Study On Acoustic Metamaterials And One-dimensional Topological Acoustics

Acoustics,as a classic field of modern science,has undergone tremendous development in the past 25 years.However,the adjustment of sound waves with traditional natural materials is often difficult to achieve and has many limitations.In the past 20 years,acoustic metamaterials have been proposed as a unique artificial structure,possessing many extraordinary and unique properties that natural materials do not possess,opening up new ways to control sound waves.At the beginning of the 21 st century,researchers realized for the first time an acoustic metamaterial with a negative equivalent mass density through a small lead ball coated with a silica gel layer.Subsequently,an acoustic metamaterial with a negative effective modulus and a double negative Acoustic metamaterials.On this basis,acoustic metasurfaces,sound-absorbing metamaterials,sound-transmitting metamaterials,acoustic stealth,and acoustic super-resolution imaging have been developed and combined with practical applications such as medicine,aerospace,and electrical appliances.Solve a series of industrial problems,such as low-frequency noise reduction,ventilation noise reduction,medical ultrasound imaging and aviation stealth and other challenging tasks.With the introduction and development of phononic crystals as a concept similar to photonic crystals,phononic crystals,as a metamaterial whose elastic modulus and mass density are periodically modulated,introduce defects in the phononic crystals to cause energy The defect of the belt,and allows the researcher to easily adjust the energy band,so as to accurately realize the control of the sound wave.The peculiar phenomena of topological physics due to the quantum Hall effect,valley Hall effect and spin Hall effect have also attracted much attention.In the Bose subsystem,similar topological phenomena can also be realized.Photonic crystals are difficult to prepare,difficult to observe and other problems,while phononic crystals have the characteristics of controllable defects,easy to prepare structures,and simple adjustments.Therefore,they are used as an important experimental platform to achieve and verify similar topological effects.Topological acoustics It has also become an important research direction in acoustic metamaterials.This paper mainly introduces the research of low-frequency acoustic absorbing metamaterials and topological acoustics.For most acoustic metamaterials,once they have been fabricated,their operating frequencies and functions cannot be adjusted,which is an intrinsic barrier for the development of realistic applications.The study to overcome this limit has become a significant issue in acoustic metamaterial engineering.Although with the advance of metamaterials in the past two decades,a series of methods such as electric or magnetic control have been proposed,most of them can only work in the condition of no fluid passage.Some metamaterials with large transmission losses have been proposed,but the sounds are essentially reflected rather than absorbed.Here,to overcome this intrinsic difficulty,we propose a ventilated sound absorber that can be manually tuned in a large range after being manufactured.During the tuning that is achieved through an intricately designed slider,high-performance absorption and ventilation are both ensured.The tunable ventilated sound absorber is demonstrated experimentally and the effective model of coupled lossy oscillators can be employed to understand its mechanism.The manually tunable ventilated metamaterial has potential application values in various complicated pipe systems that require frequency adjustment and it also establishes the foundation for future development of active tunable ventilated acoustic metamaterials.Second in the topology of acoustics,Low-energy electrons near Dirac/Weyl nodal points mimic massless relativistic fermions.However,as they are not constrained by Lorentz invariance,they can exhibit tipped-over type-II Dirac/Weyl cones that provide highly anisotropic physical properties and responses,creating unique possibilities.Recently,they have been observed in several quantum and classical systems.Yet,there is still no simple and deterministic strategy to realize them since their nodal points are accidental degeneracies,unlike symmetry-guaranteed type-I counterparts.Here,we propose a band-folding scheme for constructing type-II Dirac points,and we use a tight-binding analysis to unveil its generality and deterministic nature.Through realizations in acoustics,type-II Dirac points are experimentally visualized and investigated using near-field mappings.This deterministic scheme could serve as a platform for further investigations of intriguing physics associated with various strongly Lorentz-violating nodal points.