Abstract
Acoustic metamaterial absorbers have been built and tested with focus on low frequency airborne sound absorption in linear and nonlinear regimes. The absorbers are made up of a series of piled up flat cavities, separated by thin walls and traversed by a perforation at their centre. A model for absorber effective properties is developed and compared with experimental data. The model is used to derive simple formulae for the frequency and the peak value of the absorption coefficient at the lowest frequency resonance, depending on the geometrical parameters of the structure. Different absorbers have been built with several cavity thicknesses to allow comprehensive comparisons with the model. Nonlinear properties of the absorbers are investigated experimentally using sine wave excitation around the resonance frequency with the amplitude of the incident wave up to 250 Pa. Flow resistivity measurements at low flow rates show that the periodic set of cavities does not modify resistivity significantly when compared to a simple perforated cylinder with same thickness. As flow rate increases, the flow resistivity grows linearly according to Forchheimer's law and has a significant dependence on the absorber thickness. A numerical model is developed accounting for the linear growth of flow resistivity with particle velocity amplitude in the central perforation and compared with the measurements at high amplitudes of the incident wave.
Highlights
In recent work by Dupont et al, [1], a 3D printed microstructure was considered that contains a periodic arrangement of thin plates with central perforation separated by cavities (Fig. 1), the so-called “pancake absorber”
To investigate the effect of lateral cavities on the static flow resistivity values and on Forchheimer’s nonlinearity parameter, the measurements performed on several absorbers and on the corresponding solid cylinders with same thickness and central perforation size
An absorbing structure comprising of periodically arranged metallic plates separated by air cavities and with a central perforation traversing the periodic structure, was built and tested in both linear and nonlinear regimes
Summary
In recent work by Dupont et al, [1], a 3D printed microstructure was considered that contains a periodic arrangement of thin plates with central perforation separated by cavities (Fig. 1), the so-called “pancake absorber”. Metamaterial structures, studied in this work, are similar to pancake absorbers (and for this reason we use the same name) but made of periodically arranged metallic plates with the central perforation They are mechanically robust and do not contain any fibrous materials or foams. To investigate the effect of lateral cavities on the static flow resistivity values and on Forchheimer’s nonlinearity parameter, the measurements performed on several absorbers and on the corresponding solid cylinders with same thickness and central perforation size ( eliminating the effect of cavities completely).
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