) or dB(C). This level can be entered into the NRR Calculator, along with information about the regulations that you are following: OSHA or NIOSH as each of these uses a different calculation.

The NoiseMeters NRR Calculator uses the following calculation methods:

Regs	Freq. Wgt	Calculation

OSHA	A	Lprot = L - (NRR - 7) / 2

	C	Lprot = L - NRR / 2

NIOSH	A	Lprot = L - (NRR - 7)

	C	Foam Earplugs: Lprot = L - (NRR * 0.5)
		Other Earplugs: Lprot = L - (NRR * 0.3)
		Earmuffs: Lprot = L - (NRR * 0.75)

Where:
L is the measured level or TWA
NRR is the Noise Reduction Rating provided by the hearing protector manufacturer
Lprot is the estimated sound level at the ear when wearing the hearing protection

for more info: please see the ff: https://www.noisemeters.com/apps/occ/help/protector-nrr.asp