# Tools To Learn

Sound Source: How NRR Is Calculated  When I compare the values in the attenuation charts with the NRR on the package, the NRR is much lower than the average attenuation. How is the NRR calculated?

It is quite an ambitious calculation to measure a hearing protector’s attenuation at a variety of frequencies on a few test subjects in the laboratory, input these values into a formula and derive a single-number rating that can be applied universally to all users in all noise settings. If you have studied the attenuation charts on boxes of hearing protectors, you can see the Noise Reduction Rating (NRR) is not simply a mathematical average of the attenuation values. In 1979, the U.S. Environmental Protection Agency* standardized the NRR formula that we still use today to rate hearing protectors. Here are the significant steps used in calculating the NRR, and an explanation of why each step is important:

Laboratory Testing
At least ten subjects are tested with properly-fitted hearing protectors (called “occluded ear”) and tested again without hearing protectors (called “open ear”), across a range of test frequencies. The difference between the open ear and occluded hearing tests gives us the attenuation of the hearing protector. Each subject is tested three times, meaning there are at least thirty attenuation measurements at each frequency. The variability in these measurements (the “Standard Deviation”) is calculated and the thirty attenuation values are then averaged to give us the ”Mean Attenuation in dB” at each frequency. These Mean Attenuation values, as well as the Standard Deviations, appear in the attenuation chart on each box or bulk package of hearing protectors distributed in the U.S.

Standard Deviation
To account for individual variation in fitting hearing protectors out in the real world (remember, the laboratory only tested a minimum of ten subjects), a correction factor of two standard deviations is subtracted from each attenuation value. By subtracting two standard deviations, we can generalize the results from a small sample of ten subjects to a larger population: for a population which is properly fitted with the HPD in the same manner as the laboratory subjects, 98% of the population would be expected to achieve these same attenuation values in the lab.

Subtraction from Hypothetical Noise
To account for some differences between the laboratory test sounds and real-world noise, the adjusted attenuation values (mean minus two standard deviations) are subtracted from “hypothetical noise levels” – some standardized noise levels at each frequency band. This step is critical so that we might apply the final NRR to a hearing protector user, and not a laboratory microphone which detects sound differently than a human ear.