Noise Reduction Rating (NRR) Explained

Noise is an omnipresent occupational hazard. So many workers, especially industry and construction workers all over the US, are exposed to excessive noise levels. Long-term exposure to these noise levels can cause hearing loss but can also affect human health in other ways. That’s why it’s so important to limit the exposure to excessive noise or, if that’s not possible, to use a hearing protection device (HPD). The market is flooded with all kinds of hearing protection earmuffs and earplugs, but which one should you choose? 

The most important characteristic you should be looking for when choosing a hearing protection device is the device’s noise reduction rating (NRR). In layman’s terms, noise reduction rating describes the ability of a certain HPD to attenuate noise. However, there’s so much more to know about noise reduction ratings and about US regulations regarding occupational safety and occupational noise exposure. 

In this article, we will discuss the topic of NRR in detail. We will talk about the effects of excessive noise on human health, discuss different occupational noise exposure standards and OSHA’s hearing conservation program. We will also explain how the noise reduction rating is measured, how hearing protection reduces the noise, how combining two HPDs (earplugs + earmuffs) changes the noise exposure when used together and we will explain the difference between the advertised noise reduction rating and actual noise attenuation. 

The Effects of Excessive Noise on Human Health

The effects of excessive noise on worker’s health are numerous. For starters, you have the most obvious and most common effect – noise-induced hearing loss (NIHL). Along with hearing loss, some workers may experience tinnitus (ringing or pain in ears). Hearing issues, unfortunately, are not the only negative effect excessive noise can have on human health. The consequences of excessive noise are so much more far-reaching. 

Excessive noise exposure often affects cognitive performance, it causes sleep disturbance and annoyance. It’s been proven that it has a negative effect on cardiovascular health and can cause hypertension or even stroke. 

So, we can conclude that noise can have a massive effect on both the physical and mental health of a human. Having that in mind, it is absolutely crucial to protect anyone and everyone exposed to excessive noise in every possible way. Before we start discussing different types of hearing protection and move onto our main topic (NRR ratings), let's see how loud is too loud. 

What dB Level is Considered Excessive?

The noise is measured in decibels (dB) of SPL (sound pressure level). dB is basically a unit that describes the loudness of a certain audio source. A small change in SPL levels (2-3dBs) can make a huge difference in perceived loudness. In fact, increasing the SPL by only 10dB is usually perceived as twice as loud. As you can see in the picture below, 110dB is considered the threshold of discomfort, while the 120-140dB (depending on the author) is considered the threshold of pain. 

Pressure levels of different sounds and noises (source – National Center for Biotechnology Information)

What you know so far, based on the info you have above, is that 60dB is not excessive noise (that’s the SPL level of normal speech) and 110dB is most definitely too much. But where’s the limit? What noise exposure is considered excessive? Before we give you the answer, we have to introduce another factor – the period of exposure. So, the dB level is not the only important thing – the period of exposure is equally important. The exposure to some noise can be considered excessive if it lasts 8 hours or longer but can also be considered acceptable/permissible if it lasts only 4 hours. 

Different US occupational health and safety organizations have defined different limits but none of the limits exceed the exposure to 90dB noise during the period of 8 hours. National Institute for Occupational Safety and Health (NIOSH), which is a part of the CDC, recommends that the noise level should not exceed 85dB during the 8h period. The official occupational safety and health standards are introduced by the organization called OSHA (Occupational Safety and Health Administration). According to this administration, 90dB is the max allowed exposure during the 8h period. So, 90dB or higher is considered too much if the exposure time is 8h. Increasing the noise level by only 5dB, halves the max allowed exposure time. So, if the noise level is 95dB, the max allowed exposure period is 4 hours. 

Permissible noise exposure levels (source – Occupational Safety and Health Standards, Occupational Noise Exposure)

But what happens if a worker has to work in an environment louder than 90dB for 8 hours or more? The employer has to protect him/her somehow. 

Different Ways of Hearing Protection and Reducing the Exposure to Excessive Noise

The employer can protect his workers by using low-noise machinery and by maintaining and lubricating the equipment. If the noise level still exceeds the values recommended by the OSHA, the employer can do two things. The preferred option is separating the source of noise (machinery) from the worker by enclosing the machinery or by separating the worker from the source of noise (by introducing some kind of sound barriers). 

