Hearing Protection Devices: Fitting, Attenuation, and Fit Testing
NRR and how it is used
Every protector carries a Noise Reduction Rating derived from laboratory testing of ten subjects. The NRR describes attenuation under ideal fitting and is rarely achieved in the field, so OSHA practice derates it: estimated protected exposure = TWA(dBA) − (NRR − 7). NIOSH uses another scheme, applying a 25% derating to earmuffs, 50% to formable foam plugs, and 70% to all other plugs. Dual protection is credited by adding about 5 dB to the higher NRR, not by adding the two ratings.
Selection is not just arithmetic. Comfort, communication needs, compatibility with eyewear and hard hats, and adequate attenuation at the frequencies present all matter. A protector removed for conversation provides almost no protection; the best protector is the one worn consistently.
Fit testing - audiometric (attenuation measured with and without the protector), field microphone-in-real-ear, or controlled-attenuation methods - shows how much protection an individual actually receives and teaches better insertion.
Traps
The NRR is not subtracted directly from exposure; an NRR 25 device does not take 100 dBA to 75 dBA in practice. Overprotection in moderate noise can create communication problems. Earmuffs worn over eyewear lose attenuation. The NRR handles impulse and very low-frequency noise poorly.
Rule of thumb: 100 dBA TWA with an NRR 27 muff gives an estimated 100 − (27 − 7) = 80 dBA.