NRR, SNR, HML — Hearing Protection Ratings Demystified?

Hi all,

I know there's a lot of posting already on NRR ratings, the NRR - 7 / 2 calculation, and all that. But after reading everything online, and everything on the board, I'm still confused. Which likely means others are too... so can someone explain this all to me once and for all?

Here's what I know so far:

*NRR is the US metric; SNR is the EU metric.

* NRR goes up to 33 dB max; SNR appears to have no posted max rating (can anyone confirm?)

* NRR isn't accurate, and needs to be recalculates via that NRR - 7 / 2 formula; However, I can find *nothing anywhere* to suggest that SNR values require the same treatment. Can anyone confirm?

* NRR values are "laboratory values", while SNR values are "real world" values by untrained testers (and this ostensibly explains why NRR ratings get reduced and SNR ratings do not). Can anyone confirm?

* SNR values further come with an HML (high/med/low) metric that breaks attenuation into specific frequencies; NRR is just a global (and thus inaccurate) metric.

Now for the questions that I'm hoping someone can help with:

1. Everything online, including reputable safety equipment and workplace safety websites, indicates that SNR values *can* be trusted to dampen dBs by the rated value. Is this true? Does anyone know for sure??

2. If SNR and NRR values are both ratings of sound attenuation, as measured in decibels (i.e. they are on the same scale), why do they differ?

It seems like we have two rating scales that differ *drastically* regarding what they are suggesting. For instance, my EarPeace plugs (which I love for low level protection) have an NRR rating of 14 dB, and an SNR rating of 20 dB. If I trust the NRR rating, these plugs provide only 14 dB - 7 / 2 = 3.5 dB of protection (which is completely implausible). But if I trust the SNR rating, I get a full 20 dB of protection (which may also be implausible, but seems closer to the truth, by subjective experience).

So which is it? 3.5 dB or 20 dB? Can anyone clarify?

 

*crickets*

 

Bumping this - is it really the case that nobody knows??

 

Even if we knew for sure, it doesn't really matter. Tinnitus can get worse due to noise exposure, with or without earplugs - go with whatever level of protection makes you feel like you're not taking unnecessary risks

Don't expose yourself to unnecessarily loud noise and remember to have a good time

 

Im quite lost, but I do understand this: the decibel scale is logarythmic, and this means that on the top of the scale a small increase in decibel (maybe from 100 to 101) means a great increase in how harmful is sound, and the perceived loudness.

The 30 db rating for earplugs maybe offer 30 db protection from 0 to 30 dbs but they cannot substract 30 dbs from the top of the decibel scale, maybe they just offer a few dbs of protection, the further up you are in the decibel scale, the less protection they offer. So maybe if you are exposed to 110 dbs and have earplugs on, you are still perceiving like 105 dbs or so, not 80 dbs, wearing earplugs that attenuate by 30 dbs.

 

Hmmm... this is an interesting idea.

You're right, decibels are logarithmic. But does that really mean that plugs are less effective at higher volumes? Why couldn't they still remove 30 dB?

 

Because the 30 dbs on top of the scale mean many times the sound power of the 30 dbs at the lowest part of the scale.

 

In Australia and New Zealand, it's very straightforward; our ear protection is defined by SLC80 (Sound Level Conversion valid for 80 per cent of the wearers).

It is the "difference between the measured C-weighted sound pressure level of the workplace noise outside the hearing protector and the A-weighted sound pressure level, attenuated by the hearing protector, under the hearing protector inside the ear canals."

So pretty much, if you have the best earplugs available, which have an SLC80 of 26 dB or above (also known as Class 5), then you will get 26 dB of protection.

Also interestingly, "Noise reduction rating (NRR) The NRR system is used in the USA, but is not acceptable in Australia as the method of testing is different from the AS 1270 test method."

Source

 

Interesting that the Australians use yet another method. Not sure if that clarifies things or complicates them further though.

First: the same question still applies - why does NRR require a -7/2 formula, and SLC80 does not? Presumably *the same plugs* could have both ratings...?

Second: Juan's point about the logarithmic nature of decibels is very interesting, and makes some sense. I wish we had an expert here to answer this question about less protection at higher volumes. Would be pretty ironic for a plug intended to protect against loud noises to only be maximally effective in quiet environments!

 

Yes, I see what you're saying.

So really we need to know at what volume the plugs are tested at. Because it could be that they *do* removed 30 decibels from a 100 dB noise, and are even more effective at lower volumes. Intuitively, this makes some sense - with 30 dB plugs in, I would not expect to hear a 40 dB noise at all...?

 

You will hear it if you have hyperacusis and over-excited hair cells that amplify sound. Healthy outer hair cells work to amplify sound in quiet environments and dampen sound in loud places. Hyperacusic hair cells amplify all sounds no matter what.

When my hyperacusis was at its worst I could hear perfectly someone talking with 30 dB earplugs and Peltor Optime III earmuffs on top.

 

Interesting. Though talking is likely closer to 60 dB than 40 dB - but point taken.

Thanks for adding to this conversation Juan - you've got my brain churning.

 

More about sound, in general:

Hearing is so complex.. hyperacusis in some cases means damage, loss, or alterations of the outer haircells. The outer haircells enhance sound in quiet enviroments (that's why a clock ticking very low in a silent room is clearly audible and it can sound kind of "loud" and clear, because our hearing enhances the sound), and outer haircells together with other mechanisms and muscles also dampen the sound of loud sounds.

When outer haircells do not work properly the perception of sound is altered and their role to dampen loud sound is compromised. For some this means recruitment, for others hyperacusis, etc After a hearing shock, it is easier to experience recruitment, maybe just by listening to objects one places on a table. Just place let's say a comb on a table: if the sound is unnaturally amplified, there is something that does not work well with hearing, probably haircells not working properly.

The most obvious way to experience recruitment at louder levels is listening to a loud source of noise getting closer (let's say a car). At the beginning the sound is far away and it is comfortable, but there is a point in which the increase in volume as the car gets closer is not linear, but suddenly jumps and feels a lot louder. That can be recruitment.