Acoustic Manipulation In Spatial And Frequency Domain Based On Acoustic Metamaterials

Over the past 20 years,acoustic metamaterials have greatly contributed to the development of modern acoustics,leading to acoustic functional devices with superior performance,lighter weight,smaller bulky size,and greater freedom of acoustic manipulation.Characterized by artificial unit cells with subwavelength size,acoustic metamaterials can achieve the acoustic properties that are not inherent in natural materials and demonstrate plenty of brand-new physical phenomena.These new phenomena do not violate the laws of physics,yet they challenge our physical perception and intuition.In contrast of traditional acoustic devices,impressive acoustic manipulation have been realized by acoustic metamaterials with ingenious design of unit cells.Nowadays,acoustic metamaterials can realize a lot of spatial manipulation of the acoustic wave,making the acoustic wave propagate in the way we designed.However,there are still lack of efficient methods of manipulating the frequency of acoustic waves.Based on the acoustic metamaterials,we have conducted research on acoustic manipulation in spatial and frequency domain,demonstrated some impressive practical functions through the devices we designed,and discussed the application schemes of the devices in a targeted manner.The main contents include:In Chapter ?,the development of acoustic metamaterials is reviewed and the physical mechanism of acoustic metamaterials is expounded.Then different types of acoustic metamaterials and some functional devices related to this paper are introduced.In Chapter ?,we introduce the related theories and calculation methods.Firstly,the physical mechanism of the acoustic equation in anisotropic media and the effective medium theory are expounded from the anisotropic acoustic equation.The dispersion relation in anisotropic media is obtained by deducing the acoustic equation in curvilinear coordinates.Then,the physical meaning of evanescent wave in wave vector space is expounded from the angle spectrum expansion method.Finally,we give a brief derivation of acoustic vortex beam.In Chapter ?,a 3D broadband omnidirectional acoustic concentrator is designed to manipulate the acoustic waves in space.First of all,we design a ideal equivalent model of acoustic concentrator via ideal extremely anisotropic medium,and a perforated spherical stucture based on fractal method is proposed to substitute the extremely anisotropic medium equivalently.Numerical simulation and experimental demonstration show that our design can effectively enhance the intensity in the concentrated area in the broadband range of 1.0-1.8 k Hz.In addition,we also quantitatively analyze the influence of the parameters of the filling medium in concentrated region,the ratio of the outer radius to the inner radius and the volume filling ratio of the spherical shell on the transmission efficiency of the acoustic concentrator,and put forward the performance optimization scheme based on this.Finally,we demonstrate a three-dimensional omnidirectional broadband absorber based on the perforated spherical acoustic concentrator,which has broadband,omnidirectional and efficient performance in sound absorption and reveals the potential application prospect of perforated spherical acoustic concentrator.In Chapter ?,a 3D acoustic hyperlens is designed for super-resolution imaging utilizing methods of spatial acoustic manipulation.The 3D acoustic hyperlens based on the perforated spherical shell is designed with the perforated spherical structure with uniform size of the triangular holes on the surface that is obtained by the fractal method with the icosahedron used as the basic geometric configuration.We numerically and experimentally demonstrate that the designed 3D hyperlens can image the acoustic source images of capital letters,whose size is only about one wavelength on the inner surface of the hyperlens,on the outer surface in a broad band ranging from 4.6-9.0 k Hz with the image size magnified 4.5 times.Then,we propose a performance optimization scheme to eliminate the defects caused by structural resonance and surface wave mode.Finally,according to the characteristics the fractal method and extreme anisotropy of the designed structure,we propose and demonstrate the cascading scheme,and further propose a hyperlens suit scheme suitable for any geometric shape of the acoustic hyperlens.In Chapter ?,we propose a frequency manipulation mechanism of acoustic waves.A receiver scheme based on rotational Doppler effect in a multiplexed acoustic vortex beam channel is proposed,which can receive multiple time domain signals efficiently with only a few transducers.Then,we use acoustic metasurface based on Helmholtz resonance stucture to design an ultra-high efficiency(theoretically up to 100%)acoustic frequency shifter,which can shift the frequency1)₀to a new frequency1)₀+7)/2in the laboratory coordinate in a high efficiency.Then,we theoretically and experimentally demonstrate that the frequency shift mechanism has strong robustness to the incident acoustic wavefront.Finally,a cascaded acoustic frequency shift amplifier is designed to realize the amplification of acoustic frequency shift in the laboratory coordinate with high efficiency.The frequency of acoustic wave after frequency shift is further shifted to a new frequency.At the same time,the high efficiency tansformation of acoustic energy in the system is maintained,which makes it have plenty of application prospect.In Chapter ?,the main conclusion of this paper and the prospect of the future work are drawn.