Propagation Of Acoustic Waves In Sonic Crystal With Defects

With further research of sonic crystals(SCs), the application based on crystal defects become a hot topic of academic. By introducing defects, the propagation mode or resonant mode can be obtained in the band gap which can be used to achieve varieties of acoustic applications. In this dissertation, control of acoustic waves by sonic crystals and novel applications based on sonic defects are investigated. The main work is included as follows:Firstly, two kinds of splitting schemes for self-collimated acoustic beam are designed based on the coupling between defect modes and self-collimated modes in sonic crystals. The schemes are verified numerically by the finite element method. Controllable energy distribution can be achieved in our schemes as the ratio of energy between reflected and transmitted beams can be regulated freely by changing the defect size. By concatenating and expanding the designed structures, multichannel splitting can also be achieved which can be widely used in the integrated acoustic devices.Secondly, in the use of composite sonic crystals and three-export interferometer, a new kind of directional beam is proposed. At the same frequency, the SC with smaller equal frequency contour(EFC) can be used as a collimator to bound divergent wave. Splitting schemes based on line defects is employed to gain a three-port interferometer which makes the directional beam more collimated. In addition, we can adjust the phase difference by altering the crystal structure for enhancing the interference.Next, by combining a cavity and two waveguides which are mutually perpendicular to each other, we present a new kind of unidirectional acoustic transmission based on the match and mismatch of symmetric modes between defect and waveguide in two-dimensional sonic crystals. In certain range of frequencies, the waveguide simultaneously support even-mode and odd-mode with different wave vector. When the symmetry of eigenmodes between the waveguide and the cavity are matched, the resonance could be achieved, on the contrary is not the same. By combining both the match and mismatch, a unidirectional transmission can be obtained. Because of the sensitivity to frequency, this device also can be used as frequency filter.Finally, we present a new kind of unidirectional acoustic transmission based on channel drop filter which composed by two resonant cavity formed by two point defects and two parallel waveguides. As the ports of channel drop filter are regulated by the coupling element, the wave propagating in undesirable channels become destructive interference. And by setting a series of crystal column at one port, acoustic waves may propagate from one side to the other side, conversely it cannot.