40 C.F.R. § 211.207
Calculate the NRR for hearing protective devices by substituting the average attenuation values and standard deviations for the pertinent protector category for the sample data used in steps #6 and #7 in Figure 2. The values of -.2, 0, 0, 0, -.2, -.8, -3.0 in Step 2 and -16.1, -8.6, -3.2, 0, + 1.2, + 1.0, -1.1 in Step 4 of Figure 2 represent the standard "C"- and "A"-weighting relative response corrections applied to any sound levels at the indicated octave band center frequencies. (NOTE: The manufacturer may label the protector at values lower than indicated by the test results and this computation procedure, e.g. lower NRR from lower attenuation values. (Ref. § 211.211(b) .)
Figure 2-Computation of the Noise Reduction Rating
| Octave band center frequency (Hz) | 125 | 250 | 500 | 1000 | 2000 | 3000 | 4000 | 6000 | 8000 |
| 1 Assumed Pink noise (dB) | 100 | 100 | 100 | 100 | 100 | 100 | 100 | ||
| 2 "C" weighting corrections (dB) | -.2 | 0 | 0 | 0 | -.2 | -.8 | -3.0 | ||
| 3 Unprotected ear "C"-weighted level (dB) | 99.8 | 100 | 100 | 100 | 99.8 | 99.2 | 97.0 | ||
| (The seven logarithmically added "C"-weighted sound pressure levels of Step #3 = 107.9 dS) | |||||||||
| 4 "A"-weighting corrections (dB) | -16.1 | -8.6 | -3.2 | 0 | + 1.2 | + 1.0 | -1.1 | ||
| 5 Unprotected ear "A"-weighted level (step #1-step #4) (dB) | 83.9 | 91.4 | 96.8 | 100 | 101.2 | 101 | 98.9 | ||
| 6 Average attenuation in dB at frequency | 21 | 22 | 23 | 29 | 41 | (43 + 47)/2 = 45 | (41 + 36)/2 = 38.5 | ||
| 7 Standard deviation in dB at frequency | 3.7 | 3.3 | 3.8 | 4.7 | 3.3 | (3.3 + 3.4) = 6.7 | (6.1 + 6.5) = 12.6 | ||
| * 2 | * 2 | * 2 | * 2 | * 2 | |||||
| 7.4 | 6.6 | 7.6 | 9.4 | 6.6 | |||||
| 8 Step #5-(step #6-step #7) develops the protected ear "A" weighted levels (dB) | 70.3 | 76.0 | 81.4 | 80.4 | 66.8 | 62.7 | 73.0 | ||
| (The seven logarithmically added "A"-weighted sound pressure levels of Step #8 using this sample data = 85.1 dB) | |||||||||
| 9 NRR = Step #3-Step #8-3 dB*; = 107.9 dB-85.1 dB-3 dB*; = 19.8 dB (or 20) (Round values ending in .5 to next lower whole number). | |||||||||
*Spectral uncertainty (as defined in § 211.203 ).
The value for #3 is constant. Use Logarithmic mathematics to determine the combined value of protected ear levels (Step #8) which is used in Step #9 to exactly derive the NRR; or use the following table as a substitute for logarithmic mathematics to determine the value of Step #8 and thus very closely approximate the NRR.
| Difference between any two sound pressure levels being combined (dB) | Add this level to the higher of the two levels (dB) |
| 0 to less than 1.5 | 3 |
| 1.5 to less than 4.5 | 2 |
| 4.5 to 9 | 1 |
| Greater than 9 | 0 |
40 C.F.R. §211.207