# Tools To Learn

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

It is quite an ambitious calculation to take a hearing protector’s attenuation measurements at each frequency, input them 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 Single Number Rating (SNR) is not simply a mathematical average of the attenuation values. Here are the significant steps used in calculating the SNR*, and an explanation of why each step is important: Laboratory Testing
Sixteen subjects with normal hearing are tested with properly-fitted hearing protectors (called occluded ear test), and tested again without hearing protectors (called open ear), across a range of sound frequencies. The difference between the open ear and occluded hearing tests gives us the attenuation of the hearing protector. The variability in these attenuation measurements among subjects (the “Standard Deviation”) is calculated and the attenuation values from all subjects 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.

Calculate the Assumed Protection Value (APV)
To account for individual variation in fitting hearing protectors out in the real world (remember, the laboratory only tested sixteen subjects), a correction factor is subtracted from each attenuation value. This correction factor is based upon the standard deviation, and can be adjusted based upon the desired protection performance (that is, a higher correction will protect a greater percentage of the population). For example, we subtract one standard deviation from the laboratory attenuation measures to achieve an Assumed Protection Value that can be applied to 84% of the population; but we subtract two standard deviations to achieve an APV that can be applied to 98% of the population. By calculating this Assumed Protection Value, we can generalize the results from a small sample of sixteen subjects to a larger population.

Subtraction from Hypothetical Noise
To account for some differences between the laboratory test sounds and real-world noise, the Assumed Protection Values are subtracted from “hypothetical noise levels” – some standardized noise levels at each frequency band. This step is critical in making the SNR more relevant to a hearing protector user, and not a laboratory microphone which detects sound differently than a human ear.