Define NRR

Noise Reduction Rating. Noise Reduction Rating is used to evaluate and determine the effectiveness or fit of noise protection equipment for a given work environment. NRR is specifically the measurement, in decibels, of how well hearing protection reduces noise as specified by the Environmental Protection Agency. The actual effectiveness of any individual hearing protector cannot be determined under workplace conditions. However, OSHA’s noise standards (29 CFR 1910.95(j)(2) y 29 CFR 1926.52(b)) require that personal hearing protection be worn to attenuate the occupational noise exposure of employees to within the limits shown in Tables G-16, G-16a, y D-2, respectively. Hearing protectors are evaluated under laboratory conditions specified by the American National Standards Institute in ANSI S3.19-1974 (OSHA’s experience and the published scientific literature indicate that laboratory-obtained real ear attenuation for hearing protectors can seldom be achieved in the workplace).

OSHA’s experience and the published scientific literature have shown that laboratory-obtained real ear attenuation for HPDs can seldom be achieved in the workplace. To adjust for workplace conditions,OSHA strongly recommends applying a 50% correction factor when estimating field attenuation. This is especially important when considering whether engineering controls are to be implemented. The equations above would then be modified as follows:

• Single Protection: Estimated Exposure (dBA) = TWA (dBC) – [NRR x 50%], or Estimated Exposure (dBA) = TWA (dBA) – [(NRR – 7) x 50%]
• Dual Protection: Estimated Exposure (dBA) = TWA (dBC) – [(NRRh x 50%) + 5] , or Estimated Exposure (dBA) = TWA (dBA) – {[(NRRh – 7) x 50%] + 5}