The high, medium, and low method--a better noise reduction rating?

This paper compares hearing protector attenuations calculated using National Institute for Occupational Safety and Health (NIOSH) method No. 1 ("long method") and the HML method. The study was done on 144 combinations of 12 noises and 12 protectors. In each case, the attenuation was calculated using each of the above methods. The difference between attenuations was used to define the accuracy of the high, medium, and low (HML) method. It was found that the attenuations calculated using the HML method are almost always lower than those calculated using the NIOSH method. However, the differences between both attenuations are well within the field noise level measurement errors. Consequently, the HML method appears to be an acceptable approximation of the NIOSH method, offering the additional advantage of being easier to apply. The higher accuracy of the HML method, when compared to the Noise Reduction Rating (NRR), makes is preferable for applications in industrial hearing conservation programs.