Measurement of the local static mechanical pressure of earplugs

Earplugs are used in various critical industrial sectors, such as construction, aviation, military and defense, transportation, and healthcare. However, they present inherent drawbacks, notably discomfort that can lead to inconsistent and incorrect use, thereby leaving a significant proportion of workers vulnerable to irreversible hearing damage, ranging from tinnitus to deafness. This discomfort is closely linked to a physical parameter known as static mechanical pressure (SMP), representing the pressure exerted by earplugs on the earcanal walls. Determining the SMP is crucial for developing earplugs that prioritize comfort. However, experimental studies in this area are scarce and there is no experimental set-up capable of measuring the SMP whether in vivo or ex vivo. The objective of this paper is to fill this gap by presenting a novel experimental system designed to mimic a real earcanal, equipped with an optical probe for precise pressure measurements at various points along the earcanal. This precision-targeted methodology holds the promise of yielding a comprehensive pressure map spanning the entirety of the contact area between the earplug and earcanal, thereby offering unprecedented insights into SMP distribution. To validate our test bench, we conducted experiments using cylindrical rolled-down PVC earplugs inserted into a rigid cylindrical earcanal replica of varying radii. We compared our results with data obtained from a commercially available radial force tester commonly used in investigating stent grafts. Once validation was achieved, we expanded our investigation to measure SMP across various types of earplugs. Additionally, we designed and measured the SMP using roll-down foam earplugs in an artificial earcanal shaped to resemble the human anatomy. Our experimental findings were corroborated with data from finite element method (FEM) numerical calculations. Apart from providing the first comprehensive characterization of the static mechanical pressure exerted by earplugs, our research reveals that with roll-down foam earplugs, increased compression prior to insertion leads to reduced SMP. The versatility of our specially designed test bench allows for mapping SMP caused by earplugs across diverse earcanal shapes. Integration of this instrument into design frameworks holds promise for enhancing the physical comfort of earplugs and, consequently, improving their overall efficacy in hearing protection.