Predicting the acoustic attenuation of dual protection

When workers are subjected to very high noise levels, one way of protecting their hearing is to use dual protection, i.e. to wear earplugs and earmuffs simultaneously. The literature shows that the acoustic attenuation provided by a dual protection is much lower than the sum of the acoustic attenuations provided by each of the hearing protectors, but it does not propose any method for evaluating it, which poses a risk to safety. A review of the literature was undertaken. The acoustic attenuations of a number of combinations were collected. At the same time, tests were carried out on four combinations, in order to supplement the relatively old data in the literature with up-to-date data. On the basis of this data set, formulas were established to calculate the octave-band attenuations of the combination of two protection devices from their respective attenuations. The overall attenuation indices (SNR, H, M and L) can be deduced to select hearing protectors according to noise exposure. These indices are predicted with good accuracy, less than 3dB. This work will enable occupational health and safety professionals to assess the protection provided by dual protection, and thus to recommend it in a fully safe way.