Periodicity-Enhanced Attenuating Layers and Structures

周期性增强（Meta）材料和表面是人工结构，具有天然材料和表面中没有的特性。周期性源于主体基质中夹杂物的规则间距或表面上的粗糙度。夹杂物的范围从空气中的固体圆柱体，如“声波晶体”中遇到的多孔弹性材料中的网格框架，如用于吸声的充气泡沫。粗糙度元件可以具有各种形状和轮廓，从相同的矩形凹槽到具有分形轮廓的阵列。在没有进一步修改的情况下，结晶度增强材料阻止了一些入射波长（或频率）的通过，并增强了其他波长（或频率）的传输。通过改变表面的粗糙度，可以控制从源直接行进到表面上方的接收器的波与从表面反射的波之间的干涉。该提案涉及扩大频率范围的方法，在该频率范围内，增强的材料和表面减少了声音和振动的传播。要研究的方法包括使用局部共振夹杂物或粗糙度元素，使用多个共振，利用共振之间的相互作用和重叠，与结晶度相关的传输损耗和周期性的空间变化以及其他特性，从而产生分级系统和粗糙度轮廓。这项工作将为设计更有效的声音和振动吸收装置提供基础，这些装置重量轻，但具有高传输损耗和振动阻尼特性。由此产生的表面设计将包括传统隔音屏障的替代方案，同时允许进入和保护视线，以及保护建筑物免受地面振动的成本效益方法。

Periodicity-enhanced (meta) materials and surfaces are artificial structures that possess properties not found in naturally-occurring materials and surfaces. The periodicity stems from the regular spacing of inclusions in a host matrix or roughness on a surface. Inclusions range from solid cylinders in air such as encountered in 'sonic crystals' to a grid framework in a poroelastic material such as an air-filled foam used for sound absorption. Roughness elements can be of various shapes and profiles ranging from identical rectangular grooves to arrays with fractal profiles. Without further modification, periodicity-enhanced materials stop the passage of some incident wavelengths (or frequencies) and enhance the transmission of others. By modifying the roughness of a surface, the interference between waves travelling directly from a source to a receiver above the surface and waves reflected from the surface can be controlled. The proposal is concerned with ways of extending the frequency range over which the periodicity-enhanced materials and surfaces reduce the transmission of sound and vibration. The methods to be investigated include use of locally resonant inclusions or roughness elements, use of multiple resonances, exploitiation of interactions and overlaps between resonances periodicity-related transmission loss and spatial variation of periodicity and other characteristics thereby producing graded systems and roughness profiles. The work will provide a basis for the design of more efficient sound and vibration absorbing devices that are lightweight yet offer high transmission loss and vibration damping properties. The resulting surface designs will include alternatives to conventional noise barriers, while allowing access and preserving line of sight, and cost-effective methods for protecting buildings against ground-borne vibrations.