Research On Acoustic Transmission In Bull’s Eye Structure

Acoustic artificial structures have gained extensive attention due to their ability to exhibit many unnatural and fascinating phenomena that are impossible to obtain in natural materials. Both phononic crystals and acoustic metamaterials are belonging to the research category of acoustic artificial structures. Phononic crystals are periodic composite materials or structures which its parameters such as elastic constants and density change periodically. The lattice size of the phononic crystal is almost length of the acoustic wavelength, phononic crystals can exhibit some special acoustic characteristics for its band properties, such as negative refraction, sound focusing, control the propagation of the elastic waves, etc. Acoustic metamaterials are artificial composite structure in subwavelength scales, by modulating the structure to achieve unique physical phenomenon, such as extraordinary acoustic transmission, acoustic cloaking, acoustic super lens, etc. The dissertation is mainly focused on the study of extraordinary acoustic transmission and unidirectional acoustic transmission in bull’s eye structure theoretically and experimentally. Including: Manipulation of extraordinary acoustic transmission by tunable bull’s eye structure; Unidirectional acoustic transmission in asymmetric bull’s eye structure; Manipulation of extraordinary acoustic transmission and sound collimation by the output grating. The dissertation consists of five chapters:In chapter one, we give an introduction for the related experimental and theoretical background, and the research progress of the acoustic artificial structures. Also, we give a brief outline of the dissertation.In chapter two, extraordinary acoustic transmission (EAT) has been investigated in a tunable bull’s eye structure. We demonstrate that the transmission coefficient of acoustic waves and resonance frequency can be modulated by a grating structure. When the grating is located at a distance of 0.5 mm away from the base plate, the acoustic transmission shows an 8.77-fold enhancement compared to that by using a traditional bull’s eye structure. When the distance increases to 1.5 mm, the transmission approaches zero, indicating a total reflection. Thus, we can make an efficient modulation of acoustic transmission from 0 to 877%. At the same time, by moving the grating horizontally can also module the transmission. By changing the period length, transmission energy is nearly unchanged, but the transmission frequencies will change as followed. By changing the temperature of the water, we can control the transmission frequency corresponding to the EAT, so this structure can play a role as an acoustic filter. The EAT effects have been ascribed to the coupling of structure-induced resonance with the diffractive wave and the wave-guide mode as well as the Fabry-Perot resonances.In chapter three, unidirectional acoustic transmission has been investigated in an asymmetric bull’s eye structure, which is consisted of a subwavelength hole with concentric grooves on one side of a thin steel plate. When acoustic waves impinge normally on the groove side of the asymmetric structure, a strong acoustic transmitted energy flux is observed in the frequency range of 400-450 kHz, while there is no obvious transmitted energy flux in the same frequency range if the acoustic waves impinge normally on the other side. Thus, a remarkable unidirectional acoustic transmission behavior is exhibited by the current structure. With changing the period of the grooves, it is found that the transmitted acoustic energy flux keeps unchanged while the frequency of the transmitted waves can be modified. Coherent diffraction of the scatterings from the periodic array of grooves is responsible for the observed unidirectional acoustic transmission phenomenon. The experiments are performed and have confirmed the unidirectional acoustic transmission behavior in the asymmetric bull’s eye structure.In chapter four, manipulation of EAT and sound collimation by the output grating has been investigated in the bull’s eye structure. At first we put the same grating at the output side of the structure, by changing the distance between the output grating and the base plate, the modulation of the EAT has been realized. When input grating distance H1 is fixed 0.5 mm, output grating distance H2 is 0.5 mm the transmission T=0.4695, output grating distance H2 is 1.5 mm the transmission T=0.3315, the ratio between them is 1.41. When input grating distance HI is fixed 1.5 mm, output grating distance H2 is 0.5 mm the transmission T=0.00678, output grating distance H2 is 1.5 mm the transmission T=0.00131, the ratio between them is 5.18. So by changing the output grading distance we change the transmission too. Then we let both distance of input grating H1 and output grating H2 are 0.5 mm, by changing the thickness of the grating we realize the adjustment of the transmission. When the grating is too thin, it can’t lead to strong resonance; when the grating is too thick, it will affect the coupling between the resonances. In the end, far field collimation is illustrated in the proposed bull’s eye structure.The last chapter, we present a summary of this dissertation and the prospect for the future work.