The other, more popular (and cheaper) option, is buying protective gear for the workers. To be more specific, the second option is buying hearing protection devices (earmuffs and earplugs). The most important thing to look for when buying hearing protection devices is the NRR (noise reduction rating). 

What is a Noise Reduction Rating (NRR)?

Noise reduction rating describes the ability of the noise protection muffs and earplugs to attenuate noise to a satisfactory level. It's expressed in dB (just like the noise level) and it tells you how much noise a pair of hearing protection earmuffs or earplugs can block out. Higher NRR means that the HPD (hearing protection device) blocks out more noise. 

OSHA usually performs the screening of the working conditions, tests the noise exposure, and recommends the necessary NRR. 

How is NRR Measured?

The NRR is measured in laboratories, under ‘’ideal’’ conditions and in compliance with the ANSI/ASA S12.6-2016 standard, as required by the EPA (Environmental Protection Agency) and OSHA. In order to get the EPA label with the noise reduction rating, the hearing protection device has to be tested in an authorized facility. In reality, the testing is often done by the manufacturer (but the manufacturer has to be authorized to perform the testing). 

EPA- approved NRR label (source – HOWARD LEIGHT by Honeywell)

Earlier, the NRR measuring was done in compliance with the ANSI S3.19 standard, released in 1974. This standard required 10 human subjects (professional listeners). Each of these subjects tested the hearing protection device and used 9 frequencies to estimate the NRR. The medium value was the approved NRR value.

ANSI S3.19 was replaced by the ANSI S12.6 in 1984, which was updated in 2016. This standard defines two methods for measuring the NRR. The first one is called ‘’designated trained-subject fit’’ and it’s supposed to determine the maximum attenuation provided by a certain HPD. These NRR values can be achieved only when the device is used by a trained professional. The second method is called ‘’ designated inexperienced-subject fit” and it's supposed to approximate the max attenuation provided by a certain HPD when the device is used by an average user. As you can assume, the second gives the results that are closer to real-life results. Testing facilities can use either method but, regardless of the method, the HPD must be used the way they are supposed to be used, and the anatomy of the test subject (listener) has to reflect the average anatomic characteristics of the population.

After the testing is done, the hearing protection device is given the EPA label with a certain noise reduction rating (NRR). These ratings almost never exceed 33dB while the lower limit is around 22dB. You are supposed to choose the right hearing protection based on the screenings of the environmental noise conducted by the authorized body (OSHA). For example, if a worker is constantly exposed to 100dB noise, he needs a device that would attenuate the noise by up to 10dB (preferably 15dB). That way, the noise level will be reduced down to 90dB (or 85dB). 

NRR VS SNR

SNR is used to describe the same thing as the NRR. NRR (noise reduction rating) is used in the US and Canada, while the SNR (Single Number Rating) is used in Europe. 

SNR stands for Single Number Rating. Just like NRR, SNR measuring is done in compliance with certain standards established by the EU (EN352-1/EN352-3 and ISO 4869-2:1995 standard). Testing methods are thoroughly described in the EN 13819 standard. These standards also define the max clamping force of earmuffs (14N) and the minimum sound attenuation for different frequencies.

SNR ratings are, in general, a few dB higher than the NRR ratings. There's no formula for converting SNR to NRR and vice versa since both values are measured in labs, and the circumstances the measurements are not the same. 

NRR (US standard) VS SNR (EU standard)

Are You Really Getting the Advertised Reduction/Attenuation?

Unfortunately, many studies show that the advertised SNR and NRR values don’t really represent the real-world performance of a certain hearing protection device. The real-world attenuation depends on the fitting and many other factors. 

Most health and safety experts and organizations recommend reducing the advertised NRR and SNR values to get more accurate values of the attenuation provided by an HPD in real-life conditions. 

When it comes to Noise Reduction Rating (NRR), OSHA recommends two methods for estimating/calculating the real-world performance of an HPD (29 CFR 1910.95, Occupational Noise Exposure, Appendix B). 

If the noise level is measured with a sound pressure level meter (dosimeter) that is capable of C-weighted measurement, you are supposed to convert it to TWA (time-weighted average noise level for the 8-hour shift). This value describes the average noise level in a certain working environment. To get the real-life attenuation, you are supposed to divide the NRR value by two and then subtract that value from the measured C-weighted TWA. So, for example, if a measured C-weighted TWA is 100dB and the NRR of some earmuffs is 30dB, wearing the hearing protection for 8 hours will reduce the noise exposure to 85dB (100dB-30dB/2).

