Hearing protection devices like earplugs and earmuffs often come with various noise reduction ratings on their packaging. Understanding what these ratings mean is vital for safety professionals and PPE retailers in helping customers choose the right protection.
In this post, we will demystify three key concepts – SNR (Single Number Rating), NRR (Noise Reduction Rating), and relevant EN standards – in clear, non-technical language. We’ll explain how each rating system works, where each is used (Europe vs. U.S.), how to interpret them when selecting hearing protectors, and the key differences and limitations of each.
The goal is to help you make informed choices about hearing protection and ensure compliance with safety regulations.
What is SNR (Single Number Rating)?
SNR (Single Number Rating) is a single-value rating used primarily in Europe (including the UK) to indicate the average noise reduction provided by a hearing protection device. In simple terms, the SNR tells you roughly how many decibels (dB) of sound the protector can block out under ideal conditions.
The higher the SNR, the greater the noise reduction it offers. For example, a hearing protector with an SNR of 30 dB can typically reduce a 115 dB noise exposure down to about 85 dB at the ear. This is because SNR is an “expected attenuation value” – a 30 dB SNR means the device can lower the noise level by ~30 dB when worn correctly.
SNR is defined by international standards (specifically ISO 4869-2) and is part of the European EN 352 series of standards for hearing protectors. It is derived from laboratory tests across multiple frequencies. In fact, the ISO standard describes SNR as a single attenuation value determined from the octave-band sound attenuation data of a hearing protector.
In practice, the SNR value is used by subtracting it from the ambient noise level (measured in dB) to estimate the noise level reaching the ear when the protector is worn. Manufacturers will typically provide the SNR on the product’s packaging or datasheet, along with additional details.
H, M, L ratings: Alongside the SNR, European hearing protectors usually also list H, M, L values. These denote the device’s attenuation performance for High-, Medium-, and Low-frequency noise respectively.
In other words, H/M/L values break down how well the protector handles different pitch ranges of sound. This is useful because real-world noise can have more low-frequency rumble or high-frequency hiss. The H, M, L ratings help safety professionals pick appropriate protection for the dominant noise type – for example, a protector with a strong L value is better for low-frequency noise like heavy machinery.
Together, the SNR and H/M/L system (defined by EN 352 / EN ISO 4869) give a more complete picture of a hearing protector’s performance across frequencies.
Where is SNR used? SNR is the standard rating system in the European Union, UK, and other regions following EN standards. European regulations (the PPE Regulation and EN 352 standards) require that hearing protection devices be tested and marked with their SNR value. So, when you see a CE-marked earmuff or earplug in Europe, it will have an SNR rating (and H/M/L) to indicate its noise reduction capability.
Always ensure the product’s SNR rating meets the needs of your environment and is compliant with EN 352 – this means it has been tested according to the proper European methods. For instance, EN 352-1 covers earmuffs and EN 352-2 covers earplugs, and both require providing the attenuation data and SNR. By choosing hearing protection that conforms to EN 352 (and thus has a valid SNR), you know it’s been vetted for the EU market.
What is NRR (Noise Reduction Rating)?
NRR (Noise Reduction Rating) is a similar single-number rating, but it’s used predominantly in the United States (and some other markets) to indicate the noise reduction of hearing protectors.
The NRR is mandated by the U.S. Environmental Protection Agency (EPA) for hearing protectors sold in the U.S., and it’s measured via a standardized laboratory test. In essence, the NRR describes the average sound level reduction (in dB) provided by the hearing protector under lab conditionsosha.gov. Like SNR, a higher NRR means more noise is being blocked out – for example, an earplug with NRR 30 dB is expected to reduce noise by about 30 dB in ideal circumstances.
However, it’s important to note that the NRR is based on a specific testing protocol and includes certain built-in safety margins. The standard U.S. testing method (established by ANSI – the American National Standards Institute) involves laboratory measurements on human subjects to determine how much the protector attenuates sound.
The current NRR values you see on U.S. product labels are typically derived from ANSI’s 1974 standard (ANSI S3.19) with “experimenter-fit” procedures – meaning trained technicians ensured a best-case fit during testing. To account for variation, the calculation of NRR includes a substantial safety factor. Specifically, the EPA’s formula subtracts two standard deviations from the average attenuation in each frequency band (and even an extra 3 dB “spectrum safety” offset) to yield the NRR.
This was intended to ensure that the labeled NRR would be achieved by about 98% of users if fitted correctly. In plainer terms, NRR is somewhat conservative on paper – it’s designed so almost everyone should get at least that amount of protection (in the lab).
Where is NRR used? NRR is the required rating on hearing protectors in the United States. OSHA (the Occupational Safety and Health Administration) and other U.S. agencies refer to NRR for compliance in occupational noise reduction. The EPA requires manufacturers to print the NRR on the packaging of hearing protection.
