What Is the Noise Reduction Rating (NRR) and How to Use It?

When buying earplugs, earmuffs, and some noise-isolating earbuds, you will notice a label Noise Reduction Rating (NRR) together with a number. For example, 3M’s Peltor X5A earmuffs have an NRR of 31, while Howard Leight’s Max Lite earplugs have a comparable NRR of 30.

What is the NRR and how to use it

What is the Noise Reduction Rating (NRR)?

The NRR is a single number rating given in decibels.

Every hearing protector sold in the U.S. has to have an NRR-label as per EPA regulation. It was devised to give purchasers and users of hearing protection a guideline what level of noise protection they can expect from their earplugs or earmuffs when properly fitted.

In addition to the NRR, earplugs and earmuffs are accompanied by an attenuation table detailing by how much they reduce noise at different frequencies. This table comprises actual test results and is the basis for the calculation of the NRR.

And – it can give you a good picture of the strengths and weaknesses of the hearing protection you are considering.

Here is the attenuation table for the 3M Peltor X5A (over the head) earmuffs, NRR 31 dB:

Noise reduction by octave band earmuffs

And here is the table for the Howard Leight Max Lite foam earplugs, NRR 30 dB:

Noise reduction by octave band earplugs

Contents

How are earmuffs and earplugs tested to determine their NRR?

Ten test subjects undergo a test called real ear attenuation at threshold (REAT) to determine the average attenuation at each of the 9 frequencies detailed in the table above.

The details of the test method are specified in ANSI standard S 3.19-1974.

In a nutshell, for each frequency, subjects listen to a pulsed test noise to determine the threshold of hearing for the uncovered ear and the threshold with the hearing protector in place.

The difference between the two thresholds for a given frequency is the attenuation at that frequency. Each of the 10 subjects undergoes the test 3 times; so we obtain 30 different results for each frequency. The attenuation noted in the table is the mean of the individual results.

The NRR is then calculated from the means (using two standard deviations) in this attenuation table, assuming pink noise with a sound level of 100 dB at each frequency.

Note that while the earmuffs and earplugs in the example above have almost the same noise reduction rating, there is a marked difference between the two at different frequencies.

The earplugs seem to be doing better at lower frequencies, while the earmuffs seem to be better at the middle frequencies. Check this post for more on the relative benefits of earmuffs and earplugs.

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What are the highest NRR earplugs and earmuffs?

The highest-rated earmuffs I have come across have an NRR of 30 or 31. Most of them are passive earmuffs, that is, they don’t have electronics that amplify environmental noise while at the same time attenuating hazardous noise levels. Two exceptions are included in the table below.

The earplugs with the highest NRR I know of are foam earplugs with an NRR of 33.

Be careful when comparing noise reduction ratings provided by the manufacturer or importer.

Some companies advertise earmuffs with an NRR of 36 or 37. Upon closer inspection, they are using the SNR (Single Number Rating), which is obtained by testing according to European Standard EN 352-1:2002.

In the United States, the standard for testing hearing protection and calculating the NRR is still ANSI S3.19-1974.

As per EPA regulation, all hearing protectors sold in the U.S. must be labelled with the NRR. The SNR might be printed in addition if the same protectors are also being sold in Europe:

The SNR for most earmuffs and earplugs is several decibels higher than the NRR.

Here are some of the highest-rated earmuffs:

ModelNRRSNR
3M Peltor X5A3137
Howard Leight Thunder T33036
3M Peltor Optime 1053035
Howard Leight Leightning L33034
Howard Leight Impact Pro Industrial (electronic)3033
Howard Leight Impact Pro Shooters (electronic)3033

As for earplugs, the highest rated I have come across by reputable brands have an NRR of 33. Here is a selection. All of the following are foam earplugs:

ModelNRRSNR
Howard Leight Max 13337
3M Yellow Neons (312-1250)3336
Moldex Sparkplugs3335
Hearos Xtreme Protection33
Mack's Maximum Protection33

For earplugs much more than earmuffs individual fit matters. NRR-33 earplugs can easily turn into 10-dB earplugs if they don’t properly seal the ear canal.

The National Institute for Safety and Health (NIOSH) recommends derating foam earplugs by 50% and earmuffs by 25%. Occupational Safety and Health Administration (OSHA) recommends derating all hearing protectors by 50%.  See the section How to use the NRR below for more on this.

So far, I haven’t found earplugs that block more noise for me than the highest-rated earmuffs. This may be due to the shape of my ear canal.

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How to use the NRR?

In addition to the NRR, EPA also mandates supporting information, including an attenuation table detailing the different test frequencies and an example how the NRR can be used:

EPA supporting info including usage exampleHowever, both OSHA and NIOSH are more conservative than the EPA formula used in this example.

