Researches On Novel Physical Properties Of Unconventional Acoustic Artificial Structures

During the past two decades, artificial acoustic structures have received much attention. Three kinds of structures including phononic crystals, metamaterials and surface structures have been designed and investigated extensively. On one hand, these structures can provide a versatile platform for analysis and demonstration of many novel physical phenomena like negative refraction. On the other hand, they can be used to control and manipulate the propagation of acoustic waves and thus result in useful applications such as acoustic imaging and cloaking. Most acoustic artificial structures in research are periodic in space, so that their properties can be described by dispersion curves. Based on the elastic wave theory in periodic structures, in this thesis, we will give a demonstration of novel physical properties of acoustic transport in several new unconventional acoustic artificial structures, including gradient surface structure, quasi-periodic plate, disordered waveguide structure, ultrathin plate and time-domain phononic crystals. The main works are listed below:1. Focusing of spoof surface acoustic waves on graded surface structureWe design and manufacture one kind of surface structure drilled with a two-dimensional array of cylindrical holes. The holes array has a shape of rectangle as a whole, while the radii of the holes vary along both the +y and -y directions. Thus the surface can possess gradient refractive index along the corresponding directions. Using this flat lens in experiment, we achieve good focusing performance of SSAWs launched along x direction over a wide range of frequencies. Compared to bulk waves, the energy is more localized and the beamwidth is narrower in our device. So we think this surface device can have potential applications such as beamwidth compression and high resolution detecting.2. Imaging by a thin plate with quasi-periodic array of holes.We present designs of a thin steel plate, which is demonstrated to achieve the lens function. With either plane wave or point source incidence, hotspots with the smallest size up to 0.2 wavelength are observed beyond the evanescent region. With theoretical calculation by angular spectrum theory, we conclude that the subwavelength spots do not originate from the evanescent waves, but come from the delicate interference of the diffractive beams specific to quasiperiodic array. Furthermore, imaging with good resolution of a point source moving in the object plane is also demonstrated. This planar lens is easy to fabricate compared to traditional lens with parabolic surface. These researches are expected to open up new prospects for far-filed subwavelength imaging of sound.3. Discrete diffraction and Anderson localization of sound in acoustic waveguide arraysWe design one kind of one-dimensional acoustic waveguide arrays in which the acoustic wave evolution in arrays can be modeled within the framework of coupled-mode theory. The governing equations are similar to that of tight binding models in solid state physics and the propagation direction z-axis plays the same role with time in Schrodinger equation. The coupling constants, as the key parameters in controlling acoustic propagation in waveguide arrays, can be extracted reliably from simulations and experiments as the function of distance between neighboring waveguides. In periodic waveguide arrays, sound injection can lead to the discrete diffraction, which is dramatic and very different from the diffraction process in homogeneous media. In disordered ones, sound injection gives rise to transversal Anderson localization.4. Low-frequency resonance of flexural waves in ultrathin plateWe extend the multiple scattering theory to flexural waves in ultrathin plate and calculate the band structure of both resonant and nonresonant systems. The wave equation in ultrathin plate is a fourth-order differential equation, which results in parabolic relationship of frequency and wavevector at low frequency. In resonant systems, it’s found that the monopolar resonance can produce a stopband corresponding to negative mass density. We also design a three-component structure which has a band with negative group velocity at low frequency. It is proven that this band structure originates from the coupling of monopolar and dipolar resonance and corresponds to both negative mass density and negative rigidity of the plate.5. Multiple frequency up- and down- conversions of acoustic waves by temporal phononic crystalsWe report on the experimental demonstration of frequency conversions by temporal phononic crystals (TPCs), which are artificial structures designed with periodically time-varying density or elastic moduli. We present a design of TPCs, which is achieved by repetitively swapping of carbon dioxide and air in between the acoustic source and the probe by a rotating drum. When monochromatic acoustic waves propagate through the TPC, novel multiple up- and down-conversions of frequency are experimentally observed and the frequency interval can be modulated by changing the rotating frequency of drum. It should be noticed that this realization of frequency conversion doesn’t involve any strong field condition and material nonlinearity.