Friday, October 22, 2010

Table for Adding NRR Values

The Decibel scale is logarithmic so when using more than one kind of hearing protection, you cannot just add the NRR ratings together to arrive at the total. The table below can be used to find the combined NRR rating when two different methods are used.

+ NRR A
NRR B 192021222324252627282930313233
19 2223232424 2526272829 2930313233
20 2323242425 2526272829 3030313233
21 2324242525 2626272829 3031313233
22 2424252526 2627272829 3031323233
23 2425252626 2727282829 3031323333
24 2525262627 2728282929 3031323334
25 2626262727 2828292930 3031323334
26 2727272728 2829293030 3131323334
27 2828282828 2929303031 3132323334
28 2929292929 2930303131 3232333334
29 2930303030 3030313132 3233333434
30 3030313131 3131313232 3333343435
31 3131313232 3232323233 3334343535
32 3232323233 3333333333 3434353536
33 3333333333 3434343434 3435353636

Even moderate pressure cartriges like the 45 ACP or 38 spl will generate 155 dB near the muzzle. High pressure cartridges like the 9MM, 40 S&W or 357 magnum will produce 160 dB to 165 dB. The report from a firearm is highly directional so the sound pressure level (SPL) you as the ooperator experience out of doors may be 10 Db to 15 Db less. However, in an indoor range, reflections will intensify the sound and lengthen the time your ears are exposed to damaging sounds levels.

Also the NRR is an average value measured over a range of frequencies. Depending on what frequencies dominiate the report from your particular gun, some ear plugs and/or muffs will be more efffective than others even if the NRR is the same.

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