There are a few issues with the NRR:

  • It was obtained in a lab with the subjects sitting still.
  • The hearing protector was fitted by the experimenter (not the test subjects).

Fitting earplugs so that they completely seal the ear canal takes practice. But this is paramount if you want to get even close to their NRR. Even when fitted properly, not all earplugs will work for you.

With some, you may get a very good seal while others barely block any noise, despite them having the same NRR.

Putting on earmuffs is more straightforward, and despite some earplugs having a higher NRR, the highest rated earmuffs I own block noise better for me than all earplugs I have used.

In a nutshell, the EPA formula in the image above is in many cases way too optimistic.

Important Note: The following information is for educational purposes for do-it-yourselfers working around the house.

To stay on the safe side, for exposure limits I would generally follow the recommendations by the NIOSH.

Your work place may fall under more specific and sometimes different regulations, such as the ones enforced by the OSHA. You will have to check with safety personnel in your company and the OSHA/regulatory agency responsible for your industry.

So how can a lay person use the noise reduction rating?

First, you want to remember to stay below 85 dBA. Noise at or above 85 dBA is hazardous.

If you are running power tools at home or in your workshop, mowing your lawn, or blowing leaves, do you know how loud these tools are?

If you have access to an iPhone or iPad, I suggest you get the NIOSH Sound level meter app.

NIOSH Sound Level Meter App

Set the app as seen under Noise at Work in the following screenshot (settings):

NIOSH Sound Level Meter Settings

Run your circular saw, grinder, leaf blower, or lawn mower and check how loud they are.

If the noise level is at or above 85 dBA, get good earmuffs or earplugs or both to protect your hearing.

Suppose you have measured your lawn mower at 93 dBA.

You are considering the 3M 1100 earplugs with an NRR of 29.

We are conservative and derate foam earplugs by 50% as recommended by the OSHA and NIOSH:

We use the following OSHA formula (since we are measuring in dBA, we subtract an additional 7 dB before derating):

Noise level entering the ear = unprotected noise level in dBA – (NRR – 7) x 0.5

93 dBA – (29 dBA – 7) x 0.5 = 82 dBA.

The earplugs attenuate your lawn mower’s noise to below 82 dBA. This is below 85 dBA, and an acceptable level, provided they fit well.

Notes:

  • The OSHA and the NIOSH use slightly different formulas for derating hearing protection.
  • Also, the NIOSH derates foam earplugs by 50% and earmuffs by 25%, while the OSHA recommends uniform 50% derating of all hearing protection.
  • For the sake of simplicity, in this example, we have used the OSHA formula and uniform derating. You can find the NIOSH formulas here. (page 64)
  • For exposure limits we use the more conservative NIOSH recommendations.
  • While quite accurate, as the NIOSH points out, their app is not a certified type-2 sound level meter. As per OSHA, a type-2 sound level meter is the minimum you need to conduct official workplace measurements.
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How can you find out whether your earplugs or earmuffs are effective for you?

The NIOSH has devised Quickfit, a simple hearing test with which you can check whether your earplugs /earmuffs provide at least 15 dB of attenuation.

The reasoning is that if they don’t properly seal your ear canal, they are likely not even going to provide that level of attenuation.

  1. First you listen to a pulsed test noise without your hearing protector fitted. Use either speakers in a quiet room or over-the-ear headphones (only for earplugs).
  2. Adjust the volume so that you can barely hear the test noise, i.e., the sound level is just at the threshold of your hearing.
  3. Fit your earplugs/muffs.
  4. Listen to the second pulsed test noise. This noise is 15 dB louder than the first one. If your earplugs are fitted well, you should not be able to hear that noise.

If you do hear it, you need to refit your earplugs or muffs, and if they don’t work after repeated attempts, you need to find a different type that works for you.

Quickfit can be found at the NIOSH website. The sound files are also available for offline download.

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How to estimate your hearing protector attenuation when wearing both earmuffs and earplugs?

Unfortunately, you can’t just add the NRRs. You can only add 5 decibels to the protector with the higher NRR.

Suppose you are using earmuffs with an NRR of 31 and earplugs rated 30.

You are using an angle grinder and have measured it at 100 dBA.

Estimated exposure (only earmuffs) = 100 dBA – (31 – 7) x 0.5 = 88 dBA.

This is still a hazardous noise level, so you add earplugs with an NRR of 30 underneath.

Since the earmuffs are rated higher, you use these in the calculation and then subtract 5 dB for the earplugs:

Estimated exposure (double protection) = 100 dBA – (31 – 7) x 0.5 – 5 = 83 dBA.

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Noise Exposure and Duration

So far we have used the current noise level, measured with the NIOSH app, in our calculations.

The OSHA and NIOSH are concerned with noise exposure over time. Their exposure limits are stated as 8-hour time weighted averages (TWA).

