NRR When Doubling Up

[Larry T]

It's generally accepted that when doubling up, you don't add the two NRRs - but you add 5 to the highest NRR to get your actual NRR - assuming perfect fit, usage. So a 33 NRR plug with muffs of any reasonable number (24-30 NRR) gives you a best-case 38 NRR.

Here's the question: based on this, assuming you always double up, why would you ever choose a more expensive set of muffs for the higher NRR UNLESS you were paying for additional features and the higher NRR is a bonus OR you plan to use just muffs some of the time?

This could apply to the Peltor 300 vs 500 or even the Howard Leight Impact Sport vs Pro.

[peterb]

I always go higher, because real-world attenuation is much worse than one would expect from the published NRRs.

"The most obvious feature of Fig. 1 is the very poor correspondence between the magnitude of lab and field NRRs. Mea- sured as a percentage of the laboratory- rated attenuation, the field NRRs for ear- plugs yield only 6 to 52% of the labeled values (averaging about 25%), and for earmuffs, from 33 to 74% (averaging about 60%)."

https://pdfs.semanticscholar.org/4ae...ad59975de8.pdf

[Larry T]

This part of the article sort of addresses but doesn't answer my question...

"Of equal importance is the inherent within-laboratory and between-laboratory variability in test data.8 Considering these facts, one can state that differences in the NRR of less than 3 dB have no practical importance, and even 4- to 5-dB changes are of questionable significance unless closely controlled data are being compared..."

This seems to be saying that an advertised difference of -5 NRR is insignificant. If the formula of "Rated NRR minus 7 divided by 2" is accurate, there would be almost no difference. So a NRR of 30 vs 24 isn't a difference of 6 - but (30-7) / 2 = 11.5 vs (24-7) / 2 = 8.5... a real difference of 3db. But even an additional 3db NRR would cut about 25% more sound volume, so it seems even a 3db NRR is meaningful.

[peterb]

From reading the literature and my experience, I think it is worth paying more for a better fit. That'll get you closer to the "lab" attenuation values. Earcup shape and depth, seal shape and materials, fit over glasses, glasses temple size and shape, plug size and shape -- all of those can cause variation from "average" results.

For shooting, muffs are almost never worn without glasses, but glasses usually cause a "leak" in the muff seal which lets in noise.

David Clark -- the aviation headset manufacturer -- sells a foam seal made to help minimize glasses gaps, but I've never seen anyone use them. I tried making my own and they did seem to help.
http://www.sportys.com/pilotshop/avi...ions-pair.html

This company sells earplug assortment packs so you can find one that works well for you:
https://www.earplugstore.com/unfoamtrialp.html

One of the reasons that "real world" numbers for earplugs are low is that they are often inserted incorrectly. Here's the right
way:
https://m.youtube.com/watch?v=3S6dthcSVIM

[RJ]

It's generally accepted that when doubling up, you don't add the two NRRs - but you add 5 to the highest NRR to get your actual NRR - assuming perfect fit, usage.

I’m curious as I’ve never heard that.

Where have you seen this guideline?

[blues]

I’m curious as I’ve never heard that.

Where have you seen this guideline?

Here's one place I've seen it discussed:

http://www.noisehelp.com/double-hearing-protection.html

[Larry T]

I’m curious as I’ve never heard that.

Where have you seen this guideline?

If you google "NRR when doubling up" it will be in practically every article. "peterb"'s links are also helpful.

[TGS]

Here's the question: based on this, assuming you always double up, why would you ever choose a more expensive set of muffs for the higher NRR....

Because it's still more attenuation.

[Larry T]

From reading the literature and my experience, I think it is worth paying more for a better fit. That'll get you closer to the "lab" attenuation values. Earcup shape and depth, seal shape and materials, fit over glasses, glasses temple size and shape, plug size and shape -- all of those can cause variation from "average" results.

For shooting, muffs are almost never worn without glasses, but glasses usually cause a "leak" in the muff seal which lets in noise.

David Clark -- the aviation headset manufacturer -- sells a foam seal made to help minimize glasses gaps, but I've never seen anyone use them. I tried making my own and they did seem to help.
http://www.sportys.com/pilotshop/avi...ions-pair.html

This company sells earplug assortment packs so you can find one that works well for you:
https://www.earplugstore.com/unfoamtrialp.html

One of the reasons that "real world" numbers for earplugs are low is that they are often inserted incorrectly. Here's the right
way:
https://m.youtube.com/watch?v=3S6dthcSVIM

I've used a bunch of different plugs and like Flents Quiet Contour (green 33 NRR) the best, but will use Mack's 33 NRR if my ears get sore. The Macks are shorter than the Flents.

[RJ]

If you google "NRR when doubling up" it will be in practically every article. "peterb"'s links are also helpful.

Ok, thanks. I did not know that was a estimate published by OSHA.

I’m kind of scratching my head how you could come up with an arbitrary 5 dB; all things being equal. Two attenuators of x dB, when combined, will attenuate at x+3 dB, at a maximum. So it seems kind of fishy to me if one of the two pieces of ear pro is *less* than the other, how you get to 5 dB.

I admit I’m coming from a technologists viewpoint in terms of the math involved, and not from a background in Audiology. Maybe all things are not equal when it comes to NRR.

I definitely double up when possible, and focus more on proper ear plug fit these days.

Anyway: you learn something new every day. Thanks.