Hearing Protector Attenuation (OSHA 1910.95 App. B)
The number on the package is never the protection.
Example
You enter
- Measured 8-hr TWA (dB) 98
- Weighting of the measurement A
- Noise reduction rating on the package (dB) 29
- Method niosh-other
- Plugs and muffs worn together? no
- Dual-protection bonus (dB; editable, not from Appendix B) 5
- Target exposure at the ear (dB) 85
You get
- Derated nrr db 8.7
- Effective attenuation db 1.7
- Protected TWA db 96.3
- Label vs real db 27.3
- Spectral adjustment 7
Checked against published worked examples from: Occupational Safety and Health Administration.
Details, formula, and sources
ScopeAppendix 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. NIOSH also takes the 7 dB off the derated value for a dBA reading, so an NRR 29 pre-molded plug credits about 1.7 dB on a dBA job, not 29.
Formula or tableC-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 - and an A-weighted measurement then takes 7 dB off the derated value. Exposure at the ear = TWA - effective attenuation.
Source and editionOSHA 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).
Free access29 CFR is published in full at no cost by OSHA and the eCFR; the NIOSH criteria document is published at no cost by CDC/NIOSH.
What governsEstimate. AHJ and licensed professional govern.
API field namestwa_db, weighting, nrr_db, method, dual_protection, dual_bonus_db, target_db, derated_nrr_db, effective_attenuation_db, protected_twa_db, label_vs_real_db, spectral_adjustment_db
- C-weighted measurement the NRR is subtracted directly29 CFR 1910.95 App. B
- A-weighted measurement subtract 7 dB from the NRR, then subtract the remainder from the TWA29 CFR 1910.95 App. B
- OSHA 50% derating halves the adjusted value; enforcement guidance, not Appendix B textOSHA field guidance
- NIOSH derating earmuffs -25%, slow-recovery formable earplugs -50%, all other earplugs -70% of the labelled NRRNIOSH Criteria for a Recommended Standard, Occupational Noise Exposure (1998)
- NIOSH and the 7 dB an A-weighted exposure takes the derated NRR minus 7 dBNIOSH Criteria for a Recommended Standard, Occupational Noise Exposure (1998)
- Dual protection editable bonus, defaulted to 5 dB per OSHA Technical Manual guidance; not from Appendix BOSHA Technical Manual