Theoretical Study On Broadb And Perfect Absorption Of Acoustic Waves

Sound absorption within deep sub-wavelength space, especially in the low frequency regime, is an interesting and challenging issue. Traditional methods such as porous materials, micro-perforated plates, and sound absorption wedges show severe limitation in this respect. Recently, the rapid development of acoustic metamaterials, i.e. artificial acoustic materials with almost arbitrary mass density and modulus, has provided new possibilities to achieve sound absorption in unprecedented ways. Bestowed with the unusual parameters, acoustic and elastic metamaterials exhibit strong abilities to control acoustic and elastic waves, giving rise to novel phenomena like low frequency sound blocking, negative fraction and lensing, acoustic cloaking, etc. Very recently, acoustic metamaterials have been applied to enhance the absorption of acoustic wave energy.Through theoretical analysis and numerical simulations, we demonstrate that perfect absorption of elastic waves can be achieved in two types of ultra-thin films: one requires a large value of almost pure imaginary mass density and a free space boundary, while the other requires a small value of almost pure imaginary modulus and a hard wall boundary.When the pure imaginary mass density or modulus exhibits certain frequency dispersions,the perfect absorption effect becomes broadband, even in the low frequency regime. Our work provides a feasible approach to realize broadband perfect absorption of elastic waves in ultra-thin films.The dissertation is organized as follows:In the chapter one, we research the background and progress of the absorption and scattering. We also describe the application of metamaterials and so on.In the chapter two, we study on the optical absorption and scattering. This chapterintroduces the development of optical coherent perfect absorbers. We also study that possibility of the coherent perfect absorption is realized by metamaterials.In the chapter three, we study on the absorption of elastic waves. In this work, we analyze the mechanism to achieve broadband perfect absorption for elastic waves by using an ultra-thin film. Based on the transfer matrix theory, we find two types of ultra-thin films with such possibility.In the chapter four, we study on the condition of achieving acoustic broadband absorption. Through a simple model analysis, we demonstrate that elastic metamaterials with large damping can be designed to realize such effective media with almost pure imaginary parameters with the required frequency dispersions in certain frequency regimes,therefore providing a feasible approach for broadband absorption of elastic waves.