It’s talk about Earmuff H9A
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.