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Noise Reduction Ratings

Hearing protectors sold in the U.S. are required by the U.S. Environmental Protection Agency (EPA) to have a noise reduction rating (NRR), which is an estimate of the reduction of noise at the ear when protectors are worn properly. However, due to the discrepancy between how protectors are fit in the testing laboratory and how users wear protectors in the real world, the Occupational Safety and Health Administration (OSHA) and the National Institute for Occupational Safety and Health (NIOSH) have developed derating formulas to reduce the effective NRR.

While the NRR and the SNR (Single Number Rating) are designed to be used with C-weighted noise, which means that the lower frequencies are not de-emphasized, other ratings (NRR(SF) and NRSA) are determined for use with A-weighted noise levels, which have lower frequencies de-emphasized. The National Institute for Occupational Safety and Health recommended and the US EPA mandated that 7-dB compensation between C and A weighting be applied when the NRR is used with A-weighted noise levels.

OSHA has defined in their training manual for inspectors that the adequacy of hearing protection for use in a hazardous noise environment should be derated to account for how workers typically wear protection relative to how manufacturers test the protector's attenuation in the laboratory. For all types of hearing protection, OSHA’s derating factor is 50%. If used with C-weighted noise, the derated NRR will become NRR/2. If used with A-weighted noise, OSHA applies the 7-dB adjustment for C-A weighting first then derates the remainder. For example, an protector with 33-dB attenuation would have this derating:

• Derated NRR = (33 – 7)/2

NIOSH has proposed a different scheme for derating based upon the type of protector. For earmuffs, the NRR should be derated by 25%, for slow-recovery foam earplugs the derating is 50% for all other protection, the derating is 70%. NIOSH applies the C-A spectral compensation differently than OSHA. Where OSHA subtracts the 7-dB factor first and derates the result, NIOSH derates the NRR first and then compensates for the C-A difference. For example, a slow recovery foam earplug with a 33-dB NRR would have this NIOSH derating:

• Derated NRR = (33/2) – 7

NIOSH also has different derating percentages for various types of hearing protection. Currently, the derating factor is 70% for premolded plugs, 50% for fast expansion foam plugs (formable plugs) and for muffs is 25%. For example, to find the derated NRR for an earmuff by using the NIOSH derating system, the procedure would go as follows:

• Derated NRR = (Original NRR x (1-.25)) – 7

Expected Updates in Noise Reduction Ratings

In 2007, the American National Standards Institute published a new standard for noise reduction ratings for hearing protectors, ANSI S12.68-2007. Using the real ear attenuation at threshold data collected by a laboratory test prescribed in ANSI S12.6-2008, the noise reduction statistic for A-weighted noise (NRSA) is computed using a set of 100 noises listed in the standard. The noise reduction rating, rather than be computed for a single noise spectrum the NRSA incorporates variability of both subject and spectral effects. ANSI S12.68 also defines a method to estimate the performance of a protector in an atypical noise environment. Building upon work from the US Air Force and the ISO 4869-2 standard, the protector's attenuation as a function of the difference in C and A-weighted noise level is used to predict typical performance in that noise environment. The derating may be quite severe (10 to 15 decibels) for protectors that have significant differences between low and high frequency attenuation. For "flat" attenuation protectors, the effect of C-A is less. This new system eliminates the need for calculators, relies on graphs and databases of empirical data, and is believed to be a more accurate system for determining NRRs.