Summary

The project "Study of sound transmission through hearing protectors and application of a method for evaluating their effective efficiency in the workplace" had two parts: i) a field study to measure the effective attenuation of hearing protectors in the workplace, and ii) an exploratory study to examine the potential of finite element modeling to investigate the problem of sound transmission through hearing protectors. Considering the scope of the work carried out and the amount of information to be presented, a report consisting of two separate documents was proposed. This document is the second part of the final report for this project.

Noise reduction at source is the solution to be favoured in reducing the effects of noise on workers' health and safety in the workplace. However, due to the simplicity of their use and apparent low cost, hearing protectors (earmuffs or earplugs) remain very popular. However, there is inadequate understanding of sound transmission across hearing protectors. Answering these questions through the use of an efficient model would explain the experimental observations and guide the technical developments of the instrumentation set-up proposed in part 1 for measuring the protectors' performance (optimal position of the microphones used in the protector (mainly earmuffs) performance-measuring device, better understanding of the different transfer functions between the microphones and eardrum, effects of the directivity of the incident acoustic field on the attenuation of the protectors). Such a model could also be used to improve the acoustical design of hearing protectors.

This report presents the results of an exploratory study to examine the potential of numerical modeling for investigating the problem of sound transmission through two types of protectors (earplugs and earmuffs). Finite element models of a baffled cylindrical plug inserted in a cylindrical air-filled duct equipped with a termination characterized by its acoustic impedance and of earmuffs attached to a baffle coupled to the same cylindrical duct were established using COMSOL Multiphysics software. The cylindrical duct represents a simplified ear canal with an eardrum termination. An experimental device was developed to evaluate these models. It consists of an IEC 711 ear simulator attached at the rear of a metal plate playing the role of acoustical baffle and coupled to a moulded silicone plug or to EARMUFF 1000 earmuffs in a semi-anechoic chamber. The earplug is excited by a sound wave, while the earmuffs are excited either acoustically or mechanically.

The project's results are promising. In the case of earplugs, the model is a good representation of the physics of the problem over a large frequency range with parameters adjusted at a given temperature. The differences observed between calculation and measurement are probably due to acoustical leaks in the experimental setup. In the case of earmuffs, the model gives the good trends for the two types of excitation (acoustic and mechanical) but there are differences between measurement and calculation for both the amplitude and frequency positions of the peaks. These differences can be explained by a poor knowledge of the excitation field, the inadequacy of the cushion's vibroacoustic model, and problems related to the experimental setup. This work demonstrates the importance of properly characterizing each system component (materials, impedance of the coupler, excitation), of choosing a more appropriate model of comfort cushion for the earmuffs, and of properly controlling the leaks in the experimental device. Scenarios are proposed for improving the quality of the model.