Mesure F-MIRE en continu de l'atténuation des protecteurs auditifs : expérimentation terrain et modélisation par éléments finis

Abstract

While hearing protectors devices (HPD) are widely used to protect workers from Noisy environments, a question still needs being asked: “How well is a worker protected during his work shift, compared to the protection written on the label of his HPD ?” In order to answer this question, two main objectives were developed: (1) a field measurement method was developed and tested and (2) a numerical model was developed and validated to improve the measurement method. To meet the first main objective of the thesis, a field measurement system was developed. It consists of a microphone doublet that records simultaneously the external and the internal sound pressure of the HPD. The method is called continuous F-MIRE (Field- Microphone-in-real-ear) and the difference between the 2 microphones is called measured noise reduction (NR*). Results for earmuffs and custom molded earplugs on 24 workers in 8 different work environments are presented. The signal processing from in-house routines give useful indicators like : exposed levels, protected levels, NR*. Indicators are presented as a function of time or frequency. A field attenuation index was developed and compared to standard attenuation (ANSI S12.68 (2007)). Several observations are made about field attenuation measured: (i) as found in the literature, field attenuation values are lower than labeled, for a given worker, (ii) attenuation vary considerably as a function of time during his work shift, (iii) attenuation vary between two workers with the same type of protector, but also between the two ears of the same worker, (iv) the field attenuation index developed for this research has a strong dependence to the frequency content showing how poor the attenuation is at low frequency and how important it is to know the type of noise field the worker will be in to choose the right HPD for him. Finally, research perspectives are given to improve the method. The second main objective of this thesis is to develop and validate a numerical model of an earmuff coupled to an ATF (acoustical test fixture) that will allow studying different indicators influencing the precision of the method. This objective comes from a finding during the development of the measurement method that NR* varies, up to 20dB per third of octave band, as a function of the sound source incidence. The finite element (FE) model of the earmuff/ATF, that takes into account the scattering made by the geometry of interest, is excited by a plane wave of various incidence angle. To validate the model, a measurement protocol was developed where a sound source revolves in a horizontal plane around an instrumented earmuff on an ATF. The earmuff of interest is an EAR-1000 (3M). Regarding the evaluation of the external microphone SPL and of the NR*, the model correlates very well with measurements for frequencies below 1250Hz whatever the sound incidence. Above 1600 Hz, the FE model captures the trends, as a function of the incidence angle, but the agreement generally decreases with increasing frequency. A better correlation between the FE model and the experimental data is achieved for the variation of NR* as a function of the sound incidence. Actions such as accounting for the headband in the model, refining the modeling of the sound source, improving the cushion modeling and better describing the backplate/cushion coupling are suggested to improve the accuracy of the model. To illustrate the potential of the model to improve the continuous F-MIRE measurement method, the FE model is used to determine an optimal position of the external microphone and to obtain estimates of exposure levels using the left and right ear exterior microphones.

Date	7 Jul 2016
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Frédéric Laville (Supervisor) & Hugues Nélisse (Co-supervisor)