Acoustic test fixtures (ATFs) can be adopted to assess the sound attenuation of hearing protectors (e.g., earplugs and earmuffs) as they can facilitate measurements, and allow data acquisition under severe noise conditions. However, standardized ATFs are not realistic enough to closely capture the subjective attenuation of all types of protectors and their fitting on a large majority of human subjects. As an initial step towards a more accurate evaluation of hearing protector attenuation on ATFs, this thesis seeks to address two primary issues related to their vibroacoustic design features: (i) sound attenuation prediction of double hearing protectors (DHPs, i.e., earplugs combined with earmuffs) which involves structure-borne sound transmission through the ATF; (ii) impact of the IEC 60318-4 ear simulator acoustic impedance in the ATF earcanal on the hearing protector attenuation.
On the one hand, the DHP effect on a commercial ATF is studied through specially designed experiments by modifying the system coupling conditions or controlling the sound pressure level under the earmuff. Such an effect refers to the phenomenon where the DHP overall attenuation falls short of the algebraic sum of each single protector’s attenuation, and is particularly characterized by the decrease of the earplug noise reduction after adding the earmuff. Experimental data suggest that the DHP effect is mainly associated with the structure-borne energy transmitted from the earcup, through the earmuff cushion/ATF assembly and finally into the earcanal due to the sound radiation of the earplug and/or earcanal lateral walls, which dominates over the “direct” airborne transmission via the earplug outer surface. A finite element model is afterwards developed and experimentally validated to predict the DHP effect on an ATF. The crucial contribution of structure-borne transmission is further confirmed through the power balances simulated with selected configurations of the ATF. The DHP effect is shown to originate from the structure-borne power injected from the ATF boundaries and/or earmuff cushion. The important influence of earcanal wall vibration is highlighted when the artificial skin is accounted for. An indirect structure-borne path is identified which corresponds to the radiation of the earplug excited by the earcanal walls.
On the other hand, a transfer matrix model of an ear simulator is proposed based on a direct assessment of its geometric dimensions, and validated through numerical and experimental approaches. Compared to the lumped parameter model commonly used in the literature, this model is shown to accurately account for the thermo-viscous effects in the simulator, and represent its input impedance in a relatively wide frequency range. The transfer matrix model enables to retrieve an equivalent tympanic impedance of the simulator which is imposed as an impedance boundary condition at the “eardrum position” in the finite element model of open and occluded ATF earcanals to simulate the insertion loss of an earplug. Furthermore, a similar process is adopted to investigate the influence of the eardrum impedance inter-individual variability on the earplug attenuation. This is achieved through a Monte Carlo simulation of 1000 equivalent tympanic impedances obtained by varying the simulator dimensions in the transfer matrix model. The simulation results are deemed representative of the variability in the human eardrum impedance. Representative sets of equivalent tympanic impedance are then selected among these results, and applied as impedance boundary conditions in a finite element model to simulate the earplug insertion loss in a realistic-shaped earcanal. The variability in the equivalent tympanic impedance is shown to induce non-negligible differences in the insertion loss results, indicating that the human eardrum impedance diversity should be accounted for by the ear simulator for earplug attenuation measurements.
Overall, this thesis presents the experimental methodology and numerical models for investigating the ATF vibroacoustic features of interest, and in the long term, could serve as a foundation to guide the design and implementation of ATFs for more realistic characterization of hearing protector attenuation.
| Date | 16 Dec 2021 |
|---|
| Original language | American English |
|---|
| Awarding Institution | - École de technologie supérieure
|
|---|
| Supervisor | Olivier Doutres (Supervisor), Franck Sgard (Co-supervisor) & Hugues Nélisse (Co-supervisor) |
|---|
Luan, Y. (Author),
Doutres (Supervisor), Sgard (Co-supervisor) & Nélisse (Co-supervisor),
16 Dec 2021Student thesis: Doctoral thesis › Doctorate in Engineering: Engineering