Acoustic Manipulation In Single-phased Solid Phononic Crystals And Tunable Metasurfaces

Acoustic metamaterials,including phononic crystals(PnCs),traditional sub-wavelength metamaterials and metasurfaces,which are composed of ingeniously designed microstructures,can manipulate acoustic waves and show broadband prospective applications in many fields,such as noise isolation,design of new acoustic devices,acoustic cloaking,etc.In this thesis,we investigate the manipulation of acoustic waves in single-phased solid PnCs and metasurfaces.The acoustic negative refraction conditions in solid PnCs and the acoustic-structure impedance matching method are developed.Then single-phased solid PnCs showing acoustic negative refraction,imaging,clocking,etc.are designed and studied.In addition,a new tunability mechanism of metasurfaces is proposed;and a continuously tunable metasurface which shows a broadband tunable acoustic focusing property is designed.The main contents and conclusions include:1.The acoustic negative refraction conditions in solid PnCs are proposed;and a chiral single-phased solid PnC composed of periodic holes in a square lattice is designed.The structure shows itself distinguishing features of rotational symmetry and light weight.The acoustic negative refraction properties and the influence of the rotational symmetry of the designed single-phased solid PnC are investigated.The results show that a chiral unit cell with rotational symmetry leads to a rotational wave vector(k)space and a rotational quadrate equi-frequency surface(EFS)which result in asymmetric negative refraction and the changeable imaging effect between the deviated image and double images.Based on the special negative performance of the design chiral single-phased solid PnC,a multi-functional acoustic device with both focusing and beam splitting functions is proposed by introducing the mirror-symmetry.2.A new acoustic-structure impedance matching method is proposed to design a single-phased solid PnC with high acoustic transmittance.Combining the proposed acoustic negative refraction conditions,a single-phased triangular-latticed solid PnC with imaging of high transmittance is designed.And then the acoustic mirage is realized by gradiently changing the geometric parameter.The results show that the proposed impedance matching method is feasible in high frequency regions and can solve the low transmittance problem induced by impedance mismatch.Following the above guideline,a single-phased solid PnC with a negative band structure is designed.The strongly isotropic behavior of the unit-cell makes the refractive index close to-1,which can yield a perfect imaging with high transmittance.The gradient changing of a single geometric parameter can define a gradient refractive index which yields the acoustic mirage effect with high transmittance.3.Based on the developed impedance matching method,a gradient single-phased solid PnC is designed,and the acoustic directional clocking property is studied.Then a method to eliminate the phase mismatch in the directional clocking is proposed.The results show that the developed impedance matching method can enhance the transmittance of acoustic waves through the designed gradient single-phased solid PnC.By introducing the gradient refractive index of the hyperbolic secant distribution in the structure,the self-focusing of the longitudinal wave is realized.Then a directional cloak is obtained by reversing the gradient refractive index of the self-focusing structure.The phase mismatch problem is solved by introducing an acoustic delay device following the cloak.In addition,an axisymmetric 3D directional cloaking is obtained by rotating the corresponding 2D cloak around its axis.4.A tunability mechanism is proposed based on the screw-and-nut principle to design a tunable metasurface with the transmitted phase continuously tunable at each unit-cell.A circular metasurface for three-dimensional(3D)adjustable broadband acoustic focusing is particularly studied.An equivalent medium model is developed;and experimental measurement for a 3D-printed specimen is performed.The results show that the broadband tunable focusing ability including the tunable focusing length,alterable operation frequencies and types of the acoustic sources(an incident plane wave source or a point source)is relative to the spiral channel length which can be tuned through adjusting the screwed depth.It is shown that the numerical simulation,effective medium model and experimental measurement are generally in good agreement.The present research on the novel acoustic manipulation ability of the single-phased solid PnCs and the tunable metasurface provides a theoretical guidance for controlling acoustic wave and designing new acoustic devices.