Perception of danger acoustic signals in noise

Outline of reasons and objectives
In a noisy industrial environment, perception of sound signals indicating danger is essential to keep workers safe from accident risks. When considering making it mandatory to wear personal hearing protectors to reduce exposure of workers to noise, it should be remembered that accident risks might become greater if wearing the protector degrades the perception of hazard warning signals. The challenge with wearing hearing protectors is thus twofold: to provide effective hearing protection and also to ensure that danger signals remain properly audible so as to keep workers safe. The aim of this study was to develop methods making it possible to assess the effect of wearing hearing protectors on the perception of warning signals for all protected workers, including those suffering from hearing loss.
Approach
Two complementary methods of assessing the effect of wearing hearing protectors on perception of danger signals were applied and compared: measuring masked thresholds and calculating masked thresholds. The masked threshold of a signal is the minimum level (the “threshold”) at which it is audible in a given ambient noise (the “masker”). Measurement of these thresholds is necessary, in particular, because of the potentially large differences between the real noise reduction of the hearing protectors and their theoretical reduction ratings given by the manufacturers, leading to erroneous calculation of the masked thresholds when such calculation is based on the theoretical noise reduction ratings. As regards calculating the masked thresholds, such calculation makes it easy to study the influence of various parameters (attenuation or reduction of the protectors, hearing losses, and hearing filters) independently or in combined manner.
Main results
Through industrial collaboration involving about one hundred workers and two hearing protectors, the masked threshold measurements showed that wearing a protector improved perception or had no effect, except when the ambient noise was predominantly low-frequency. As regards hearing damage, the calculations of masked thresholds showed that, for high levels, the increase in the thresholds is due more to broadening of the hearing filters than to hearing losses. Finally, concerning the comparisons between measured thresholds and calculated thresholds, small differences were obtained for the tests without protectors. Conversely, calculation did not always reproduce the effect of the protector that was observed on the basis of the measurements.
Discussion
At the end of this survey, INRS had tested and validated methods of assessing, both by measurement and by calculation, the effect of wearing hearing protectors on the perception of hazard warning signals. These methods make it possible, henceforth, to respond to external requests and to assist companies in choosing protectors that are appropriate both to their sound situations and to the hearing statuses of their workers. Various prospects can also be explored for improving the robustness of the methods used: conducting other tests by also measuring the real attenuation or noise reduction of the hearing protector; estimating the repeatability of the threshold measurements; and, finally, proposing developments of the model for calculating masked thresholds so as to take better account of the hearing protection effect.