Investigation On Acoustic Barrier And Acoustic Insulation Pipe Based On Acoustic Metamaterial

Acoustic metamaterials which have a series of abnormal physical properties that natural materials do not possess are a new type of artificial material that emerged at the end of the 20th century.By utilizing these exotic properties of metamaterials,acoustic waves can be controlled freely so that the research of functional devices based on acoustic metamaterials is attracting more and more attention from researchers.It is expected that new acoustic metamaterials can be used to design acoustic devices that are superior to traditional materials in all aspects.The problem of sound insulation has attracted much attention in the fields of environmental noise control and architectural acoustics while the traditional sound insulation methods have more or less inherent defects caused by the principle itself.This paper focuses on the issues of barrier and pipeline sound insulation and proposes a broadband sound barrier based on the partial bandgap of phononic crystals,an omni-directional ventilated acoustic barrier based on the Fano resonance and an ultra-thin sound insulation pipe based on acoustic metasurface.In Chapter I,the preface mainly introduces the development of photonic and phononic crystals,electromagnetic metamaterials and acoustic metamaterials.Afterwards,the related theories of metamaterials such as wave equations and equivalent media are briefly introduced.In Chapter II,a two-dimensional rectangular lattice phononic crystal is proposed,which not only has a complete band gap but also a partial band gap.The use of the partial band gap for the regulation of the acoustic wave front allows the plane wave incident on the structure surface to propagate in the opposite directions in the structure,and when it emerges from the phononic crystal it is split into two plane waves having a certain spacing and the same wavevector direction,thereby creating an infinite length of local silent area and then a broadband ventilated sound barrier is realized.In addition,placing the two phononic crystal structures symmetrically with a certain distance in the vertical direction can also make the split plane wave acoustic beams combine together again to form a plane wave.In this condition,the invisible acoustic cloak is finally realized.In Chapter III,we first calculate the vibrational displacements and frequencies of the mechanical systems of two oscillators connected by a spring from the perspective of mechanical analogy.It is found that they have the same Fano resonance phenomenon as found in the fields of atomic physics and nanophotonics before.After that,by changing the damping of the two oscillators,it's found that when the damping of the oscillator is large enough,the anti-resonance point of the Fano resonance disappears,and the asymmetric Fano resonance profile degenerates into a symmetric Lorentzian type of profile.Then,the acoustic Fano resonance is realized by constructing the coupling between the discrete state and the continuous state of the sound field,and the omnidirectional ventilated acoustic barrier is designed by using the structure of coiled-up space and a hollow pipe.In Chapter ?,numerical simulations are performed to verify that the reflection and refraction angles of an acoustic wave impinging normally on the interface of two different medias satisfy the generalized Snell's law.Using the equivalent parameter model to construct a reflective type metasurface,the numerical simulation results show that when the acoustic wave impinges obliquely on the metasurface with periodic phase gradient distribution,the high order diffraction term caused by the Bragg scattering of periodic structure needs to be introduced into the generalized Snell's law to explain the occurred negative reflection or negative refraction.When the system of metasurface ignores the loss effect,energy of the scattered waves radiates into the direction of negative reflection or negative refraction.However,when the system considering the loss effect,the scattered field of reflected or transmitted energy corresponding to the higher-order diffraction orders will be significantly reduced and mainly exists in the form of surface waves.When neglecting the higher-order diffraction orders in the system with loss effect,the scattered field is nearly the same as the condition of lossless system which has the same abnormal reflection phenomenon and the reflected angle is all the same.Finally,a metasurface with a phase gradient is constructed by the coiled-up space to realize the abnormal reflection of incident sound waves.Placing a well-designed symmetrical metasurface between the inner walls of the pipeline can prohibits the propagation of the acoustic wave through the interior of the pipeline and then totally reflect to the incident side of acoustic wave.Finally,Chapter V gives the main conclusions of this study and the outlook for the future.