"Caution: Excessive Noise Warning" Sign

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Noise Reduction Rating (NRR) is the primary unit of measurement that is used today in order to determine what kind of hearing protection may be needed in any given environment. The goal of using the NRR system is to achieve a safe noise level in various activities that we all take on every day. This allows us to be able to reduce noise exposure levels to less than 85 dB, thereby lowering or eliminating the risk of hearing damage occurring.

The Problem with Using NRR Straight Up

Noise Reduction Ratings come from tests performed in a laboratory – not in real life. In real life the full NRR is seldom achieved all the time.

The problem is that many people don’t use the NRR system accurately. They take the NRR dB reduction rating and apply it directly to the decibel exposure. If your lawn mower is producing 100 dB, for example, and your hearing protection has an NRR rating of 33 dB, this doesn’t mean you reduce your sound exposure to 75 dB.

Here’s how you’d need to calculate the effectiveness of the NRR rating for your hearing protection to get the adequate exposure.

Step #1: Take the NRR decibel rating and subtract 7 from it. (NRR 33 – 7) 

Step #2: Now divide this result by two. (NRR 33 -7) ÷ 2 = 13 dB ‘real-life’ reduction 

Step #3: Take this answer and reduce your environmental dB level. Lawn mower at 100 dB – 13 dB protection = 87 dB exposure. 

As you can see, the NRR 33 dB rating hearing protection for your 100 dB lawn mower will reduce your decibel exposure to 87 dB. It should be noted that this would not bring your exposure down into the lowest risk levels. 4 hours of mowing at this exposure could create permanent hearing damage despite wearing the protection. 

If you do not make these adjustments, then you will be significantly overestimating the amount of protection that your preferred earplugs or earmuffs are able to provide. For high noise workplaces, this means workers and their employers could be exposing people to high-risk environments without adequate protection and not even realize it. 

This is why it is vitally important to input this calculation into the noise reduction rating of any hearing protectors. Most protection falls within a range of 0-30, but some products do offer greater effectiveness than 30 if you’re willing to pay for a custom product. 

How to Improve Your NRR 

If mowing the lawn at 87 dB is not something you want to do, then you do have the option of combining two hearing protection items to increase the overall sound reduction benefits that you’re receiving. In this instance, let’s say that your NRR 33 dB device are earmuffs and now you’re going to add a set of silicone earplugs that have an NRR rating of 29 dB.

You don’t add the two NRRs together to get a final result. You simply add +5 to the NRR of the highest rated device. You would not add +5 to your final answer.

Your equation in this circumstance would look like this: (NRR 33+5-7) ÷ 2. Using left to right mathematics rules, your new answer would be 15.5. This means combining the two items for mowing the lawn would reduce your decibel exposure to 84.5 dB – or just below what would be considered the start of a high-risk exposure.

How Loud Is Your Environment?

As a general rule of thumb, your high risk of hearing loss due to loud noise begins at 85 dB. For every 3-5 decibel increase from this base figure, the amount of exposure that you can safely receive before permanent hearing damage occurs is halved. The maximum amount of time at the 85 dB noise level is 8 hours. This means at 88 dB, your time is reduced to 4 hours. Once you reach the 110 dB level, the amount of exposure is reduced to under an hour. It doesn’t take much more noise to reduce your exposure to just minutes or seconds.

So how can you tell what your decibel exposure is going to be? You could install an app on your smartphone that can tell you what your current exposure happens to be, but not everyone has that luxury. Here are some examples of everyday activities and what the noise level associated with them tends to be on average.

  • 150 dB. Concerts, some firearms, some community warning sirens.
  • 140 dB. Jet engines.
  • 130 dB. Construction equipment, football stadiums, amplified music.
  • 120 dB. Headphones at the highest volume, orchestras, car stereos.
  • 110 dB. Many forms of indoor home machinery.
  • 100 dB. Motorcycles, snowmobiles, pneumatic tools.
  • 90 dB. Lawn mowers, other miscellaneous DIY tools, urban traffic, trains.
  • 80 dB. Ambient urban noise, alarm clocks, home generators.
  • 70 dB. Vacuum cleaners, ambient driving noise.
  • 60 dB. Person-to-person conversation, dishwashers, washing machines.
  • 50 dB. Rain falling.
  • 40 dB. Ambient indoor environment.
  • 30 dB. Whispering. 

From this list, you can then take your NRR calculation, apply it to the average decibel exposure for your activity, and then shop for hearing protection that can reduce your exposure below 85 dB. This will give you the lowest possible risk in determining what your noise exposure levels are going to be. 

But Wait… There Are Different Weights to Decibels

Determining what your NRR is going to be also depends on what the decibel exposure is weighted to be. The calculations above, including the common activities listed, are measured using what are called “A-Weighted Decibels”. Sometimes that is abbreviated as dBA. This means you’re measuring the expression of loudness that is picked up by the human ear. The only problem with this measurement scale is that low sounds, such as those below 1000 Hz, are discounted because the human ear is less sensitive to low audio sounds.

If you are measuring your sound exposure based on B-Weighted Decibels, then you would need to take your NRR calculation and increase it by 50%. Your ambient noise levels for common activities would remain the same, but your sound reduction benefits would increase. In essence, you would take the “7” in the equation and turn it into “3.5.”

It would look like this. (NRR 33 – 3.5) ÷ 2 = 14.75

As you can see, if the B-Weighted measurement is used, the hearing protection with an NRR 33 rating would just about be enough to get your sound exposure out of the danger zone.

C-Weighted decibels involve the lowest frequencies. In this circumstance, you wouldn’t adjust the sound rating at all. This is the only circumstance where the NRR is considered an accurate reduction of a person’s sound exposure. You would subtract 33 dB from the 100 dB exposure to receive a 67 dB compensated sound exposure.

How to Protect Your Hearing in High Noise Environments

If you attend one concert per year and your maximum exposure is 120 dB just momentarily, then there is still a low risk of hearing loss. If you are a musician who tours and plays 200 concerts per year, that constant noise exposure can result in hearing loss rather quickly – even with hearing protection in place. The same is true for airport workers, some professional athletes, and construction workers.

To ensure that hearing can be preserved for these high-risk occupations, medical providers offer what is called a “hearing conservation program.” This program creates a treatment plan for your hearing much like a doctor would create a treatment plan for an illness. After a hearing evaluation by a qualified audiologist or physician, a specific NRR can be determined that will help to protect against occupational hearing harm.

This allows high-risk workers a chance to shop for the most effective products possible. It will also let them know if they will need to have multiple forms of hearing protection in place to avoid peak noise exposure damage.

In Conclusion 

It is important to remember that the NRR for any hearing protective device is based on average ratings that are achieved in laboratory conditions. A minimum of 10 people are tested with the hearing device on to determine what their exposure happens to be. These results are then averaged to produce the NRR. If you have equipment that is rated NRR 25, then you may experience personal results that are much lower or much higher than this rating.

Hearing protection must be perfectly worn for the maximum NRR to be achieved.

The bottom line is this: use the NRR system as a guide to determine what your noise exposure will be after your hearing protection is in place. If the average NRR doesn’t bring you into a low-risk zone, then look for better equipment. Calculate your actual exposure with the equations above to ensure the best possible results. Follow all the manufacturer’s instructions for wear and you will be able to achieve the noise reductions you may need.