NRR stands for “Not Really Realistic”

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.

About AU Bankaitis

A.U. Bankaitis, PhD is a clinical Audiologist with extensive clinical, research, and business experience within the hearing industry. Dr. Bankaitis created this blog to educate her colleagues on viable product solutions for their patients and/or clinical practice.
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5 Responses to NRR stands for “Not Really Realistic”

  1. Dan Schwartz says:

    Oops! You didn’t write out the fractional part of the algebraic formula correctly! Using the order of calculating the parenthetic value first, then working outward calculating the multiplies and then the additions, when you wrote

    “dBAw – 1/2(NRR-7)” when solved, you actually get for the right hand term:

    2 * (NRR-7)

    The proper notation should be:



    — (NRR-7)

    • Dan Schwartz says:

      Or, on a single line,


      with the full equation:

      = dBAw – (NRR-7)/2

    • aubankaitis says:

      Thanks for the comment. It appears that WordPress did not recognize my half as 1 over 2. The text explains the formula in more detail so hopefully most people will recongize that 1/2(NRR-7) is “one half of” (NRR-7).

      • Dan Schwartz says:

        Yeah, WordPress tries to “interpret” what it thinks you want… In fact, it drives me nuts, because a lot of EE equations use Greek characters, subscripts, special symbols, etc.

        If you don’t have an equation editor handy — Like if you’re on a public terminal or someone else’s PC, one trick I use is to log into Wikipedia, go to my “sandbox” page, use the equation editor in wiki markup (it’s easy); then, when I see that it displays the way I want, I simply view the page HTML source code it generated. On IE,
        File –> Source
        On Opera,
        File –> Developer Tools –> Source
        and then a text page pops up. Copy the relevant section; then back in WordPress switch to HTML view, and paste the HTML code into place~

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