Hearing protector NRR (estimated exposure)
Used on the ASP, CSP and CHST. The worked example below is computed by code and checked against the source.
Single, A-weighted: TWA − (NRR − 7)
Single, C-weighted: TWA − NRR
Dual, A-weighted: TWA − [(NRRh − 7) + 5]
TWA = the employee's 8-hour TWA; NRR = the protector's noise reduction rating; NRRh = the higher NRR of the two protectors worn together. For OSHA's 50% safety factor, halve the field-adjusted NRR before adding the 5 dB.
Worked example
OSHA example: 110 dBA TWA, earplugs NRR 29 and earmuffs NRR 25 worn together.
110 − [(29 − 7) + 5] = 83 dBA
The source prints this result, and our code reproduces it.
Single protector, same TWA, NRR 29 (A-weighted): 110 − (29 − 7) = 88 dBA (1910.95 Appendix B).
Run your own numbers: hearing protection NRR calculator.
Where it appears on the exam
ASP: ASP11 Domain 1, Mathematical Calculations
Blueprint: ASP11 Domain 1, task 5: noise hazards
CSP: CSP11 Domain 6, Occupational Health and Applied Science
Blueprint: CSP11 Domain 6, task 1: occupational exposures (noise)
Practice: Occupational Noise
CHST: CHST5 Domain 1, Hazard and Risk Identification and Control
Blueprint: CHST5 Domain 1, knowledge 18: noise exposure
Source
The formula comes from
OSHA Technical Manual, Section III, Chapter 5 (Noise), Appendix F(opens osha.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”
Questions
- What is the formula for hearing protector NRR (estimated exposure)?
- Single, A-weighted: TWA − (NRR − 7); Single, C-weighted: TWA − NRR; Dual, A-weighted: TWA − [(NRRh − 7) + 5]. TWA = the employee's 8-hour TWA; NRR = the protector's noise reduction rating; NRRh = the higher NRR of the two protectors worn together. For OSHA's 50% safety factor, halve the field-adjusted NRR before adding the 5 dB.
- Will this formula be given on the BCSP exam?
- BCSP says the formula is included in the question whenever one is needed, unless the question is testing whether you know the formula. You can't bring a formula sheet into the exam, so learn what each formula does and when to use it.
BCSP on formulas at the exam
BCSP, What to Expect When Sitting for a BCSP Exam (item 7)(opens bcsp.org in a new tab)“For any question requiring a mathematical formula, the formula will be included within the exam question (unless the question is specifically testing your knowledge of a formula). You will not be provided or permitted to bring a formula sheet.”
Related
- Noise reduction rating (NRR)
- Noise dose
- Noise TWA from dose
- Reference duration for a sound level
- Adding decibels
- Sound level at a distance
- ASP formula sheet
- CSP formula sheet
- CHST formula sheet
Checked 2026-10-16. Spot a problem? Email hello@passcited.com.