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This study systemically applies porous acoustic metamaterials in an inverted wedge shape on the planar interface. The porous acoustic metamaterials have been studied analytically, numerically, and in the laboratory. Results indicate that the new porous acoustic metamaterials based on melamine foams demonstrate anomalous reflection phenomena and possess better sound absorption properties in a wide range of frequencies and incidence angles than uniform melamine layers and the classic wedge shape absorbers with the same weight. Several metasurfaces with different periodical lengths are also discussed to improve their sound absorption properties further. In the porous acoustic metamaterial design, the wave manipulation can be well achieved regardless of the properties of porous materials as long as the desired reflected phase shifts are realized on the reflected surface, which creates a new way to increase the sound absorption properties of classic absorbing foams. © 2020 Elsevier Ltd

This thesis investigates acoustic properties of porous acoustic metamaterials (AMs) including both planar and circular-interface types of AMs (PAMs and CAMs) and ultra-thin nano-fibrous membranes backed with melamine foams. PAMs, including porous triangle-shaped AMs and porous labyrinthine type of AMs (LAMs), are studied analytically, by simulation and by laboratory tests. The triangle-shaped AMs significantly improve the sound absorption coefficient of the uniform melamine foams at the frequency of interest. Furthermore, the porous LAMs simultaneously demonstrate better sound absorption coefficient (more than 55%) and transmission loss property (more than 300%) from 1, 000 Hz to 3, 000 Hz than original melamine foams. The sound absorptivity of ultra-thin nano-fibrous membranes backed with melamine foams is evaluated over a broad frequency range from 500 Hz to 6,400 Hz. The effects of the thickness of the nano-fibrous membrane, the nanofiber diameter and the melamine foam thickness are experimentally studied, which are also analysed by several analytical models. The proposed thin sound absorber possesses an excellent potential to improve sound absorption coefficients, especially for thin melamine foam samples. Porous CAMs have been studied analytically, numerically and in a laboratory. An analytical expression of the sound refraction through the CAMs is derived based on the principle of the stationary phase. In the numerical and experimental study, porous LAMs are applied on circular interfaces. Results indicate that two parameters, which are the angular distance over a period of CAMs and the ratio between the radius of a circular interface and the incident wavelength, are determining factors in the excitation of high-order wave modes in the scattered sound pressure fields. This study provides an engineering method to apply CAMs in industrial applications.

This study systemically applies acoustic metamaterials based on wedge-shaped melamine foams on the planar interface. A new phase shift acoustic porous metamaterials have been studied analytically, numerically and in the laboratory. Results indicate that the new acoustic porous metamaterials demonstrate anomalous reflection phenomena and possess better sound absorption properties in a wide range of frequencies and incidence angles than the uniform melamine layers and the classic wedge absorber with the same weight. Several metasurfaces with different design parameters are also discussed to improve sound absorption properties. As the melamine foams can be replaced by other acoustic foams, it creates a new way to increase the sound absorption properties of classic absorbing foams. © 2019, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

A number of planar-interface types of acoustic metamaterials (PAMs), including tapered labyrinthine structures and porous materials, have been proposed based on the generalized Snell's law. Circular-interface types of acoustic metamaterials (CAMs), however, have not been studied extensively. These can find applications on aero-engine intakes and aircraft fuselages. In this study, CAMs are studied analytically and numerically. An analytical expression of the sound refraction through the CAMs is derived based on the principle of stationary phase. In the numerical study, several fibrous metal foams with various parameters that generate a periodical linear phase gradient on the transmitted interface, are applied on circular interfaces. Results indicate that two parameters, which are the angular distance over a period of CAMs and the ratio between the radius of a circular interface and the incident wavelength, are determining factors in the excitation of high-order wave modes in the scattered sound pressure field. The high-order wave modes will disappear in the refracted sound pressure field when these two parameters are at critical values. The analytical expression is derived based on the assumption that the distance between two incident sound wave paths is infinitesimal. The assumption leads to the differences between the critical values given by analytical predictions and numerical simulations, which can be significantly reduced with the increase of ratios between the radius of a circular interface and the incident wavelength. This study offers a technical method in extending the industrial applicability of acoustic metamaterials from a planar interface to a circular interface. Copyright © (2018) by International Institute of Acoustics & Vibration. All rights reserved.

A number of planar-interface types of acoustic metamaterials (PAMs), including tapered labyrinthine structures and porous materials, have been proposed based on the generalized Snell's law. Circular-interface types of acoustic metamaterials (CAMs), however, have not been studied extensively. These can find applications on aero-engine intakes and aircraft fuselages. In this study, CAMs are studied analytically and numerically. An analytical expression of the sound refraction through the CAMs is derived based on the principle of stationary phase. In the numerical study, several fibrous metal foams with various parameters that generate a periodical linear phase gradient on the transmitted interface, are applied on circular interfaces. Results indicate that two parameters, which are the angular distance over a period of CAMs and the ratio between the radius of a circular interface and the incident wavelength, are determining factors in the excitation of high-order wave modes in the scattered sound pressure field. The high-order wave modes will disappear in the refracted sound pressure field when these two parameters are at critical values. The analytical expression is derived based on the assumption that the distance between two incident sound wave paths is infinitesimal. The assumption leads to the differences between the critical values given by analytical predictions and numerical simulations, which can be significantly reduced with the increase of ratios between the radius of a circular interface and the incident wavelength. This study offers a technical method in extending the industrial applicability of acoustic metamaterials from a planar interface to a circular interface. Copyright © (2018) by International Institute of Acoustics & Vibration. All rights reserved.