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
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| | | | | | | | | |
| Octave band center frequency (Hz) | 125 | 250 | 500 | 1000 | 2000 | 3000 | 4000 | 6000 | 8000
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| 1 Assumed Pink noise (dB) | 100 | 100 | 100 | 100 | 100 | | 100 | | 100
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| 2 “C” weighting corrections (dB) | −.2 | 0 | 0 | 0 | −.2 | | −.8 | | −3.0
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| 3 Unprotected ear “C”-weighted level (dB) | 99.8 | 100 | 100 | 100 | 99.8 | | 99.2 | | 97.0
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| (The seven logarithmically added “C”-weighted sound pressure levels of Step #3 = 107.9 dS)
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| 4 “A”-weighting corrections (dB) | −16.1 | −8.6 | −3.2 | 0 | + 1.2 | | + 1.0 | | −1.1
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| 5 Unprotected ear “A”-weighted level (step #1-step #4) (dB) | 83.9 | 91.4 | 96.8 | 100 | 101.2 | | 101 | | 98.9
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| 6 Average attenuation in dB at frequency | 21 | 22 | 23 | 29 | 41 | | (43 + 47)/2 = 45 | | (41 + 36)/2 = 38.5
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| 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
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| | × 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
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| (The seven logarithmically added “A”-weighted sound pressure levels of Step #8 using this sample data = 85.1 dB)
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| 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).
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| Difference between any two sound pressure levels being combined (dB)
| Add this level to the higher of the two levels (dB)
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|---|
| 0 to less than 1.5 | 3
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| 1.5 to less than 4.5 | 2
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| 4.5 to 9 | 1
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| Greater than 9 | 0 |
source: 44 FR 56127, Sept. 28, 1979, unless otherwise noted.
cite as: 40 CFR 211.207