How to Calculate NRR (Noise Reduction Rating) of Hearing Protection?

I'm seeing conflicting information on how to calculate the decibel reduction of hearing protection using NRR system.

The OSHA website shows the following formula:
NRR - 7

Sources:
[1], [2]

However, other sources (including manufacturers of hearing protection) use the this formula:
(NRR - 7) / 2

I'm also unsure about double protection. Do all earmuffs simply add 5 dB of reduction regardless of their NRR rating? I cannot find any reference that a 31NRR pair of earmuffs will reduce more or less than a 21NRR pair when used with earplugs.

 

https://www.tinnitustalk.com/posts/567682/

 

So with double protection I’m assuming I can get smaller earmuffs, and they will provide the same amount of extra protection as my large earmuffs.

 

Yes, from that article @ajc referenced that appears to be the case as long as the muffs have the lower NRR of the two forms of protection.

 

I'm still unsure about the formula discrepancy between OSHA and other sources. It should probably be a no-brainer to stick with what OSHA says, but I'm wondering why other sources are saying to divide by 2.

 

This is on the CDC website in regard to NIOSH standards:

“However, studies have repeatedly demonstrated that the NRR does not accurately predict noise reduction performance for an individual in the workplace. Research shows that most workers achieve less than half of the sound attenuation predicted by the NRR. The difference between the NRR and actual worker attenuation varies widely across different types of protectors and different ear canal sizes and shapes.”

https://www.cdc.gov/niosh/docs/2015-181/default.html​

This site says that both OSHA and NIOSH /2 at some point in their calculation. Though that conflicts with your original source which is OSHA directly:

“NIOSH recommends dividing the NRR by 2 and then subtracting 7 if using a TWA measured in dBA. OSHA prefers to subtract 7 and then divide by 2.”

https://www.protectear.com/us/blog/2011/03/31/whats-wrong-with-nrr-choose-a-or-b/​

If you read this discussion of the ANSI standards it discusses two testing methods for calculating protection:

“Method-A is designated trained-subject fit, and it is meant for devices fitted by “individually trained and well-motivated users”, while Method-B, called inexperienced-subject fit, is for devices that are for groups of occupational users.”

https://blog.ansi.org/2017/08/measuring-noise-reduction-hearing-protective-ansi-epa/​

So possibly the main OSHA listing is for individually trained and well motivated users and the /2 is for standard users. Just speculation on my part. Hope that helps.

 

I've also been thinking about the NRR - 7 part as well. Earasers sells a 5NRR earplug. Are those the same as wearing no earplug then? 5NRR - 7 = -2. What's the point of giving something an NRR rating and then saying "oh but now you have to subtract 7 from it." What's up with that?