The Noise Reduction Rating (NRR) is a laboratory-derived single-number rating designed to characterize a hearing protector's noise reduction capabilities. The level of noise entering a person's ear, when the hearing protector is well-fitted and worn as directed, is approximated by the difference between the environmental noise level and the NRR (A-weighted environmental noise measurements must be corrected by subtracting 7 dB). A more precise estimate of field performance can be provided by Individual Fit Testing. This personalized test is performed under typical wear conditions and results in a Personal Attenuation Rating (PAR) for each worker.