Abstract
Low frequency sound attenuation is a challenging task, because of the severe mass, stiffness and volume constraints on the absorbing and/or reflecting barriers. Recently, significant improvements in low frequency sound attenuation has been achieved by introducing the acoustic metafoam concept, which combines the mechanism of conventional acoustic foams - high viscothermal dissipation - with the working principle of locally resonant acoustic metamaterials - wave attenuation at low frequencies. However, the attenuation improvement provided by periodic materials containing identical resonators is confined to a narrow frequency range. To overcome this limitation, graded acoustic metafoams are proposed and studied here, where a distribution of local resonators with varying properties (mass and stiffness) is introduced. It is demonstrated that, through a suitable design of mass and stiffness distribution of the resonators, the broadening of the frequency attenuation ranges can be effectively achieved. Graded acoustic metafoams are, therefore, a natural development direction for achieving broad frequency attenuation zones.
Highlights
Low frequency noise is a significant problem, especially in urban environments that are saturated with sources of unwanted sounds [1]
The recently proposed acoustic metafoams [7], which consist of a poro-elastic material endowed with local resonators, might be a promising approach, since this foam based microstructure can be potentially manufactured via wellcontrolled foaming processes
As demonstrated in Lewinska et al [7], acoustic metafoams combine the benefits of standard acoustic foams and locally resonant acoustic metamaterials (LRAMs), exhibiting improved attenuation at low frequencies while preserving the visco-thermal dissipation effects typical of standard foams
Summary
Low frequency noise is a significant problem, especially in urban environments that are saturated with sources of unwanted sounds [1]. Most of the matamaterials are still not suitable for mass production [6] Along this line, the recently proposed acoustic metafoams [7], which consist of a poro-elastic material endowed with local resonators, might be a promising approach, since this foam based microstructure can be potentially manufactured via wellcontrolled foaming processes. In the field of locally resonant acoustic metamaterials (LRAMs), which are a promising solution for low frequency sound attenuation problems [5], it has been demonstrated that the use of functionally graded microstructures can address the main application limit of these materials, i.e. the narrowness of the band gaps. The analysis is performed by means of computational homogenisation, which enables the identification of macroscopic effective parameters that control the acoustic response of the material The dependence of these parameters on the stiffness and mass variation of the resonators is computed and used to design functionally graded metafoam models.
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