NRR on hearing protection

[AAChang]

I've noticed on most hearing protection, there's a NRR (Noise Reduction Rating?) I was wondering how that is measured, such as what's the scale and how is it in relation to other types of noise (aircraft, loud music, etc). Also If you wear two forms of hearing protection does it add directly (example: if you have foam earplugs in that have a NRR of 10 and then the over the ear cover kind like ear muffs that have a rating of 25, does it equal to a NRR of 35?) Sorry if this post is a little confusing but I hope someone out there can help.

[hksigwalther]

[Speculation]

I'm pretty sure the NRR is on the decibel scale. So, for example, a .22lr at 120 dB will be reduced to 95 dB with 25 NRR plugs.

I don't see why the ratings would not be added if using two forms of protection at the same time. If the first line, like ear muffs, has an NRR of 30 the .22lr would sound like 90 dB on the inside of the muff. When those sound waves hit the second line, say foam ear plugs, with an NRR of 25 at 90 dB, then the side noise should be reduced to 65 dB when it hits your eardrum. This would be the same as NRRs of 30 + 25. 120 dB - 55 = 65 dB.

[RHC]

No, decibels work more like the earthquake scale, where the value is going up ten times for each number. I don't think you can just add up the numbers.

I'm sure there's a good web site that explains all this. According to my dictionary,

"A bel is ten decibels. The number of bels difference between two amounts of power is the logarithm to the base ten of the ratio of one amount to the other."

Well, that clarified it for me!

[David Park]

It took me a while, but 25dB SPL + 10dB SPL = 26.4dB SPL. SPL stands for Sound Pressure Level. A decibel has to be a ratio of something, so a dB SPL is a ratio of the sound in question and the quietest sound humans can hear (defined as 20 micro-Pascals of sound pressure).

Keep in mind that, because dBs work on a log scale, a small numerical difference can make a big perceptive difference. The 1.4dB above may not seem like a lot, but the difference between 25dB and 26.4dB is much greater than, say, 10dB and 11.4dB. In fact, that 1.4dB above 25dB is worth 10dB by itself. In other words, muffs plus plugs are worth the effort.

The formula for the above addition is:
sum = 20 log ( 10^(dB1/ 20) + 10^(dB2/20) )

There may be an easier way to get the same number. There are some neat things about decibels, such as any sound "twice is loud" is always 6dB louder. For example, if one gunshot is 120dB, two gunshots at the same time is 126dB. You can check this with the above formula.

Some links:
http://www.norsonic.com/web_pages/in...ng_the_db.html
http://www.phys.unsw.edu.au/music/dB.html

[hksigwalther]

Whoop, yup you guys are right. Forgot the decibel scale was logarithmic.

[part swede]

I differ with the above analysis. It takes a log of a fraction involving decibels; that does not make sense. A decibel is a logarithm rescaled by 10 to account for the "deci". If hearing protectors always reduce incoming power by the same ratio, regardless of the power level, then it is correct to add the decibel ratings to get 35. That follows from the fact that the multiplying two ratios, and taking the log, equals in the sum of the logs of the ratios. But this is only true assuming that the NRR is independent of the sound dB level.

[Spackler]

The actual reduction rating of plugs plus muffs doesn't really matter that much, because both together are better than one or the other alone, and that's what counts. The highest NRR of a plug that I'm aware of is the Howard Leight "MAX", which is rated at NRR 33. The classic yellow cylindrical E*A*R plugs have an NRR of 29. Combine either with a good set of muffs with an NRR of 21, or a set like the Dillon Ultimate 10, which has an NRR of 29, and you are doing all you can do. Nothing out there on the market right now will do a better job.

[Skorzeny]

Pardon me for my ignorance, but doesn't the NRR rating have mostly to do with sustained sound (pulse?), like drilling sound, rather than instantaneous ones like gun report?

My understanding is that the NRR rating does not accurately predict protection for sounds like gun report. Supposedly, the protection is actually better than claimed by the NRR for the latter.

If someone can clarify, I'd appreciate it as, I am sure, others here would.

Skorzeny

[Spackler]

Continual exposure to high noise levels will cause problems a lot faster than brief exposure to be sure. Permanent damage can occur in less than two hours, if you are exposed to noise over 100dB for that period of time. What that means to me is, I'm still feeling the effects of that Damn Yankees/Bad Company concert back in '90.

[johnbt]

Skorzeny - Now that you mention it, my Peltor Ultimate 10 package said something to the effect that the rating did not apply to sounds such as gunshots. As best I remember, it didn't explain it whether it was better or worse.

John

[Double Naught Spy]

As I recall, sound pressure doubles with every three decibel units. There is twice as much pressure at 103 db over 100 db.

I wear two sets of ear protection. The first are the plugs rated aroud 20 and then Leighting muffs with a rating of 31. Sure enough, both together work much better than either one alone. What I find the true benefit of wearing two sets is that if I forget to put on my muffs, the plugs keep damage from occuring or certainly not nearly as much damage as going without. Wearing a second set of ear protection is something like carrying a backup.

And yes, the NRR rating is for continuous sound, such as loud equipment like newspaper presses, lawn mowers, etc. and is not for pulses noice such as comes from gun shots. Even so, they seem to work just fine.

Reasons why muffs can offer only so much protection is because the materials out of which they are made, how good of a seal they have to prevent leakage of undampened sound from going straight into the ear, and because much of the sound transferred to the ear drum comes from conduction of the noise through the bones of the cranium. So no matter how good the muffs are and how many layers you wear over your ears, the ear drum will still feel the sound vibrations and that signal will be interpreted by the brain as sound.