It is estimated that around 500,000 of the 3.7 million workers in Quebec are daily exposed to noise levels which may cause damage to hearing. When it is impossible to reduce the noise at source or to limit the propagation of sound, the use of hearing protection devices (HPD), such as earmuffs, remains the solution of necessity. While seen as a short term solution, HPDs are commonly used since they are low cost, easy to employ and readily portable to most operations in noisy environments. However, HPDs can prove both inadequate for workers and their environment as well as uncomfortable, thereby limiting their wearing time and reducing effective protection.
To address some of these issues, a collaborative research project on hearing protection between the École de technologie supérieure (ETS) and the Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST) was launched in 2010. As part of this research program, this Ph.D. thesis focuses specifically on hearing protection using passive earmuffs, whose use is associated with three specific problematics described below.
The first specific problematic concerns the discomfort caused, for example, by the static pressure induced by the headband clamping force, which can reduce the recommended wearing time.
The second specific problematic is the assessment of the actual protection provided by the protector. The REAT (Real Ear Attenuation Threshold), also seen as a "golden standard", is used to quantify the noise reduction but generally overestimates the performance of the protectors. Field measurement methods, such as the F-MIRE (Field Measurement in Real Ear) are seen as better tools to assess the individual attenuation. While such techniques exist for earplugs, they must be adapted and improved for earmuffs by determining the optimal location of the acoustic sensors and the individual compensation factors linking the sound pressure level measured at the microphone to the sound pressure level at the eardrum.
The third specific problematic is the optimization of earmuff design, which is generally based on empirical criteria. So far, there has been very little investigation of predictive Tools or modeling and this area merits further study. Virtual prototyping would optimize the design before production, accelerate the product development phase and reduce costs.
The principal objective of this thesis is to address these issues by modeling the sound attenuation of commercial earmuffs. Given the geometrical complexity of these protectors, the use of the finite element method (FEM) was chosen. To achieve this principal objective, three specific objectives were established, as briefly detailed below.
The first specific objective is the development of an acoustical test bench to assess the sound attenuation of earmuffs, with two sub-objectives: (1) to pinpoint the correct modeling level for each component based on an assessment of the sound transfer path through the earmuff components, and (2) to validate the developed model.
The second specific objective is the development of the FEM model based on knowledge of geometric entities, material parameters and boundary conditions applied to the modeled earmuffs.
The third specific objective is to utilize the results of measurements and models to (i) better understand the vibroacoustic behavior of the earmuffs, (ii) propose design improvement solutions, and (iii) assist in developing the F-MIRE method.
Chapters 1-7 present the methodology used and the results attained. This thesis contains three scientific articles written in English, submitted, accepted or published in peer reviewed journals. Additional chapters, in French, have been added to provide further details and developments.
Outcomes from this research can be divided into three categories: scientific outcomes, technology outcomes and outcomes related to health and safety in the workplace.
From a scientific point of view, the modeling efforts combined with the experimental work provided a better understanding of the vibroacoustic behavior of earmuffs. Moreover, while the predicted attenuation did not perfectly match the measurements, a closer correlation between the model and the experimental validations was obtained compared with earlier published research.
From a technology point of view, the developed models can be used as predictive tools to virtually prototype earmuffs and speed up the product development phases. This may be done by running parametric studies on the geometric entities, material parameters of the different components of the earmuff, to identify design optimization priorities. Also, the model's ability to predict the sound field inside the earmuff make it an ideal tool to help develop field measurement methods, such as the F-MIRE, by determining ideal locations for the measurement microphones as well as compensation factors.
Finally, in terms of health and safety at work, the use of modeling to optimize earmuff design and to advance field measurement methods will, in the long term, lead to better hearing protection devices, with enhanced performance, and better adapted to both the user and the environmental work noise. Greater protection for workers is therefore anticipated, while ensuring their physical and acoustic comfort and reducing the risks of occupational hearing loss.
The thesis concludes by proposing various work perspectives, several of which emphasize a refinement of the existing model to improve the correlation between simulations and validation measurements. Other perspectives suggest broadening the research to other types of earmuff than the one studied here. It is further proposed to extend the scope of the modeling by adding a coupling to an artificial test fixture, by combining the earmuff model with one developed for earplugs to study the dual protection, and also by adapting the model to transient noises.
| Date | 15 Jul 2015 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Frédéric Laville (Supervisor) & Franck Sgard (Co-supervisor) |
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