If you’re in North America, you’ll likely see NRR on products instead of SNR. For example, a popular foam earplug might be labeled “NRR 33 dB” – indicating it can reduce noise by 33 dB in lab tests. Always ensure hearing protectors comply with the relevant U.S. standards (ANSI and EPA requirements) if you are using them in the U.S. workplace.
In practice, most quality hearing protectors designed for industrial use will have either an NRR or SNR (or both) depending on where they’re sold.
Using NRR in practice: One crucial point for safety managers is that the lab NRR may overestimate the protection workers get in the real world, especially if protectors are worn improperly. OSHA provides guidance for applying NRR to actual workplace noise. A common rule of thumb is the so-called “NRR – 7” correction.
If you have a noise measurement in dB(A) (A-weighted decibels) and an NRR value, OSHA suggests subtracting 7 dB from the NRR, then subtracting that remainder from the ambient noise level to estimate the protected levelosha.gov. For example, if noise is 95 dBA and the earplug is NRR 31: subtract 7 to get 24, then 95 – 24 = 71 dBA estimated under the protector. (This formula accounts for the difference between the test method and real-world A-weighted.)
Additionally, OSHA and NIOSH note that real-world factors (imperfect fit, movement, etc.) can further reduce effectiveness. In fact, NIOSH has recommended derating NRRs by 50% or more for field estimates in some cases. The key takeaway is that NRR is a useful comparative rating, but one should not assume you’ll get the full NRR attenuation in practice without adjustments.
Proper training on fitting earplugs/earmuffs and possibly using fit-testing systems can help ensure users approach the laboratory performance.
EN Standards and the European Approach
When we talk about SNR, we’re inherently talking about the European framework for hearing protection standards. EN standards (EN stands for European Norm) lay out the requirements and test methods for safety equipment, including hearing protectors. In Europe, hearing protection devices must meet the EN 352 series of standards to be CE-certified for sale.
The EN 352 standards have multiple parts (EN 352-1 for earmuffs, EN 352-2 for earplugs, etc.), but all ensure that a product meets certain minimum safety and performance criteria – from physical construction to acoustic attenuation. As part of this, manufacturers have to determine the device’s attenuation across frequencies (per EN 13819 / ISO 4869-1 test methods) and then calculate the SNR and H/M/L values (using ISO 4869-2).
This harmonized approach means that any hearing protector sold in Europe will come with a comparable SNR rating that you can use to judge its effectiveness.
It’s important to understand that EN ratings are brand-neutral and standardized. An SNR of 30 on one brand should represent a similar level of protection as an SNR of 30 on another brand, since both were tested to the same criteria. The EN standards ensure consistency. Furthermore, Europe provides guidance on how to select appropriate hearing protection for a given noise environment through standards like EN 458 (which offers advice on selection, use, and maintenance of hearing protectors).
EN 458:2016, for instance, gives recommendations to avoid “overprotection” and under-protection by matching SNR to the noise level. It suggests aiming to attenuate noise at the ear to a range of about 75–80 dB and warns that reducing the level below ~70 dB may be counterproductive. We’ll discuss more on that in the next section.
In summary, EN standards ensure that when you see an SNR and HML values, those numbers are backed by a defined testing methodology. Always look for the EN 352 marking on hearing protectors in Europe – it indicates compliance with these standards. If you’re a PPE retailer or safety officer in Europe, you should favor products that are EN 352 certified (which all legally sold protectors will be) and use the SNR as a guide to their performance.
Also be aware of any specialized EN 352 parts if you deal with electronic earmuffs, active noise cancellation, or communication headsets (there are specific parts of EN 352 that cover those features). For basic passive hearing protectors, though, SNR and HML are the main ratings to look at.
SNR vs. NRR – Key Differences and Limitations
At first glance, SNR and NRR might seem interchangeable – both are single-number ratings in decibels meant to tell how much a hearing protector can reduce noise. However, these ratings are not directly comparable and have some important differences due to how they are derived. Here are the key points to understand:
- Region and Standards: The most obvious difference is geographic. SNR is the rating used in Europe (per EN/ISO standards), whereas NRR is used in the United States (per EPA/ANSI requirements). If you see a product with an SNR, it’s likely a European-market item; if it shows NRR, it’s for the U.S. market. Some global products might list both. Always use the rating applicable to your region’s regulation (EN 352 for EU, ANSI S3.19 for USA).
- Test Methodology: SNR and NRR are calculated from laboratory test data, but the test protocols and calculations differ. In the U.S., NRR testing historically uses an experimenter-fit approach with highly trained subjects, and then applies a hefty statistical cushion (subtracting 2 standard deviations from the mean attenuation) to ensure a high confidence leve.
