The Study Of Acoustic Energy Manipulation Based On Acoustic Metamaterials

In recent years,research on metamaterials and metasurfaces of electromagnetics and optics has attracted widespread attention.By artificially designing the structural parameters of the sub-wavelength unit,the metamaterials can show a macroscopical parameter distribution that does not exist in nature,such as anisotropic material constant,negative dielectric constant,negative permeability,etc.Due to the similarity between the acoustic wave equation and the electromagnetic wave equation,the special parameter distribution of acoustic metamaterials,such as negative elastic modulus,negative mass density,etc.,can also be obtained by designing sub-wavelength unit.This method of constructing material parameter distributions that do not exist in nature greatly enriches people's ability to manipulate acoustic waves to constructs many novel acoustic phenomena.For example,by controlling the phase and amplitude distribution of the transmitted and reflected waves on the metasurface,abnormal refraction(reflection),acoustic super lens focusing and acoustic holography imaging can be realized.The construction of near-zero refractive index materials can realize the acoustic tunneling in narrow areas and acoustic stealth.Construct a multi-frequency resonance structure to achieve broadband sound absorption.And more,acoustic metamaterials also have great application prospects in noise controlling,acoustic particle manipulating and acoustic communicating.These methods of manipulating acoustic waves mostly control the propagation direction or distribution of acoustic energy in space.In addition,acoustic energy manipulation also includes harvesting energy carried by acoustic waves in space.Unlike common energy sources such as solar energy,water energy,nuclear energy,and biological energy,the power density carried by acoustic waves in the air is too low,due to the impedance mismatch during the energy conversion process,acoustic energy is habitually ignored and being wasted.But in some special conditions,such as near aircraft engines,noisy intersections,etc.,the energy carried by acoustic waves in these environments can be harvested and used.For some MEMS devices,the energy required is so small that acoustic energy can be harvested to power these microelectronic devices to replace batteries.These examples illustrate the potential application value of research on acoustic energy harvesting in the air.At this time,most researches focus on harvesting vibration energy in solids,but not harvesting acoustic energy in the air.Because there is a serious impedance mismatch while the acoustic waves enter the transducer from air to solid,which brings great difficulties to harvesting acoustic energy in air.In order to solve the problem of impedance mismatch,the existing method is to use resonance,such as cantilever beam to improve the acoustic-electric conversion efficiency.However,using resonance inevitably leads these acoustic energy harvesting devices can just work efficiently at a single frequency,and have a poor efficiency in low frequencies.These disadvantages have limited the feasibility of these acoustic energy harvesters.Combining with the development of acoustic metamaterials in recent years,this paper studies the method of using acoustic metamaterials to manipulate acoustic energy.One is the use of acoustic metamaterials to construct acoustic waveguides to control the acoustic energy flow,and the other is to design two acoustic metasurfaces that can localize acoustic energy and use them to harvesting acoustic energy in the air.This paper mainly includes the following chapters:Chapter I is the introduction.First,it introduces the basic principles and design methods of acoustic metamaterials and metasurfaces,lists the researches of acoustic metamaterials in recent years,and introduces the generalized Snell's law that guides the design of acoustic metasurfaces.After that,the principles and methods of acoustic energy harvesting are introduced,the calculation and design methods of acoustic energy harvester are descripted in detail,and some representative researches are listed and analyzed.Chapter II proposes a method of constructing all-pass acoustic waveguides using acoustic metamaterials.In this chapter,the theory of acoustic waveguide is briefly introduced,and the problems existing in the application of traditional waveguide are explained.Later,a soft boundary waveguide using acoustic metamaterials as the boundary was proposed.This boundary constructed by metamaterials can efficiently reflect acoustic waves in a thin layer of air,and it can be regarded as a perfect soft boundary.It is worth mentioning that this kind of “virtual” boundary constructed by metamaterials does not spatially divide the continuous media on both sides of the boundary,but only divides the acoustic field on both sides.The acoustic waveguide constructed by this boundary also does not hinders the material exchange of the background fluid inside and outside the waveguide.Since the "virtual" boundary of the metamaterial structure has a band-stop frequency response,it allows us to use multiple all-pass waveguides with different parameters to separate and mix acoustic waves of multiple frequencies and guide acoustic energy flow to different directions.In order to illustrate the results,we demonstrate the design in finite element numerical simulations and experiments,and excites the first-order and second-order modes in the waveguide to illustrate the stability and applicability of the waveguide.Chapter III proposes and introduces an ultra-thin broadband acoustic energy harvesting metasurface based on coupled Helmholtz resonators.In this chapter,first introduces the resonance type acoustic metasurface,introduces the principles of several common resonance structures in detail,and then proposes and introduces this type of acoustic energy harvesting device based on the coupled Helmholtz resonator in detail.Two kinds of Helmholtz resonators with different resonance frequencies are alternately arranged to form a metasurface,so that the reflected acoustic field of this metasurface presents a dipole field.The air's vibration phases in adjacent cavities are