If the noise level is measured with an SPL meter (dosimeter) that is capable of A-weighted measurement, you are supposed to convert the A-weighted value to TWA. To calculate the real-world attenuation, you should retract 7dB from the advertised NRR value and then divide it by two. In the end, subtract the calculated value from the TWA and that’s the expected real-life attenuation. So, for example, if the A-weighted TWA is 100dB, and the advertised NRR is 30dB, the hearing protection device should reduce the noise level to 88.5dB (100dB- (30dB-7)/2).

Some manufacturers, like 3M, also recommend this method of calculating the expected noise exposure. 

The advertised NRR VS real-world attenuation (source – 3M)

SNR values, just like NRR values, are often overestimated and different official services and bodies, like the Health and Safety Executive in the UK, recommend subtracting 4dBs from the advertised SNR value to get the actual attenuation. 

The Effects of Having Dual Noise Protection (Earplugs + Earmuffs)

If you need a better noise attenuation than your hearing protection earmuffs can provide, you can try combining earmuffs and earplugs. One thing to know about dual protection is that the combined NRR rating is not just a simple sum of two noise reduction ratings. So, if the earplugs have the NRR rated at 30dB and the earmuffs have the NRR rated at 29dB, the combined NRR is not 59dB. It’s only 35dB. The general rule when calculating the combined NRR of dual noise protection is to add 5dB to the higher NRR. Based on the combined NRR, you can calculate the real-life attenuation. 

For example, if you only have earmuffs with the NRR rated at 29dB and if the TWA is 100dB (A-weighted), the real-life noise level will be reduced to 89dB (100dB-(29dB-7dB)/2). 

When using dual protection (NRR1-29dB, NRR2-30dB), the combined NRR is 35dB. If the TWA is still 100dB, the real-life noise level will be reduced to 86dB (100dB-(35dB-7dB)/2).

It’s recommended that, when the TWA noise exposure exceeds 100dB (A-weighted), the worker should use dual noise protection (earmuffs + earplugs). Some organizations, like the National Institute of Occupational Safety and Health, suggest that even dual noise protection doesn’t help when the TWA exceeds 105dB (A-weighted). 

In the end, it’s important to emphasize that the attenuation provided by the hearing protection device depends highly on the characteristics of the device but also on the way the worker uses it. According to OSHA's research, an HPD that has the potential of providing 30dB attenuation during an 8h shift will deliver only 15dB if the worker doesn't wear it for only 30mins. So, it's important to wear hearing protection correctly and it's important to wear it all the time.

This is the end of our article on noise reduction ratings. Hopefully, it helped you understand the meaning and importance of the NRR. In case you have some more questions, we have prepared a section with short and accurate answers to the most common questions about noise reduction ratings (NRR).

FAQs

Q: What is a good NRR rating?

A: The only good NRR rating is the one that’s high. The higher the NRR the better the noise attenuation. NRR rating values vary between 20dB and 33dB. If you want the best possible noise reduction, you should go for 30dB+.

Q: Is a higher or lower NRR rating better?

A: Higher NRR rating is definitely better than lower. If you are choosing between 28dB and 33dB, go for 33dB. 

Q: How is the NRR rating calculated? 

A: NRR is actually measured, not calculated. ANSI/ASA S12.6-2016 standard defines the procedures and methods for measuring the NRR. The manufacturers are required (by the EPA and OSHA), to test their noise protection equipment in authorized facilities in order to get the label with the NRR rating.

Q: Is NRR 22 good for shooting?

A: Well, any protection is better than no protection at all but you should probably use something with higher NRR. Armed Defense Training Association recommends using NRR 28 or higher. 

Q: How loud is a gunshot?

A: All guns are loud but some are louder than others. The average gunshot SPL is rated at 140dB which is, as you may know, often considered the threshold of pain. Some experts even say that the threshold is significantly lower (130dB). Some guns can be much louder than 140dB. For example, Glock G17 9mm reaches 162dB, while the AR-15 16’’ reaches 165dB. Some guns can even produce noise over 175dB. Considering these values, it's highly recommended to use a hearing protection device with a high NRR for shooting. 

Q: What is the best hearing protection?

A: The highest noise reduction rating (NRR) for earmuffs is 31dB, while the earplugs provide slightly better noise attenuation (up to NRR 33dB). If you want the best possible hearing protection, you should use dual protection (earmuffs + earplugs). 