In Europe, the SNR typically uses a slightly different fitting approach (often supervised fits too) but usually subtracts only 1 standard deviation from the mean attenuation in the calculation. In other words, NRR has a larger built-in safety margin (aiming to cover ~98% of users) while SNR’s calculation uses a smaller margin (about 84% of users).
This means an SNR value will often come out a bit higher than the corresponding NRR for the same product tested under each system. - Typical Values: Because of the above differences, SNR values are usually a few dB higher than NRR values for the same hearing protector. There isn’t an exact conversion formula, but commonly you’ll find SNR is about 2–5 dB greater than NRR. For example, if a certain earmuff is rated NRR 25 dB in the U.S., its European rating might be SNR ~28 dB.
This doesn’t mean one is “better” than the other – it’s just a result of the different calculation methods. Do not directly add or subtract a fixed number blindly; if you need to compare, treat these as approximate differences. The best practice is to stick to using NRR within the context of U.S. regulations and SNR within the context of EU regulations, rather than trying to convert between them. - Usage and Application: Both ratings ultimately serve the same purpose – to help predict how much a protector will reduce noise exposure. Both assume the protectors are worn correctly and consistently. And both are based on lab conditions which may differ from real-world usage. Neither SNR nor NRR guarantees you’ll get that exact reduction in practice.
They are comparative indicators or guides. For instance, if Protector A has SNR 30 and Protector B has SNR 20, you know A offers higher potential attenuation than B under test conditions. Similarly, NRR 33 is higher protection than NRR 25. But whether you get 30 dB or 20 dB of reduction depends on fit, wear time, and the noise spectrum. All rating systems acknowledge this.
The ISO standard notes that all such methods (SNR, HML, or octave-band calculations) are only valid if the protectors are worn correctly, in the same manner as during testing, and that individual fit can vary widely. In other words, the real limitation is human factors – if a user doesn’t insert the earplugs properly, the actual attenuation could be far below the SNR/NRR. - Limitations of Each: A limitation of the NRR system, as historically implemented, is that it was based on an older test method that might not reflect average real-world user technique (hence why OSHA and NIOSH often insist on derating it for calculations).
There has been work on improving the rating system – for example, a proposed NRR(SF) “subject-fit” rating that would use more realistic fitting conditionsosha.gov. Until any new system is officially adopted by regulators, the NRR on packages remains the lab-based value, so just remember to be cautious with it.
SNR, on the other hand, already often uses a slightly less conservative calculation (one standard deviation), but it is still a lab value with well-fitted subjects. Both systems do not account for intermittent use, improper fit, or individual anatomical differences. That’s why training and fit testing are important in hearing conservation programs.
Interpreting the Ratings for Selection of Hearing Protection
Knowing what SNR and NRR mean is only half the battle – the ultimate goal is to use these ratings to select the right hearing protection for a given noise environment. Here are some practical guidelines on interpreting and applying these ratings:
- Match the Rating to the Noise Level: First, determine the noise level that workers or users are exposed to (usually in dB(A) as an 8-hour TWA or the peak levels). As a rule of thumb, you want the protector to bring the exposure down to a safe level. In occupational settings, “safe” is typically below 85 dB(A) for an 8-hour exposure (this is the exposure limit in many regulations).
However, simply aiming for just under 85 dB might not be ideal. Many experts and standards (like EN 458) suggest an optimal protected level of about 70–80 dB at the ear. This range ensures protection from hearing damage while still allowing the wearer to hear conversation or alarms reasonably well. Overprotection – where noise is reduced too much – can be a problem because it isolates the worker and can lead to safety risks or communication difficulties.
If someone is overprotected (below ~70 dB), they may feel the need to remove their protection to hear, which defeats the purpose. So, ideally choose a protector with a rating that puts the noise at ear in the 70–80 dB zone. - Using SNR in Selection (EU method): If you have the workplace noise level, you can subtract the SNR to estimate the level under the protector. For instance, if noise is 100 dB and you have an earplug with SNR 25, the level at the ear is roughly 75 dB (100 – 25). That would be in a good zone. If the SNR were 35, the ear level would be 65 dB, which is perhaps too low – indicating overprotection.
In fact, UK’s HSE guidance provides a simple table for using SNR: for noise 95–100 dB, an SNR of 25–35 is suitable; for noise 85–90 dB, an SNR of 20 or less might suffice. You generally shouldn’t pick an extremely high SNR device for a moderate noise area because it might unnecessary isolate the user. Conversely, don’t pick a very low SNR for a high noise area.
Every CE-marked protector will list its SNR, so use that as a guideline alongside the noise measurements. If the noise has a lot of low-frequency content (like loud booms or engine rumble), pay attention to the L value as well – sometimes a device with a higher SNR but poor low-frequency performance might not protect well against low-frequency noise.
