An empirical formula for predicting the attenuation given by double hearing protection (earplugs and earmuffs)

In some specially noisy environments, double protection (generally earplugs and earmuffs) can be needed for efficient worker audition protection. Some data has already been published on attenuation provided by several combinations of commercially available plugs and muffs. In the present study, data were collected from different laboratories and completed by our own tests. On the basis of these data, an empirical formula was derived, which expresses the global attenuation ENR 80 (according to ISO/DIS 8353) given by a couple of protectors from the global attenuation given by each of them. The accuracy of the assessments made by this formula is around ±4 dB (95% confidence interval) when considering all the protectors, and ±3 dB when considering those protectors having an attenuation higher than 15 dB. These confidence intervals have to be increased by 1 dB when considering ENR 90 instead of ENR 80 .