Posted on: October 3rd, 2016 by mobilear

What Does the Noise Reduction Rating (NRR) Mean?

You’ve probably seen the NOISE REDUCTION RATING (NRR) on the package ear plugs or ear muffs. Have you ever wondered what this number means?nrr-inset

As you read in the previous blog post, noise is measured in decibels. The NRR is a number which predicts the amount of protection a given hearing protector will provide. The higher the NRR, the more protection a user will be provided.

What’s important to realize is that this number is derived from laboratory testing and doesn’t necessarily take into account some of the real life situations that occur with hearing protection users. A significant loss of protection occurs when hearing protection is not worn correctly.

Have you seen employees with the foam plugs barely in the ear canals? If these employees were to bend over, the plug would probably fall to the ground. What about an ear muff user who also is wearing a baseball cap and safety glasses, and has a beard? You can almost put a finger in the gap created between the ear muff and the cap, glasses and beard.

Appendix B of 29 CFR 1910.95 Occupational Noise Exposure; Hearing Conservation Amendment; Final Rule provides formulas to determine the protected noise exposure. Unfortunately, these formulas do not take into account that there’s a good chance the hearing protectors are not going to be worn exactly as they would in the laboratory. In fact, it is noted in the Hearing Conservation Amendment “The employer must remember that calculated attenuation values reflect realistic values only to the extent that the protectors are properly fitted and worn.”

As a result, manufacturers of hearing protectors are recommending the following formula used to assess protected exposure. The formula is presented below with an example:

NRR of hearing protector: 31
dBATWA: 95

Step 1: Subtract 7 from the NRR 31 – 7 = 24
Step 2: Divide this number by 2 24/2 = 12
Step 3: Subtract that number form the dBATWA: 95 – 12 = 83

Using this calculation, the employee’s protected noise exposure would be 83 dBATWA, which is below OSHA’s Action Level of 85 dBA. OSHA states an employee’s protected noise exposure should be 90 dBATWA or less. Since the promulgation of the Hearing Conservation Amendment, research indicates the human ear is likely more sensitive to noise and using 85 dBA dB as the target protected noise exposure would be a more conservative and smarter choice.

Now, let’s say you’re wearing ear plugs and ear muffs together. You don’t simply add those two NRR numbers together. You simply add 5 dB to the higher of the two NRRs. So if the NRR of ear plugs is 30 and the NRR of the ear muffs is 25; add 5 to 30 and use 35 dB as the NRR in the calculation.

NRR of hearing protector with higher NRR: 30
Add 5 for hearing protector with lower NRR: +5
New NRR 35
dBATWA: 95

Step 1: Subtract 7 from the NRR 35 – 7 = 28
Step 2: Divide this number by 2 28/2 = 14
Step 3: Subtract that number form the dBATWA: 95 – 14 = 81

Using this calculation, the employee’s protected noise exposure would be 81 dBATWA.

Overall, hearing protection works as well as the user’s ability to insert them properly and to wear them at all times when in the noise.

Take a walk through the plant to see how hearing protection is being used.

Does it look like this?


Or more like this?