The attenuation of hearing protectors against high-level industrial impulse noise; comparison of predicted and in situ results

Three earmuffs, each with a different cup volume and type test attenuations were measured by the MIRE technique in drop forging-workshops. The cup volumes were rated as small, medium and large. The small size was planned for military use against shots and blasts from firearms and explosions. In the drop-forging plant the unweighted peak sound pressure levels were around 140 dB, which is considered the upper limit for the linearity of the attenuation of hearing protector devices (HPD). Our results indicate that none of the earmuffs satisfied the hearing conservation purposes in the selected plant. The peak levels inside the HPDs and effective to the ear were 120–130 dB, and may be considered acceptable if the number of impulses is low. The energy equivalent levels effective to the ear were between 84 and 97 dB, i.e. far from satisfactory protection. The best protection was provided by the medium volume HPD, whereas the HPD intended for military use gave the lowest protection. The application of the HML method showed that the large volume earmuff underestimated the MIRE attenuation by 2–3 dB, whereas the medium and small volume earmuffs provided 5–11 dB lower attenuation, respectively. This difference can be explained by the clearly higher headband force in the large volume earmuff than in the medium and small volume earmuffs. The protection efficiency can be improved by wearing earmuffs and earplugs simultaneously.