The exploration of transmission property by using the circular-interface types of porous acoustic metamaterials

Based on the generalised Snell's law, many planar-interface types of acoustic metamaterials have been proposed and extensively used. However, circular-interface types of acoustic metamaterials (CAMs) that could be applied to aero-engine intakes and aircraft fuselages have not been extensively explored. Porous CAMs have been explored theoretically, numerically, and experimentally to investigate their acoustic properties, such as wave-front modulation and insertion loss, in this research. The analytical equation that governs the sound refraction of cylindrical waves through CAMs is derived based on the principle of stationary phase for the first time. In the numerical study, two types of porous CAMs with various geometries that generate periodically linear phase gradients are applied to circular interface structures. Results indicate that the excitation of high-order wave modes in the transmitted sound pressure field is dependent on the angular distance over one period of porous CAMs. When this parameter goes below critical values, the high-order wave modes in the transmitted sound pressure field will be converted into evanescent surface waves. Consequently, the sound insertion loss of porous CAM is significantly enhanced compared to that of uniform melamine foam of the same thickness. Based on the analytical and numerical study, technical guidance is proposed for designing porous CAMs with defined radii and dominant frequencies, taking compensation factors into account. This study offers a technical method for extending the industrial applicability of acoustic metamaterials from a planar interface to a circular interface. © 2022 Elsevier Ltd.