Hearing protection NRR calculator
At or below 90 dBA: meets 1910.95(j).
OSHA uses this when deciding whether engineering controls are needed.
Formulas and sources
- dBA: exposure = TWA − (NRR − 7)
- dBC: exposure = TWA − NRR
- Plugs and muffs: exposure = TWA − [(NRRh − 7) + 5]
- 50% factor: exposure = TWA − [(NRR − 7) × 50%], or − {[(NRRh − 7) × 50%] + 5} for dual
The formula comes from
29 CFR 1910.95, Appendix B (mandatory)(opens ecfr.gov in a new tab)“(iii) When using a sound level meter set to the A-weighting network: (A) Obtain the employee's A-weighted TWA. (B) Subtract 7 dB from the NRR, and subtract the remainder from the A-weighted TWA to obtain the estimated A-weighted TWA under the ear protector. (iv) When using a sound level meter set on the C-weighting network: (A) Obtain a representative sample of the C-weighted sound levels in the employee's environment. (B) Subtract the NRR from the C-weighted average sound level to obtain the estimated A-weighted TWA under the ear protector.”
The formula comes from
OSHA Technical Manual, Section III, Chapter 5 (Noise), Appendix F(opens ecfr.gov in a new tab)“Determine the hearing protector with the higher rated NRR (NRRh) and subtract 7 dB if using A-weighted sound level data. Add 5 dB to this field-adjusted NRR to account for the use of the second hearing protector. Subtract the remainder from the TWA. It is important to note that using such double protection will add only 5 dB of attenuation. ... Estimated Exposure = 110 - [(29 - 7) + 5] = 83 dBA”
The formula comes from
OSHA Technical Manual, Section III, Chapter 5 (Noise), Appendix F(opens ecfr.gov in a new tab)“When applying the 50% safety factor for estimating field attenuation (required when considering whether engineering controls are to be implemented), the above equations are 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}”
Source
29 CFR 1910.95(j)(2)-(3)(opens ecfr.gov in a new tab)“(2) Hearing protectors must attenuate employee exposure at least to an 8-hour time-weighted average of 90 decibels as required by paragraph (b) of this section. (3) For employees who have experienced a standard threshold shift, hearing protectors must attenuate employee exposure to an 8-hour time-weighted average of 85 decibels or below.”
FAQ
How do you calculate NRR for hearing protection?
With an A-weighted TWA, subtract 7 dB from the NRR and subtract the remainder from the TWA: protected exposure = TWA − (NRR − 7). With a C-weighted level, subtract the NRR directly. Both methods are in mandatory Appendix B to 29 CFR 1910.95.
Why subtract 7 dB from the NRR?
The NRR is meant to be subtracted from a C-weighted level. The OSHA Technical Manual says that if C-weighted data isn't available, A-weighted data can be used by subtracting a 7 dB correction factor from the NRR. Appendix B to 1910.95 sets out the same step.
How do you calculate NRR for double hearing protection?
Take the higher of the two NRRs, subtract 7 dB if your data is A-weighted, and add 5 dB for the second protector. OSHA's example: TWA 110 dBA, plug NRR 29, muff NRR 25 gives 110 − [(29 − 7) + 5] = 83 dBA. Never add the two NRRs together.
What is OSHA's 50% NRR derating?
The OSHA Technical Manual applies a 50% safety factor to the field-adjusted NRR when deciding whether engineering controls are needed: TWA − [(NRR − 7) × 50%]. Example: 98 dBA with an NRR 25 protector gives 89 dBA. For judging whether protectors are adequate under the hearing conservation rules, OSHA uses the 7 dB subtraction alone (98 − 18 = 80 dBA).
How much attenuation do hearing protectors need?
Under 1910.95(j), enough to bring the worker's 8-hour TWA to 90 dBA at least, or to 85 dBA or below for a worker who has had a standard threshold shift.
Need the TWA first? Use the noise dose and TWA calculator, or the decibel addition calculator to combine sources. Calculated attenuation only holds when protectors are fitted and worn correctly, as Appendix B itself notes. This tool is for study and estimation.