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		19	20	21	22	23	24	25	26	27	28	29	30	31	32	33
	19	22	23	23	24	24	25	26	27	28	29	29	30	31	32	33
	20	23	23	24	24	25	25	26	27	28	29	30	30	31	32	33
	21	23	24	24	25	25	26	26	27	28	29	30	31	31	32	33
	22	24	24	25	25	26	26	27	27	28	29	30	31	32	32	33
	23	24	25	25	26	26	27	27	28	28	29	30	31	32	33	33
	24	25	25	26	26	27	27	28	28	29	29	30	31	32	33	34
	25	26	26	26	27	27	28	28	29	29	30	30	31	32	33	34
	26	27	27	27	27	28	28	29	29	30	30	31	31	32	33	34
	27	28	28	28	28	28	29	29	30	30	31	31	32	32	33	34
	28	29	29	29	29	29	29	30	30	31	31	32	32	33	33	34
	29	29	30	30	30	30	30	30	31	31	32	32	33	33	34	34
	30	30	30	31	31	31	31	31	31	32	32	33	33	34	34	35
	31	31	31	31	32	32	32	32	32	32	33	33	34	34	35	35
	32	32	32	32	32	33	33	33	33	33	33	34	34	35	35	36
	33	33	33	33	33	33	34	34	34	34	34	34	35	35	36	36

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.