completely opposite.The piezoelectric cavity wall is subjected to the "push-pull" effect from the cavities on both sides to produce bending deformation and output electric energy.This "push-pull" effect exists in the frequency band between the resonance frequencies of the two Helmholtz resonators,so the acoustic energy harvesting device has a certain working bandwidth.Not only that,sinse this metasurface has translational and rotational symmetry,different piezoelectric transducers are completely equivalent in space,so the output voltage has exactly the same phase,there is no need for rectification or phase modulation when cascading for reducing the complexity of subsequent circuits.The feature of deep subwavelength(1/20?)in the thickness direction also reduces the directional sensitivity of this metasurface and further improves its practicability.Chapter IV proposes an acoustic metasurface that can transform acoustic propagation waves into surface acoustic waves,and uses it for air acoustic energy harvesting.In this chapter,we first introduced the acoustic propagating waves and surface waves in the air and reviews the basic theory of acoustic surface waves.Then,we introduced how to transform the propagating waves into surface waves.Based on recently research of propagating wave-surface wave conversion metasurface,an acoustic metasurface that can convert propagating waves into surface standing waves and harvest acoustic energy has been report in this chapter.The metasurface formed with two function parts,one is a thin tube with a gradient distribution of depth.Its function is to convert the acoustic wave irradiated on the metasurface in the normal direction into a standing surface wave,so that the sound energy is localized near the surface as a standing surface wave.The other part is piezoelectric transducer,whose function is to convert the air vibration energy caused by standing surface waves into electrical energy.In the design,the resonance frequency of the piezoelectric transducer is consistent with the working frequency of the acoustic surface,which further improves the harvesting efficiency.Chapters III and IV respectively put forward acoustic energy harvesting metasurfaces based on different principle,which provides a new possibility for the application of acoustic metamaterials in different field,and also enriches the method of acoustic energy harvesting in the air.Chapter V proposes an active acoustic metamaterials based on a tunable piezoelectric composite cavity.Using the acoustic-electric coupling properties of piezoelectric materials,by controlling the electrical boundary conditions of the piezoelectric materials,it is possible to manipulate the acoustic impendence of the material and manipulate the wave front to form special acoustic field.Chapter VI summarized this paper and puts forward the prospect of future work.The main innovations of this paper are as follows:1.Constructing a new type of acoustic waveguide with acoustic metamaterials to control acoustic energy flow.It is difficult to find a perfect soft boundary in nature,and the usual acoustic boundary is hard boundary or impedance boundary.These boundaries usually divide the space of the medium in which they are located and hinder the exchange of materials on both sides of the boundary.Not only that,because the characteristic impedance of the materials constituting the boundary are similar to the impedance of the medium,these boundaries cannot reflect sound waves perfectly.The waveguide constructed by these boundaries cannot realize the exchange of materials inside and outside the waveguide,and there is still a problem of acoustic leakage in media with similar acoustic impedance.In this paper,an acoustic soft boundary based on metamaterials has been reported.The thickness of the boundary is sub-wavelength,and the acoustic soft boundary will not hinder the exchange of the media on both sides of the boundary.Its function is close to the acoustic soft boundary,and its reflection frequency response curve is banded.The structure of the acoustic waveguide can achieve the frequency division effect.This paper demonstrates the soft acoustic boundary and the frequency division effect of the constructed acoustic waveguide in theory,numerical simulation and experiment.2.Using acoustic metasurface to local acoustic energy and acoustic harvesting.Due to impedance mismatch,it is difficult to transform the acoustic energy into electric energy in the air.In order to solve this problem,the traditional method is to use cantilever beam structures,which are large and have a single working frequency.The study of acoustic metasurfaces has proposed a lot of new possibilities and methods for sub-wavelength structure acoustic absorption and exbending the response band.In this chapter,two types of acoustic energy harvesting metasurfaces are designed,which are based on the principles of coupling resonance and propagating wave-surface wave conversion.These designs increase the acoustic-electric conversion efficiency and exbending the working frequencies.And more,these works enrich the potential application possibilities of acoustic metamaterials,and also provide new ideas for acoustic energy harvesting.3.Active acoustic metamaterials based on piezoelectric materials.Wavefront manipulation mainly uses active phased array methods and artificial structure methods,both of which have significant advantages and disadvantages.The phased arry is actively and adjustable,but the circuit is complicated and costly;the latter is cheap and stable but has a single function.In order to solve this problem,we propose a method of designing acoustic artificial structures using multi-physics coupling materials.Using the piezoelectric materials' acoustic-force-electric coupling characteristics,we can manipulate the acoustic impendence by changing the electrical boundary conditions of the piezoelectric materials,thus the reflected wave and transmitted wave also can be manipulated.In the numerical simulation,we demonstrated two design methods to control the reflected wave and the transmitted wave.At the same time,we also conducted experiments on the unit to verify the idea that the acoustic impedance can be controlled by adjusting the circuit load.