Quote:
Originally Posted by Brian Pfleuger
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Third, NRR ratings don't stack mathematically, at least by addition. If you are wearing plugs rated at pretty much the highest possible rating (33) and muffs at the same rating, the combined NRR isn't 66. OSHA (29 CFR 1910.95 and 29 CFR 1926.52) recommends adding 5db for the second set of protection, so 33+33 becomes 38, at best.
It's also worth noting that OSHA has a formula for "real world use" that indicates an actual protection level of less than half (subtract 7 and divide by 2) so 38 becomes more like 15-16.
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The decibel scale is logarithmic, so a 3 dB reduction is 10 raised to the -0.3 power, which is 1/2 the sound pressure level. So adding in another 3 dB reduction is very significant, even though the numbers don't look too impressive. Using both muffs and ear plugs probably cuts the sound pressure level in half or more, and is definitely noticeable.