Q: What is the loudest sound on earth?

A: According to the Discover Magazine and Science Alert, the eruption of the Krakatoa volcano in 1883 was the loudest sound ever heard. For many people, that was also the last sound they've ever heard. The shockwave was so strong that it ruptured eardrums of people living miles away from the volcano. Some sources claim that the scientists living 100 miles away from the volcano registered the SPL of 172dB.

Q: What is the highest dB for earplugs?

A: The highest NRR a pair of earplugs can have is 33dB. You can get higher protection than that only if you combine earplugs with earmuffs. 

Q: How many decibels can kill you?

A: As mentioned in our article, 120dB is the threshold of discomfort while the 140dB is considered the threshold of pain. 150dB could rupture your eardrums. Also, constant exposure to noise under 100dB could cause some serious damage (hearing loss, tinnitus, increased blood pressure, all kinds of psychological issues). It is estimated that the sound higher than 200dB could kill a human being. In short, that kind of loudness could cause lung embolism which travels to your heart and results in death. 

Q: Can you go deaf from a gunshot?

A: There are some guns that are loud enough to cause eardrum rupture. In some cases, gunshots can cause tinnitus or short-term hearing loss, but they can also cause permanent damage. That’s why it’s crucial to use hearing protection earmuffs/earplugs for shooting – the higher the NRR, the better. 

Q: Which is better – earplugs or earmuffs?

A: They both offer similar protection but, the highest level of protection provided by the earmuffs is 31dB (NRR 31), while the highest level of protection offered by the earplugs is 33dB (NRR). So, you can say that the earplugs offer slightly better protection than the earmuffs. If you want the best possible protection, use them together.

Q: What is the difference between SNR and NRR?

A: SNR (single number rating) and NRR (noise reduction rating) are two standards for measuring the same thing – noise attenuation provided by a hearing protection device (earmuffs or earplugs). NRR is the US standard while the SNR is the EU standard. SNR ratings are usually just a little bit higher (2-3dB) than the NRR ratings for the same product. 

Q: Are silicone or foam earplugs better?

A: If we are only discussing the noise reduction/attenuation, one study showed that the foam plugs provide a slightly higher level of attenuation across a wide range of frequencies (30dB-43dB) than silicone plugs. However, the difference is not huge and your choice will depend mostly on your taste. Some people simply don’t like foam and wax plugs because they have to be inserted into the ear canal. 

Q: Are silicone earplugs safe?

A: In general, all kinds (wax, foam, silicon) of earplugs are safe but they still have some side effects. Using the earplugs regularly could cause the earwax buildup and that could lead to tinnitus and temporary hearing loss. These are not serious or complicated medical states but are very unpleasant and require intervention. 

Q: At what dB do you need hearing protection?

A: According to OSHA, exposure to 90dB noise over an 8-hour period is acceptable and shouldn't cause any damage. If the noise exposure level exceeds 90dB, some kind of hearing protection is necessary. Other organizations, like the NIOSH, recommend that workers should use hearing protection if the noise exceeds 85dB. So, if you want to be safe, wear hearing protection even if the noise level is at 85dB. 

Q: Can you fire a gun without ear protection?

A: You can do whatever you want, but the real question is – is that going to cause some damage? Police officers don’t wear hearing protection while patrolling but they do wear hearing protection for indoor shooting. Shooting without ear protection regularly can definitely cause permanent hearing damage.

Q: What does SNR 35dB mean?

A: SNR is the EU standard for noise attenuation. It’s the equivalent of the NRR in the US. The SNR 35dB indicates that the hearing protection device (HPD) is capable of eliminating up to 35dB of environmental noise (under ideal conditions). In real-life conditions, this device will be able to eliminate 31dB of noise (according to the UK’s Health and Safety Executive recommendations). 

Q: What is the best material for ear defenders?

A: Ear defenders (earmuffs and earplugs) are designed to protect the wearer’s hearing from the excessive noise. The earplugs are made from foam, wax or silicone. The earmuffs usually have metal construction and headband frame, thermoplastic cups, and paddings made from acoustic foam that’s supposed to absorb the sound.

Q: How do I choose the right hearing protection?

A: The first thing to know is how much noise reduction you need. So, you have to measure the noise levels in your working environment and determine what kind of NRR you need. After that, you can look for the right NRR and pick a few models. Try them, if possible, and determine which one is the most comfortable and most convenient.