NPSF™ -- Are you trying to understand the way ear plugs are rated for noise reduction?
| Overview |
The way ear plugs are rated depends, first of all, on the purpose of the ear plugs. Water ear plugs, for instance, usually do not have a noise rating since their primary function is keeping water out of the ear rather than keeping noise from causing hearing damage. Similarly, sleep ear plugs, flying ear plugs, and motorcycle ear plugs usually do not have a noise reduction rating since these activities are not usually dangerously loud (though some people who sleep with some snorers might well disagree!) If there is no rating provided, and you are planning to use a hearing protection product for protection against noise, our advice is to choose a product that has a rating of some sort. Once you suffer hearing damage, it can never be repaired, so do not take chances. Use only rated products when your goal is hearing protection.
Another factor in noise ratings for ear plugs comes from where the ear plugs are made, or more accurately, where they are intended to be sold. Different countries use different systems for rating the effectiveness of ear plugs. In the US and many other countries, the rating system results in an NRR (Noise Reduction Rating), in the European Union countries and some others, the rating is called the SNR (Single Number Rating,) while in Australia and New Zealand ear plugs and muffs are rated by a class system called SLC80 (Sound Level Conversion.) While NRR and SNR tell you what amount of noise reduction the product can provide when used properly, the SLC80 system tells you, instead, against what maximum noise level the device will provide adequate protection. For example an SLC80 class 1 ear plug is good for noise up to 90 dB, class 2 up to 95 dB and so on, in 5 dB increments. The SLC80 system really does not need further explanation. You just need to determine what is the level of noise against which you need protection to choose based on SLC80 ratings. See the section below for how to determine the noise level. The other two rating systems do need a little more discussion as follows below. |
| NRR/SNR |
NRR and SNR ratings are based on separate, unique, testing protocols and computations of the data collected during these tests, which means the two ratings are not convertible, one to the other. The only generalization that can be accurately made is that SNR generally will be higher than NRR for the exact same hearing protector. Beyond that, there is no direct conversion possible.
Ideally the NRR or SNR rating should simply be subtracted from the ambient noise level against which you are seeking protection, to see if the remaining exposure level is safe or not. For instance, if the ambient noise level is 100 dB, and the NRR rating of the ear plug is 29 dB, your exposure level will be 71 dB, a perfectly safe noise level. The trouble is that most users do not get the ear plugs properly inserted and so they do not get the full rated performance from their ear plugs. This is especially true for roll-down foam ear plugs, the most commonly used type of ear plug. Accordingly, guidance ranges from derating the ear plugs by subtracting 6 dB from the rating, to cutting the rating in half, in order to estimate the real world rating. For example if you derate the NRR in our example by half, to 14.5 dB, and subtract that from 100 dB, you get 84.5 dB, just barely in the safe level of long term exposure.
Reusable ear plugs are easier to use than are foam ear plugs, but they still vary from use to use in their effective level of protection, so it is a good idea to derate these as well in order to choose an ear plug with a high enough NRR rating.
Custom ear plugs eliminate the derating issue completely, so even though they may be rated at a maximum of 26-29 dB, you will generally get better protection from a custom ear plug than you will with universal fit ear plugs. You will always get the full rated performance of custom ear plugs, because they can be fitted just one way into your ears. This is one of the most important benefits of a custom ear plug. |
| Determining what rating you need |
Before you can determine what rating you need in an ear plug or ear muff, you need to know what level of noise you are trying to protect against. That is often not a simple matter. Large industrial companies use expensive SPL meters, noise monitoring systems, and/or outside consultants to determine the effective ambient noise environment, so in such workplaces, the level of protection is usually well established and all you have to do is ask.
In your home shop or on the riding mower, though, you will need to take your own measurements. You can easily do this by downloading a smartphone app and taking your own readings. The term to search in the app store is "spl meter" or "sound pressure level meter."
Once you know the ambient noise level, choosing a hearing protector based on the rating is fairly straightforward. |
| Avoiding overprotection |
We often hear from customers who want "artificial deafness," that is, they want to block all sound, to hear nothing but silence. This is almost always a terrible idea.
First, when your brain is deprived of any sound inputs, it will nearly always begin to produce its own noises, commonly called tinnitus, which can be more annoying than the noise you were trying to avoid.
Second, if you cannot hear anything, you are at greatly increased risk of injury. People and vehicles can "sneak up on you" causing a startle response that results in you injuring yourself, or even causing you to get run over by an inattentive driver.
When trying to avoid hearing damage, the rule is, use only the amount of hearing protection you need to bring your exposure into the safe zone (<85 dB for 8 hours in a 24 hour period). This will ensure that you will nearly always be able to still hear what you need to hear to remain safe. This is referred to as "situational awareness," and is always a good thing to maintain. |
| Rating dual protection systems |
For impulse noises, such as gunfire, massive industrial presses, and constant noises over 110 dB, it is difficult to get overprotected. In such cases, more protection is what you should go after, including the use of dual protection systems; wearing ear plugs plus ear muffs over them.
To determine the effective noise protection rating of any dual protection system you use, add 6 dB to the higher rating of the two devices to tell you the effective rating of the combination. For example, if you wear an NRR 33 ear plug, with an NRR 26 ear muff over those, your nominal NRR will be about NRR 39 (33 + 6.) To be conservative though, you should first derate the ear plugs by half, suggesting the combination would be 16.5 dB from the plugs and 26 dB from the muffs, or an effective total of 32 dB (26 + 6).
Why only 6 dB improvement from adding the second layer of protection? Glad you asked. Sound waves can travel into your inner ear where it is converted to what we call noise, via three pathways: via air conduction through the ear canal, via bone conduction through the bones in your head, and via soft tissue conduction through the torso and up through the neck. The first pathway, through the ear canal, is the easiest to block. An NRR 33 ear plug essentially blocks all sound from entering through the ear canal. About 95% of all sound enters our ears through the canal, with most of the remaining 5% getting there through bone conduction through the head. Only a trivial portion of noise gets there through the soft tissues.
Since most of the noise has already been blocked by the ear plugs, the ear muffs can only block a little more, mainly by dampening some of the bone conduction that would occur without the muffs.
I would like to add that while 6 dB may not seem like much, it can make a tremendous difference in extreme noise environments, and especially in impact noise environments. A convenient rule of thumb is that a 10 dB increase in attenuation results in a reduction in volume of 50%! So, a 6 dB increase in attenuation you get from adding muffs over your ear plugs can result in a 30-40% decrease in the volume to which you are exposed.
Be safe. Use the dual protection system any time you think you should. |
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