Control of Sound Directivity Based on Metamaterials

From the perspective of adjusting boundary conditions, acoustic radiation pattern based on acoustic metasurface is studied. By changing the locations of the periodic Helmholtz resonators(HRs), we can obtain dipole-like radiation pattern with arbitrary direction in the semi-infinite plane area, the scheme also has wide working band. The cause of such phenomenon is the existence of periodic HRs has changed the effective acoustic impedance at the corresponding spacial position, i.e., the effective boundary condition has been altered. By virtue of such method, the wave-vector can be tailored at different directions, which contributes to the desired radiation pattern with strong directivity. This method has the ability to compress the side lobes, which improve the usage of sound energy. Besides, owing to the analogy to surface plasmons (SPs), we add periodic acoustic gratings to the rigid surface with HRs and successfully excite the collective acoustic surface wave, which leads to long distance acoustic collimation beam. All these proposals are both feasible under two-dimensional and three-dimensional circumstances. Meanwhile, we also discuss the impact of the size of acoustic gratings on the direction of radiation mode. The work will have theoretical guidance on design and development of small scale integrated sound devices.