Low frequency finite element models of the acoustical behavior of earmuffs

Abstract

This paper compares different approaches to model the vibroacoustic behavior of earmuffs at low frequency and investigates their accuracy by comparison with objective insertion loss measurements recently carried out by Boyer et al. [(2014). Appl. Acoust. 83, 76-85]. Two models based on the finite element (FE) method where the cushion is either modeled as a spring foundation (SF) or as an equivalent solid (ES), and the well-known lumped parameters model (LPM) are investigated. Modeling results show that: (i) all modeling strategies are in good agreement with measurements, providing that the characterization of the cushion equivalent mechanical properties are performed with great care and as close as possible to in situ loading, boundary, and environmental conditions and that the frequency dependence of the mechanical properties is taken into account, (ii) the LPM is the most simple modeling strategy, but the air volume enclosed by the earmuff must be correctly estimated, which is not as straightforward as it may seem, (iii) similar results are obtained with the SF and the ES FE-models of the cushion, but the SF should be preferred to predict the earmuff acoustic response at low frequency since it requires less parameters and a less complex characterization procedure.

Low-Frequency Noise Reduction by Earmuffs with Flax Fibre-Reinforced Polypropylene Ear Cups

“…Generally, analytical models are developed based on the concepts of lumped parameters modelling. Boyer et al [28], for instance, provided a comprehensive review in this aspect. In their review, they stated that analytical models would only provide an approximation of the acoustical performance for a given earmuff up to about 1 kHz and before the occurrence of the first acoustic or structural resonance.…”

“…Numerical methods are generally suitable to approximate the acoustical performance of earmuffs up to about 5 kHz [28] where the limitations now lie on the mesh density of the model and the computational resources required for high-frequency prediction. Although numerical methods could address the limitation of analytical models, challenges remain-to date-where the material properties and the interaction between each component of the earmuff must be correctly specified.…”

“…Evidently, the air cavity enclosed by the ear cup would take the boundary profile of a highly irregular shape. However, Boyer et al [28] and Paurobally and Pan [29] mentioned that the acoustic stiffness of the air cavity could be approximated by considering it as a hemicylindrical volume. In this case, the acoustic stiffness of the air cavity air is given by [28] …”

“…Boyer et al [6] revealed that the pumping motion of the ear cups would highly influence the acoustical performance of the earmuff at low frequency. Separately, Boyer et al [28] mentioned that the pumping motion of the ear cups would typically occur at around 100-300 Hz. Hence, it could be inferred that the IL dip at around 100-200 Hz was attributed to the pumping motion of the ear cups.…”

“…To minimise such leakages, the headband force could be increased, applying more pressure on the temporal bone. For instance, Boyer et al [28] demonstrated that a higher headband force would achieve a better sealing of the cushion pads, leading to an improved low-frequency IL of the earmuffs. However, the resulting high pressure would compromise comfort and might discourage the user from wearing the earmuff over an extended period of time [8].…”