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. Sec. 211.211(b).)
Figure 2--Computation of the Noise Reduction Rating
Octave band center frequency (Hz).................................. 125 250 500 1000 2000 3000 4000 6000 80001 Assumed Pink noise (dB).......................................... 100 100 100 100 100 ..... 100 ..... 1002 ``C'' weighting corrections (dB)................................. -.2 0 0 0 -.2 ..... -.8 ..... -3.03 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 ..... -1.15 Unprotected ear ``A''-weighted level (step 1-step 4) (dB)...... 83.9 91.4 96.8 100 101.2 ..... 101 ..... 98.96 Average attenuation in dB at frequency........................... 21 22 23 29 41 ..... (43+47)/2=45 ..... (41+36)/2=38.57 Standard deviation in dB at frequency............................ 3.7 3.3 3.8 4.7 3.3 ..... (3.3+3.4)=6. ..... (6.1+6.5)=12.6
7
x2 x2 x2 x2 x2 ..... ............ ..... ...............
-----------------------------------------
7.4 6.6 7.6 9.4 6.6 ..... ............ ..... ...............8 Step 5-(step 6-step 7) develops the protected ear ``A'' 70.3 76.0 81.4 80.4 66.8 ..... 62.7 ..... 73.0
weighted levels (dB)..............................................(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 Sec. 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. ------------------------------------------------------------------------
Add this
level to
Difference between any two sound pressure levels being the higher
combined (dB) of the two
levels (dB)------------------------------------------------------------------------0 to less than 1.5......................................... 31.5 to less than 4.5....................................... 24.5 to 9................................................... 1Greater than 9............................................. 0------------------------------------------------------------------------