金属圆柱结构中声聚焦效应的研究

Study on acoustic focusing effect in metal cylinder structure

研究浸没在水中金属空心圆柱结构的声聚焦效应及其物理机制,并探索声聚焦频率与圆柱结构参数之间的关系.研究表明:当超声波源从外侧入射到圆柱结构表面时,在圆柱结构中心会出现声聚焦效应.这种声聚焦效应源于金属圆柱结构中的位移本征模式,与之前研究利用负折射机制实现声聚焦效应完全不同,是一种全新的物理机制.在圆柱结构的位移本征模式作用下,入射声波的传播方向在圆柱结构内部发生重建,均沿着圆柱径向指向圆心,同时相位波形也呈现出同心圆分布,从而在圆柱结构中心聚焦成一个完美的圆形焦点,焦点在水平方向与垂直方向均表现出优异的聚焦性能.此外,基于声散射理论推导得到系统的特征方程,计算出声聚焦的特征频率,理论与数值结果符合很好.结果表明:圆柱结构可在多个频率出现声聚焦效应,相邻的两个声聚焦频率的间隔相同,且焦点的尺寸随着聚焦频率增加而变小.当金属圆柱结构的内外半径同时增大时,圆柱结构的声聚焦频率会减小,且声聚焦频率与圆柱的尺寸成反比关系.本研究为新型声聚焦器件的研制提供有效的设计思路及理论依据.

In this paper,we study the exotic acoustic focusing effect through a simple brass cylinder structure immersed in water.Firstly,we calculated the acoustic pressure distributions with and without the brass cylinder,and the incident acoustic wave was emitted from the plane and cylindrical source,respectively.We found that thenbsp;propagation directions of the acoustic waves were reconstructed by the cylinder and the acoustic waves were focused on a prefect point at the center of the cylinder structure.Meanwhile,the phase distributions were also reconstructed inside the cylinder,and the phase waveforms were transformed into a series of concentric circles with the same values and eventually focused on a point at the center of the cylinder.Besides,it was noted that the excellent acoustic focusing characteristics appear in both longitudinal and transverse directions,and the focal spot in the cylinder was a perfect point rather than an elliptical spot in other acoustic focusing devices.In addition,we presented the deformation of the displacement distributions in the cylinder.It shows that the acoustic focusing arises from a special eigenmode in the cylinder structure at some eigenfrequencies,which is essentially distinct from the previous studies originating from the negative refraction.Finally,we derived the eigenvalue equations of the system according to the acoustic scattering theory,and theoretically calculated the eigenfrequencies.The numerical results agree well with the theoretical results.It is obtained that the acoustic focusing effect appears at a series of eigenfrequencies in the cylinder,and the difference between the two adjacent eigenfrequencies is a constant.Besides,the size of the focal spot decreases gradually with the increase of the focusing eigenfrequency.Furthermore,we investigated the influences of the structure parameters of the cylinder on the eigenfrequency,and found that the eigenfrequencis decreased with the increase of the inside and outside radii of the cylinder.Therefore,we conclude that there is an inverse relationship between the focusing eigenfrequency and the size of the cylinder.Our finding should have great potential applications in acoustic focusing devices.