The Study And Application Of Acoustic Transmission Based On Acoustic Metamaterials

In the past decades,as the emergence and development of acoustic metamaterials,the classical research field of acoustics has witnessed a magnificent development course.As a type of artificial composite materials,acoustic metamaterials exhibit abnormal acoustic properties,including negative mass density,negative bulk modulus,near zero parameters and anisotropic parameters,etc.Acoustic metamaterials have greatly improved the ability to manipulate sound waves,such as negative refraction,cloaking and illusion,super resolution imaging and super absorption,etc.This thesis will focus on the project of the sound wave propagation.From the aspects of reflection,transmission and absorption,we will discuss and realize arbitrary manipulation of sound waves by designing acoustic metamaterials.As shown in the following:1.Breathable and soundproof devices based on acoustic metamaterialsWe introduce the concept of metamaterials into the field of sound insulation,and we propose two kinds of breathable and soundproof devices.Firstly,by designing the structure with open holes and hidden bypass space coiling tunnels(BSCTs)connected to the holes,we realize a three-dimensional soundproof acoustic metacage.We find such acoustic metacage can produce a quite large low-frequency band gap along the direction of the central hole,and the maximum fractional bandwidth of the band gap can approach 2/3.Such particular band gap can be well demonstrated by the effective medium theory.Such soundproof acoustic metacage can not only block sound efficiently from all directions,but also allow air to access the cage freely due to the open holes.We experimentally observe the soundproof effect,which is not influenced by the airflow by a fan.Secondly,by designing a two-dimensional acoustic complementary medium of water,we realize a kind of acoustic illusion device,with the functionalities of sound insulation with open holes.The complementary medium is composed of core-shell rubber materials,and possesses the exact double negative values of effective medium at working frequency.Such illusion device can perfectly block the sound waves,and let other entities go through.With amount of material loss,the device works robustly in a wide frequency range2.Broadband and wide-angle acoustic impedance matching effectFirstly,we propose a general approach to design acoustic metamaterials with the functionality of broadband and wide-angle impedance matching with background medium.Then,we come up with a new nonlocal effective medium theory for acoustic materials to analyze the periodic artificial structures.And such an unusual acoustic impedance matching characteristic can be well explained by the nonlocal effective medium theory.Then,we design two kinds of acoustic metamaterials in water,and demonstrate its impedance matching effect in a wide range of incident angles and in a broad frequency band.Then,based on the unique impedance matching effect,we propose the theory of broadband and wide-angle perfect absorber in water.Finally,we extend the theory to airborne sound waves,and demonstrate a kind of non-reflecting fence.The metamaterial fence is composed of an array of rigid rods with alternating layers of different diameters,featuring the perfect impedance matched with air in a wide range of incident angles and in a broad frequency band.Through the experimental measurements,we verify that such metamaterial fence can robustly prohibit reflection and reverberation,and the reflection approaches zero.Compared to other types of fence,such as simple fence,our proposed non-reflecting metamaterial fence is characterized by a significant reflection decrease.3.Wave-front manipulation based on acoustic metasurfacesFirstly,we propose a general approach to inhibit acoustic scattering in an environment in which obstacles are necessary.Such approach can perfectly guide sound waves pass around the obstacles without distorting the wave-front.Then,we propose the corresponding acoustic metamaterial,which is constructed with subwavelength tunnels and phase manipulation structures.Both full-wave simulations and experimental measurements clearly show that the proposed acoustic metamaterial is effective in inhibiting scattering and maintaining integrity of sound waves over an ultra-broad working frequency band.Such method has no limits not only on the incident angles or the working frequencies of the source,but also on the shape or the size of the obstacles.Then,based on acoustic metasurface,we propose a kind of acoustic holographic device,which can control both transmitted and reflected fields.By utilizing inversion calculations and the target fields,we can design the acoustic holographic device,and the full-wave simulations demonstrate its functionality.