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Old August 25, 2013, 07:32 AM   #6
Senior Member
Join Date: July 26, 2006
Location: Deerfield,New Hampshire
Posts: 498
I just googles NRR and I was surprised about the addition of two
devices on each ear:

How does wearing dual hearing protectors change NRR?
When dual protectors are used, the combined NRR provides approximately 5 - 10 decibels more than the higher rated of the two devices. For example using disposable ear plugs (NRR 29dB) with ear muffs (NRR 27dB) would provide a Noise Reduction Rating of approximately 39 decibels
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