Hearing Protector Attenuation (OSHA 1910.95 App. B)
The number on the package is never the protection. Appendix B subtracts the NRR straight off a C-weighted reading, but requires taking 7 dB OFF THE NRR FIRST where the reading is A-weighted -- which is what every dosimeter gives you, and it is the most commonly skipped step in hearing conservation. On top of that sits derating, which is guidance rather than Appendix B text: OSHA field guidance halves the adjusted value, and NIOSH cuts the labelled NRR by type -- 25% for muffs, 50% for formable plugs, 70% for all other plugs. An NRR 29 foam plug credits about 8.7 dB by NIOSH, not 29.
Formula and source
C-weighted measurement: attenuation = NRR. A-weighted measurement under OSHA methods: attenuation = NRR - 7. OSHA field guidance halves that adjusted value. NIOSH derating applies to the labelled NRR by protector type - x0.75 earmuffs, x0.50 slow-recovery formable earplugs, x0.30 all other earplugs. Exposure at the ear = TWA - effective attenuation.
OSHA 29 CFR 1910.95 Appendix B (Methods for Estimating the Adequacy of Hearing Protector Attenuation), a US federal regulation in the public domain; derating factors from NIOSH, Criteria for a Recommended Standard: Occupational Noise Exposure (1998).
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