Noise reduction rating (NRR)
The lab-tested attenuation printed on a hearing protector, used to estimate the wearer's exposure.
Definition, in the source's words
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.”
What it means
Subtract 7 dB when you're working from A-weighted measurements; with C-weighted measurements, subtract the NRR directly. OSHA's Technical Manual adds a 50% derating when deciding whether engineering controls are needed, and credits only 5 dB more when two protectors are worn together.
Example
TWA 100 dBA, earmuffs NRR 33. Estimated exposure = 100 − (33 − 7) = 74 dBA. With OSHA's 50% safety factor: 100 − (33 − 7) × 0.5 = 87 dBA.
Where it appears on the exam
ASP: ASP11 Domain 6, Industrial Hygiene and Occupational Health
CSP: CSP11 Domain 6, Occupational Health and Applied Science
Practice: Occupational Noise
CHST: CHST5 Domain 1, Hazard and Risk Identification and Control
Questions
- Why subtract 7 from the NRR?
- Appendix B of 1910.95 says to subtract 7 dB when your measurements are A-weighted; with C-weighted measurements you subtract the NRR directly.
Related
- Action level
- Standard threshold shift (STS)
- Formula: Hearing protector NRR (estimated exposure)
- Formula: Noise TWA from dose
- Regulation: 29 CFR 1910.95
Checked 2026-10-16. Spot a problem? Email hello@passcited.com.