Summary

Many workers are exposed every day to noise levels that could damage their hearing. Because of this, measuring sound exposure is essential to identify and assess remedial solutions to adequately protect workers. However, current methods and technologies to measure exposure do not generally allow for accurate determination of the noise exposure experienced by individuals in their workplaces. In-ear dosimetry is a promising solution to a number of problems associated with conventional measurement methods, but this methodology has three weaknesses: (1) the sound exposure measured in the ear canal can be strongly influenced by noise disturbances induced by the wearer of hearing protectors; (2) acoustic correction is required to compare the noise levels measured in the ear canal with the exposure limits defined in various standards and regulations; (3) questions remain as to the effect of occlusion of the ear canal on hearing sensitivity.

This paper is a four-part study, the first three of which address the three issues mentioned above. Firstly, the results of loudness equalization tests with 18 human subjects suggest that the acoustic load applied to the ear canal has no effect on hearing sensitivity, and that the risk of hearing damage caused by sound pressure on the eardrum is thus not influenced by whether or not hearing protectors are worn. Next, methods and instrumentation were developed to enable individual calibration of in-ear dosimeters in the field. A series of experiments involving human participants showed, for the first time, that these methods and the prototypes could be used to determine individual hearing correction by in-ear dosimetry, in the open ear, or under earplugs or noise-canceling earmuffs. Finally, these same prototypes were used to develop an approach to detect, and exclude if necessary, noise disturbances emitted by an individual while wearing an in-ear dosimeter. This approach, tested in the laboratory with 14 human subjects, was found to be especially effective when measured under earplugs. The fourth part deals with the design and fabrication of in-ear measurement devices and the real-time implementation of measurement algorithms developed during this research project for use in the workplace.

In general, this report presents the methods, tools and knowledge developed to enable effective individual in-ear measurement of sound exposure in the workplace. By significantly reducing the number of constraints associated with in-ear dosimeters, such work should have considerable repercussions in terms of preventing work-related hearing trauma. The results obtained show that all of the scientific developments completed can now be considered in the context of field measurements applied to workers equipped with earplugs, and are particularly suitable for individuals working in predominantly reverberant environments who are subjected to a low proportion of impulsive noise. The perspectives for further work outlined in this report will enable these fields of application to be significantly extended to cover a wide range of noise conditions and the wearing of hearing protectors.