Porogranular materials composed of elastic Helmholtz resonators for acoustic wave absorption

Abstract

We present a theoretical and experimental study of the acoustic absorption of granular porous media made of non-cohesive piles of spherical shells. These shells are either rigid or elastic, possibly drilled with a neck (Helmholtz resonators), and either porous or impervious. A description is given of acoustic propagation through these media using the effective medium models proposed by Johnson (rigid particles) and Boutin (rigid Helmholtz resonators), which we extend to the configurations studied in this work. A solution is given for the local equation of elasticity of a shell coupled to the viscous flow of air through the neck and the micropores. The models and our simulations are compared to absorption spectra measured in reflection in an impedance tube. The effective medium models and our measurements show excellent agreement for configurations made of rigid particles and rigid Helmholtz resonators that induce an additional peak of absorption at low frequency. A shift of the Helmholtz resonance toward low frequencies, due to the softness of the shells is revealed by our experiments for elastic shells made of soft elastomer and is well reproduced by our simulations. We show that microporous shells enhance and broaden acoustic absorption compared to stiff or elastic resonators.

Sound attenuation optimization using metaporous materials tuned on exceptional points

“…; Electronic mail: benoit.nennig@supmeca.fr is by multi-layering 15,16 . Other alternative solution is to embed periodic inclusions 17,18 or subwavelength resonators [19][20][21][22][23] to create metaporous materials (resonant inclusions embedded in a porous matrix). Thanks to this approach, it is possible to get a total absorption 24 below the quarter wavelength frequency and to widen the absorption in a specified frequency range.…”

Numerical and experimental investigations on the acoustic performances of membraned Helmholtz resonators embedded in a porous matrix

“…The resonance frequency of the obtained Helmholtz resonator can be shifted down in comparison to the resonance frequency of a rigid resonator. Griffiths et al [23] presented a comparison between absorption measurements and theoretical models of porogranular media made of soft elastomer elastic HRs. It has been demonstrated that the use of a material with a Young's modulus on the order of 1 MPa or less leads to lower the Helmholtz frequency due to the softness of the elastic resonator.…”

Additional Sound Absorption Within a Poroelastic Lamella Network Under Oblique Incidence

“…For instance, double porosity materials that use the pressure diffusion effect between the micro and the macro pore networks have been proposed [3,4,5,6]. Another approach is to excite trapped modes between a rigid inclusion and a rigid wall [7,8].…”

“…Instead of fluid resonators, membranes [19], thin plates [20] and shell resonances [21,6] have also been considered. These elastic resonators usually resonate at lower frequencies than their fluid counterparts, and avoid the boundary layer thickness constraint that can typically limit the Helmholtz resonator neck diameter.…”

“…In order to enhance low frequency sound absorption, quarter wavelength [9,10] (eventually coiled [11]), split ring and Helmholtz resonators [12,13,14,15,6] have been embedded in porous materials. Such resonators can dramatically boost sound absorption provided that the resonator frequency is above the Biot frequency (i.e., transition from a viscous to an inertial regime).…”