Quasi-total absorption peak by use of a backed rigid frame porous layer with circular periodic inclusions embedded

TL;DR

This paper demonstrates that embedding circular inclusions in a porous layer backed by a rigid plate can induce quasi-total acoustic absorption peaks below the fundamental resonance frequency, explained by trapped modes or increased viscous losses.

Contribution

It introduces a novel structure with embedded circular inclusions that achieves quasi-total absorption below the fundamental resonance, supported by numerical analysis and mode-matching methods.

Findings

Quasi-total absorption peaks occur below the fundamental resonance frequency.

Multiple inclusions per period generate additional absorption peaks.

Numerical results agree across mode-matching and finite element methods.

Abstract

The acoustic properties of a porous sheet of medium resistivity backed by a rigid plate in which are embedded a periodic set of circular inclusions is investigated. Such a structure behaves like a multi-component diffraction gratings. Numerical results show that this structure presents a quasi-total absorption peak below the essential spectrum, i.e. below the frequence of the fundamental quarter-wavelength resonance of the porous sheet in absence of inclusions. This result is explained either by the excitation of a complex trapped mode, or by the increase of viscous loss associated with a larger velocity gradient inside the layer at the modified quarter-wavelength resonance frequency. When more than one inclusion per spatial period are considered, additional quasi-total frequency peak are observed. The numerical results as calculated with the help of the mode-matching method described in the paper agree those as calculated with the help of a Finite Element method.