On the use of structured poroelastic materials for noise control at low frequency

Abstract

Porous materials are widely used for noise control due to their light weight and excellent sound absorption, except in the low frequency range. This paper presents two configurations where a poroelastic material is structured to address this limitation by taking advantage of a skeleton resonance at low frequency. The design allows easy adjustment of the targeted frequency without increasing the mass density or sacrificing space. First, the acoustic propagation in a duct is considered. It is controlled by an array of poroelastic lamellae, which provides an increased sound attenuation around the first bending resonance of the lamellae. Secondly, the weaknesses of sound transmission through a single or double panel partition are addressed. They are mitigated by a network of poroelastic clamped beams, the first bending resonance of which is adjusted to the first mode of the finite size plate or to the mass-spring-mass resonance of the double panel. For both configurations, experimental proofs of the concept are shown. Finally, a numerical model, which can take into account the periodicity of the structure, makes it possible to analyze the dissipation mechanisms and the effect of the governing parameters, such as the mass ratio, the Young's modulus and the airflow resistivity.

Acoustic Properties of Surfaces Covered by Multipole Resonators

“…So, the multipole scatterer can be considered as a resonator and below we will focus on this specific case. However, Equations (13)(14)(15) remain valid for any scatterer, not just those that are resonant, and can be used to describe multipole scatterers regardless of how they are formed. The main goal is to determine the impedance 𝑍 , which is strongly influenced by the structure of the scatterer.…”

“…The acoustic metasurface can be created using built-in Helmholtz resonators [10]. A single Helmholtz resonator has only one mode, but if two resonators are placed close to each other, they have two modes [14]. Figure 6a shows two resonators built into the rigid surface at a distance a that is small relative to the wavelength, i.e., ka ≪ 1.…”

“…Also, the dipole resonator can be implemented by a membrane in a ring oscillating at the first eigenmode [12] or a membrane embedded in a rigid baffle oscillating at the second eigenmode [13]. Alternatively, the dipole-type metasurface can be formed by a set of short beams clamped on one side [14]. Two coupled Helmholtz resonators have a mode for which they oscillate with opposite phases [15]; hence, two monopole resonators can form the dipole one.…”

Acoustic properties of surfaces covered by multipole resonators

“…The special boundary condition in form of a tangential impedance is needed for the equivalent surface with uniform properties [20]. Alternatively, the dipole-type metasurface can be formed by a set of short beams clamped on one side [21].…”