In its publication Occupational Noise exposure, the NIOSH recommends an exposure limit (REL) of 85 dBA (as an 8-hour time-weighted average = TWA). Exposures at or above 85 decibels are hazardous.

Note: The NIOSH’s recommended exposure limit and exchange rate are more conservative than the limits currently permitted by the OSHA.  You can find the OSHA limits in the brochure Hearing Conservation.

Suppose the noise your angle grinder emits doesn’t vary over time. Then, if you were using it for 8 hours, your unprotected TWA would be 100 dBA. This would be 15 dB above the limit recommended by the NIOSH. You would very likely incur hearing loss over time.

The NIOSH recommends a 3 dB exchange rate when determining the duration a worker should not equal or exceed given a certain noise level. For every three dB more, the duration is cut in half.

For example, if for 85 dBA you are supposed to stay under 8 hours then for 88 dBA you have to stay under 4 hours.

So three decibels is a big thing!

The following table excerpt (from the NIOSH publication) shows the duration for other noise levels:

Exposure Level (dBA)HoursMinutesSeconds
8025240
8120100
821600
8312420
841050
85800
866210
87520
88400
893100
902310
91200
921350
931160
94100
970300
1000150
1030730
1060345
1090153
1120056
1150028
1180014
121007
124003
127001

If you were using the grinder ( you measured at 100 dBA) without any hearing protection (don’t do this!), you would have to limit yourself to less than 15 minutes, provided you weren’t using any other noisy equipment during your working day.

Daily Noise Dose

The daily noise dose (D) is a useful way to combine time periods with different noise levels. D must be kept < 100%.

The formula to calculate D for multiple noise exposures is: D = [C1/T1 + C2/T2 + Cn/ Tn] × 100

Cn is the total time of exposure at a specified noise level, and Tn is the duration you have to stay below for this noise level (see table above).

Let’s say you have mowed the lawn (93 dBA) for 60 minutes and used your grinder (100 dBA) for 10 minutes. (both without wearing hearing protection)

According to the table, at 93 dBA we must stay below 76 min. So we get dose D1: (60/76) x 100 = 79%.

At 100 dBA, we have to stay below 15 min: D2: (10/15) x 100= 67%.

D= D1+ D2 = 79%+67% = 146%.

D exceeds the maximum allowed daily noise dose. (It must be kept at < 100%).

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How can you measure the noise level you are exposed to over time?

Sound level meter set to dBA, slow response.

You can measure the noise level by using either a sound level meter or a noise dosimeter.

The NIOSH app mentioned above can be used as a sound level meter and a noise dosimeter, but it is not approved for official workplace use. (See below)

A sound level meter measures the noise level at a certain point in time and does simple averages over shorter periods of time.

If the noise you are exposed to is fairly constant for longer time periods, a sound level meter can be used to estimate the 8-hour time-weighted average noise exposure (TWA).

If you move around a lot and/or are exposed to different noise sources throughout a working day, it becomes difficult to calculate the TWA.

A noise dosimeter, which you wear throughout the day, records noise exposure over time and automatically calculates the noise dose and the TWA.

Most sound level meters and noise dosimeters can be switched between dBA (A-weighting) and dBC (C-weighting). Meters also allow switching between slow and fast response.

When measuring noise, A-weighting (slow response) is generally used.

To perform noise measurements at your work place, you need at least a type-2 sound level meter or noise dosimeter. Type-2 meters have an accuracy of +/- 2 dB and have to fulfill additional requirements as per ANSI standard S1.4-1983. The requirements for type-1 meters are even higher. They have an accuracy of +/-1 dB.

The meter has to be calibrated before and after use and in addition at least annually according to the manufacturer’s instructions.

While very useful for home use, the NIOSH app doesn’t fulfill all conditions required of a type-2 meter. To fulfill the OSHA’s requirements, you need a calibrated type-2 meter.

However, you can use the app to get an idea whether your workplace may be too loud and alert safety personnel in your company.

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What are A-weighting and C-weighting?

Our ear‘s sensitivity varies with the frequency: it tends to be most sensitive at 4000 Hz and least sensitive for low frequency sound.

A-weighting corresponds roughly to our ear’s sensitivity at a moderate sound level (40 dB at 1000 Hz) and tends to deemphasize lower frequencies when compared to C-weighting.  (e.g., for the 125 Hz band, the difference is about 16 dB.)

Because it provides nearly equal weight at all frequencies, using C-weighting is safer if the noise is dominated by low-frequency components.

When selecting earmuffs or earplugs, you can use dbC to calculate whether the hearing protectors provide sufficient protection. If only dbA are available (as in the example above), subtract an additional 7 dB from the hearing protector’s NRR.

Noise level entering the ear = unprotected noise level in dBC – NRR x 0.5

Noise level entering the ear = unprotected noise level in dBA – (NRR – 7) x 0.5

 

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