In such cases, the HML method or octave-band method can be used for more precise calculation, but for simplicity, if you suspect very low-frequency noise, choose protectors known to handle low frequencies (this might be indicated by a relatively high L value in the HML ratings). - Using NRR in Selection (US method): In the U.S., OSHA’s approach would be to apply the NRR to ensure compliance with exposure limits. For example, if a worker is exposed to 100 dBA noise, using a hearing protector with NRR 33 (and applying the NRR-7 formula) gives an estimated protected level of about 74 dBA: 100−(33−7)=74100 – (33-7) = 74100−(33−7)=74 dBA.
This would be a good result (under 85 dB). If the NRR was only 15, then 100−(15−7)=92100 – (15-7) = 92100−(15−7)=92 dBA – not sufficient, as 92 dB at the ear is above safe limits. So you’d need a higher-rated protector or maybe dual protection. NIOSH recommends even more conservative adjustments (they might suggest using half the NRR for a more realistic estimate, especially for earplugs).
As a simple guide: choose a protector with an NRR that comfortably covers the difference between the noise level and the target safe level. If you want to get down to 80 dB and the noise is 100 dB, you need about 20 dB of real reduction – which might mean an NRR in the high 20s or 30 (since not all of it will be realized). Remember that doubling up protection (earplug + earmuff) doesn’t double the NRR – you get diminishing returns (approximately +5 dB extra protection when combining devices). - Consider Comfort and Wear Time: A protector can only provide its rated attenuation if it is worn 100% of the time in noise. Removing an earplug “just for a moment” in loud noise can dramatically reduce the effective protection over a shift. Therefore, it’s crucial to choose hearing protectors that users find comfortable and will keep on for the duration of exposure.
Sometimes a slightly lower SNR that is comfortable and stays in place is better than a high SNR device that the worker keeps fiddling with or takes off because it’s uncomfortable. Training users on proper insertion of earplugs or adjustment of earmuffs is equally important – even the best-rated protector won’t help if it’s worn incorrectly (for example, an earplug not fully inserted). - Special Situations: If communication or hearing signals is important, consider protectors with built-in level-dependent functions (some electronic earmuffs) or choose a protector that just meets the required attenuation but not much more, to avoid isolating the person. On the other hand, if noise levels are extreme (above 105–110 dB), double protection (earmuffs over earplugs) might be recommended.
In such cases, you can use one rating as primary and add ~5 dB as mentioned to account for the second device. Always ensure that even with dual protection, the user isn’t overprotected to the point of total isolation (many high-noise jobs have communication solutions like radios or signals to counter this). - Verify and Reassess: Hearing conservation programs often include periodic checks – audiometry tests for workers and noise measurements. If you find people still showing hearing threshold shifts, it might indicate the protection in use is not sufficient (maybe due to fit or inappropriate rating). At that point, consider re-evaluating the protector choice or refitting training.
Conversely, if the environment noise was reduced (through engineering controls) and people now have very high attenuation protectors, you might downgrade to lighter protection to improve comfort and communication. The key is that SNR and NRR are starting points – actual safety is confirmed through on-site measurements and health surveillance.
Conclusion
In summary, SNR and NRR are essential tools for understanding and comparing hearing protection, but they must be used in context. SNR (Single Number Rating) is Europe’s simplified way to show how many decibels a protector can attenuate on average, tested to EN/ISO standards. NRR (Noise Reduction Rating) is the U.S. counterpart, tested to ANSI standards and mandated by the EPA.
Both serve a similar role – helping buyers and safety professionals gauge the level of protection – but due to different testing methods and safety factors, an SNR is usually a few dB higher than the equivalent NRR. Always look for the appropriate rating for your region (e.g., EN 352 compliant products with SNR in Europe, or NRR-labeled protectors meeting ANSI standards in the U.S.). Use these ratings to choose protectors that will bring hazardous noise down to safe levels without overprotecting.
Crucially, remember that any rating is only as good as the fit and usage – a high SNR/NRR means little if the plug is not inserted properly or is taken off frequently. Provide training to users on how to wear their protectors and why it’s important.
In more advanced programs, you might use fit-testing systems to personally verify the attenuation each worker gets (this can reveal if someone’s ear canals, for instance, don’t get as much protection from a given earplug as the ratings would suggest).
By decoding these rating systems and understanding the EN standards behind them, you can make better decisions in selecting hearing protection. Whether you’re a PPE supplier advising customers or a safety manager writing a hearing conservation policy, leveraging SNR and NRR information will help ensure that people’s hearing is adequately safeguarded.
In the end, the goal is to preserve hearing health while maintaining comfort and awareness – and the proper use of these ratings, combined with practical know-how, goes a long way to achieving that. Stay informed, choose wisely, and you’ll help prevent noise-induced hearing loss on the job.