Study On The Effects Of Acoustic Focusing Based On Acoustic Resonance Metamaterials

This thesis studies the acoustic focusing based on resonance acoustic metamaterials, which mainly includes three parts:(1) The related theories of resonance acoustic metamaterials; (2) Extraordinary focusing and its controlling based on acoustic metamaterials with single parameter resonance; (3) The realization and simulation of acoustic Luneburg Lens based on acoustic metamaterials with multi-parameter resonance. The content of each part is described briefly as following:1. The related theories of acoustic resonance metamaterialsAcoustic metamaterials are a kind of acoustic artificial materials. They have many acoustic properties that nature materials don’t possess because of their sub-wavelength structure. Interests in acoustic metamaterials have grown rapidly with the realization of the concepts underlying the wave physics of electromagnetic meta-materials, such as negative index of refraction, sub-wavelength imaging, transformation optics, etc., which have counterparts in acoustics.In the second chapter of the thesis, we study the propagation property of acoustic wave in homogeneous media with numbers of acoustic resonators. First, we prove that the Fano resonance of a localized state with a continuum of propagation modes is the reason why there is a band gap. Then we build a theoretical model and find the dispersion relation in such homogeneous media with periodic structures. Evidently, in chapters of the third and fourth, it is proved the correctness of the model.2. Extraordinary focusing and its controlling based on single parameter acoustic resonance metamaterialsIn 1873, the limit of diffraction theory was putted forward by German scientist Abel. Sound, just as light, is subjected to diffraction and interference effects. Diffraction limits the imaging resolution in acoustics and optics to about half of a wavelength. This limit arises from the fact that spatial details thinner than half a wavelength are carried by evanescent waves, which stick to their source, their amplitude decreasing exponentially with distance, and are negligible about a wavelength away from itIn the third chapter of the thesis, firstly we introduced the sub-wavelength focusing achieved by Fabrice Lemoult using time reversal technique. And based on it, we show that equally sharp focusing and its controlling can be achieved experimentally by arranging transducers around a nearly circular array of soda cans, but without time reversal. It shows that the nature of such an extraordinary focusing is from a more fundamental point of view. At last we used finite element method to test and verify the experiment results.3. The realization and simulation of acoustic Luneburg Lens based on acoustic resonance metamaterials with multi-parametersFor centuries, the conventional approach to design and fabricate optical lens has been to grind the surfaces of a uniform material, by which the light rays are refracted to sculpt the paths as they transit through the interfaces. Refractive lenses formed by this procedure of bending the surfaces can be of extremely high quality, but are nevertheless limited by geometrical and wave aberrations that are inherent to light refraction at the interface between both media. Conceptually, a more natural-but usually less convenient-approach to lens design would be to vary the refractive index throughout a local space. The local resonance structure to achieving the varying of refractive index throughout the lens is used in the experiment.In the fourth chapter of the thesis, first we elucidated the theory basis of the gradient index lens. And then we used the acoustic resonators in third chapter as the basis structure of the gradient index lens. We designed the effective refraction index by changing the parameter of single acoustic resonators. And also the soda cans are used as acoustic resonators in the experiment to completed the Luneburg Lens; At last finite element method is used to test and verify the experiment part.Finally, the conclusions and the prospects of the future work are presented.