Sound absorption based on acoustic metasurfaces

Abstract

High-efficiency and broadband sound absorption based on ultra-thin structures is of great interest in physics and remarkable application values in engineering communities, but it remains challenging for conventional sound absorbers. Conventional sound absorbers including porous/fibre materials and perforated-panel absorbers, generally require thick structures whose sizes are on the same order of magnitude with their longest working acoustic wavelengths, which inevitably brings a barrier in settling low-frequency noise for practical applications with restricted space. Since the 2010s, acoustic metasurfaces, which exhibit unprecedented acoustic properties by modulating their sub-wavelength geometries, have shown fascinating abilities to manipulate acoustic waves. This thesis aims to realize highly efficient and ultra-thin sound absorbers based on acoustic metasurfaces taking advantage of their compact structures and superb capacity of wave modulation. Starting by pursuing perfect narrow-band sound absorption with deep subwavelength thickness, this thesis firstly investigates a structure composed of a curled channel and an embedded neck, where the former element supports a large phase delay within a thin structure and the latter element provides outstanding adjustability of the structure. Subsequently, to further improve the adjustability, the effect of the embedded neck is strengthened by extending the length and the curled channel is changed to a straight cavity for reducing the complexity of the structure. However, limited by the dispersive nature of resonances, the above designs generally feature narrow-band working performances, leading to a major obstacle in the implementations suffering from broadband noise. To solve broadband noise, this thesis further focuses on investigating high-efficiency broadband sound absorbers based on coupled acoustic metasurfaces. In the field of metasurface-based broadband absorbers, one commonly-used and straightforward technique employed in previous studies is to arrange multiple narrow-band units with quasi-perfect or perfect sound absorption and to assemble a wide sound-absorbing band. In contrast, this thesis presents a counter-intuitive but more efficient concept of design that is based on a series of coherently coupled "imperfect components". Although the individual imperfect components can only realize inferior sound absorption performance, by suitably tuning the coherent coupling among them, a broadband high-efficiency absorption can be achieved thanks to their mutual interactions. Subsequently, the general acoustic responses of sound-absorbing materials are investigated, which reveals two guidelines, the over-damped condition and the reduced excessive response, for constructing broadband sound absorbers approaching the optimal thickness restricted by the causality constraint. Following the two guidelines, a coupled sound-absorbing metasurface is presented and successfully approaches the minimal thickness. Furthermore, inspired by the design concepts and the impedance-modulation techniques presented in constructing the metasurface-based sound absorbers above, this thesis investigates acoustic devices related to noise control engineering and impedance engineering, including the devices capable of sound attenuation in flow ducts, asymmetric absorption, extreme sound confinement/absorption, and strong emission enhancement of sound sources.