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About this tool
Turn an NRR or SNR label into an estimated protected level using the OSHA, OSHA 50% and NIOSH derating methods, including dual protection.
The Noise Reduction Rating Calculator estimates the A-weighted sound level actually reaching a worker's ear by applying the published field-adjustment formulas to a hearing protector's labelled NRR or SNR. It covers the three methods safety officers are asked for: the plain OSHA formula (dBA − [NRR − 7]), OSHA's recommended 50% safety factor (dBA − [(NRR − 7) × 50%]), and the NIOSH deratings from Publication 98-126 that cut the label by 25% for earmuffs, 50% for formable foam earplugs and 70% for all other earplugs. It also handles dual protection using the 5 dB allowance added to the higher-rated device, then checks the resulting level against the OSHA permissible exposure limit of 90 dBA and the NIOSH recommended limit of 85 dBA.
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Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
The comparison table runs the same protector through the plain OSHA formula, the OSHA 50% safety factor and the NIOSH device derating, so you can see how far apart the estimates are before choosing a device.
The 7 dB spectral correction exists only because NRR is derived from C-weighted test data — so it is applied to a dBA measurement and skipped entirely for a dBC one, instead of being subtracted blindly.
A level below 70 dBA at the ear is called out as overprotection, because a worker who cannot hear alarms, speech or machine cues is not safer for wearing a higher-rated protector.
For a level measured in dBA, estimated exposure = dBA − (NRR − 7). The 7 dB is subtracted because NRR is derived from C-weighted laboratory data. For a level measured in dBC, no correction applies and estimated exposure = dBC − NRR. Example: NRR 33 plugs in 100 dBA give 100 − (33 − 7) = 74 dBA before any safety factor.
Because laboratory NRR values are obtained under trained, experimenter-supervised fitting that real workplaces do not reproduce, so OSHA's directive CPL 02-02-035 recommends halving the field-adjusted value: dBA − [(NRR − 7) × 50%]. The same NRR 33 plug in 100 dBA then gives 87 dBA rather than 74 dBA — under the 90 dBA permissible limit, but above the 85 dBA NIOSH limit.
NIOSH Publication 98-126 says to subtract 25% from a labelled earmuff rating, 50% from a formable (slow-recovery foam) earplug rating and 70% from all other earplug ratings, then apply the 7 dB A-weighting correction. NRR 33 foam plugs therefore give 33 × 0.5 − 7 = 9.5 dB of usable attenuation, so 100 dBA becomes 90.5 dBA.
No. Both OSHA and NIOSH allow only 5 dB to be added to the higher of the two labelled ratings, because bone conduction sets a practical ceiling of roughly 40 to 50 dB regardless of what is on the ear. NRR 33 plugs under NRR 25 muffs are treated as NRR 38, not 58, before the chosen derating method is applied.
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