NRR Formula

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The NRR (Noise Reduction Rating) Formula

The NRR is a single number rating which is required by law to be shown on the label of each hearing protector sold in the United States. The NRR is specified by 40 CFR code of Federal Regulations, Part 211, Product Noise Labeling, Subpart B - Hearing Protection Devises. It (the NRR) is independent of the noise spectrum in which it is applied.

The values of sound attenuation used for calculation of the NRR are determined in accordance with ANSI S3.19-1974, "American National Standard for the Measurement of Real-Ear Hearing Protector Attenuation and Physical Attenuation of Earmuffs." The NRR calculation assumes a pink noise with octave-band levels of 100dB. Thereafter various correction factors for C-weighting scale and A-weighting scale are introduced. The octave-band noise levels are logarithmically summed to obtain the overall sound level in dB(C) and dB(A). The NRR is computed by subtracting 3 dB from the difference between the unprotected C-weighted and the protected A-weighted levels at the ear.

Computation of the Noise Reduction Rating

Octave band center Frequency, Hz	125	250	500	1000	2000	3000	4000	6000	8000	Log Sum
1. Assumed pink noise (dB)	100	100	100	100	100		100		100
2. C weighting corrections (dB)	-0.2	0	0	0	-0.2		-0.8		-3
3. Unprotected ear C-weighted level	99.8	100	100	100	99.8		99.2		97	107.9
4. A weighting corrections (dB)	-16.1	-8.6	-3.2	0	1.2		1.0		-1.1
5. Unprotected ear A-weighted level	83.9	91.4	96.8	100	101.2		101		98.9
6. Average attenuation at each frequency (example)	21	22	23	29	41	43	47	41	36
	21	22	23	29	41		45*		38.5*
7. Std. deviation in dB at each frequency (example)	3.7	3.3	3.8	4.7	3.3	3.3	3.4	6.1	6.5
	x2	x2	x2	x2	x2
8. Two standard deviations	7.4	6.6	7.6	9.4	6.6		6.7**		12.6**
9. Complete APV-98 in dB at each frequency. (Line 6 - line 8) (APV = Average Protection Value)	13.6	15.4	15.4	19.6	34.4		38.3		25.9
10. Protected ear A-weighted level, (average attenuation minus two std. deviations develops the A-weighted levels (line 5 - line 9))	70.3	76.0	81.4	80.4	66.8		62.7		73.0	85.1
11. NRR is unprotected ear "C" level (line 3) minus protected ear "A" level (line 10) minus 3 dB										19.8

* Average attenuation at 3000 and 4000 Hz and at 6000 and 8000 Hz.
** Summed standard deviation for 3000 and 4000 Hz and 6000 and 8000 Hz.
Source: NIOSH Hearing Protector Compendium
When C-weighted sound level measurement is available, the following formula should be used to calculate the required NRR
Noise Level in dB(C) - Protector NRR = 90 dB(A) or less.
When A-weighted sound level measurement is available, the following formula should be used to calculate the required NRR.
Noise Level in dB(A) - (Protector NRR - 7 dB) = 90 dB(A) or less

Click here to read an official document explaining the NRR and SNR calculations.

The NRR (Noise Reduction Rating) Formula

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