In the United States, hearing protection devices such as earplugs and earmuffs require a Noise Reduction Rating (NRR) to be listed directly on product packaging. The NRR is a dB value reflecting the level of attenuation a hearing protection product provides as evaluated under laboratory conditions. The higher the NRR value, the greater the attenuation.
Since the NRR is a laboratory derived numerical estimate of attenuation, the actual amount of attenuation provided by a specific hearing protector in real-life work situations will most likely not correlate with the NRR. As outlined by the American Academy of Audiology’s position statement on preventing noise-induced occupational hearing loss, various studies have documented that labeled NRR bears little resemblance to actual NRR achieved in practice. To better estimate actual attenuation values, the National Institute of Occupational Safety and Health (NIOSH) recommended several procedures with the following formula representing the most straightforward approach:
Estimated Actual Attenuation = dBAw – (NRR-7)/2
In the above formula, dBAw represents measured occupational noise levels of a particular occupational work setting whereas the NRR is a value taken directly from product packaging. Assuming that a work setting’s noise levels have been measured at 90 dBA and an employee is wearing earmuffs with an NRR of 29 dB, the values are entered into the formula as follows:
Estimated Actual Attenuation = 90 dBA – (29-7)/2
The next step is to automatically subtract 7 dB from the NRR value listed on product packaging. In the above example, it is necessary to subtract 7 dB from 29 dB which equals 22 dB. This correction factor is a safety margin to account for spectral differences in C- versus A-weighted dosimeter measurements. Next, that derived value of 22 dB must then reduced to half or divided by 2. This 50% reduction is another safety margin designed to account for field versus lab differences in attenuation. So, in this particular work environment, when using hearing protection labeled 29 dB NRR, the estimated actual level of attenuation is 11 dB. For more information on noise conservation, check out the library of courses offered through AudiologyOnline including Clinical Evaluation of Hearing Protectors by Tom Thunder.