New Acoustical Technology Of Sound Insulation Based On Acoustic Metamaterials

Acoustic metamaterial (AMM) is one of the hottest topics in acoustic realm inrecent years. Due to the dramatical property of controlling the propagation path ofacoustic waves, the AMM is considered as one of the most promising materials torealize acoustical functional devices.This dissertation is aimed at providing some methods to investigate new soundinsulation technology with AMMs. Based on the properties of the acoustic scatteringfield, the sound insulation device (SID) can be classified as two kinds, which arescattering case and non-scattering case, respectively. For the case of non-scattering, theacoustic waves neither transmit into the insulation area, nor reflect back into the hostmedium. On the other hand, the function of the scattering device is simpler. Theinsulation shell can only prevent the incident wave penetrating into the inner area, butcan not eliminating the acoustic reflection. The main contents and findings of thisdissertation include:1. On the basis of the layered structure of cylindrical acoustic cloak, therealizability of acoustic cloaks with complex shapes is analyzed. A layeredelliptical-cylindrical acoustic cloak with the same focus is designed based on theeffective medium theory. Furthermore, an approximation approach is proposed forrealizing arbitrarily shaped acoustic cloak. Based on the effective medium theory, thedesigned cloak is a discrete layered structure using homogeneous isotropic materials.The performance of the cloaks is simulated. The results demonstrate that the cloakpossesses properties of low-reflection outside the cloak and wavefront-bending in thecloak shell. In this section, we solve the problem of realizing a complicated shapedacoustic cloak with normal materials.2. Based on the non-scattering parameter-matched conditions on the interfacebetween the isotropic material and the metamaterial with an anisotropic mass density, amethod to design the acoustic scattering cancellation cloak for acoustic sensors isproposed. The cloak is a transparent structure while canceling the scattering of theacoustic sensors which can receive the outside information without any scattering. Thesimulation results indicate that the acoustic cloak designed by transformation acousticsis only suitable for acoustic sensors with an identical mass density equaling to the hostmedium. The reduced cloak is one of the possible approaches to avoid this restriction.Reduction coefficient is introduced to the material parameter expressions, by which theproperties of the sensor cloak are totally changed. The reduced sensor cloaks areadapted to acoustic sensors with arbitrary parameters only by adjusting the values thereduction coefficient. 3. The properties of defect mode, the energy localization effect and the resonantmodes in a one-dimensional Helmholtz resonantor (HR) metamaterial with point defectare researched analytically and experimentally. Defect mode corresponds to a narrowpass band in local resonant forbidden band. At the frequency of the defect mode, energycan be localized around the defect resonator. The frequency of the defect mode isassociated with the defect parameter; however, it can further affect the degree of thelocalized energy. The ratio of the resonant frequencies of defect reonator and perfectreonantor can influence the position of the localized energy in the structure. There aremany resonant modes in the HR metamaterial with point defect. Different modescorrespond to different distributions of the acoustic energy. By studying thedistributions of the phase, local negative parameter is observed. The negative dynamicmass density not only exists in the forbidden band, but also appears in the pass bandbrought by the defect mode.4. A low-frequency wideband SID with inbuilt elements is designed andimplemented. The embedded structure can insulate the incident acoustic energy above it,which is a possible scheme to realize acoustic defence structure with no obstacle. Theexperimental results show that the acoustic energy is reflected by the resonant HRswhen they are resonating, and a large low-frequency area is formed by the rounded HRs.Wideband insulation area can be achieved by introducing much more HRs withgradually changed resonant frequencies.In conclusion, we studied two kinds of new sound insulation devices, and carriedout experiments on the low-frequency wideband sound insulation device. Boththeoretical and experimental results all verified the feasibility of the methods proposedin this dissertation. This work is helpful for realizing new acoustic sound insulationdevices with acoustic metamaterials, and also significative for pushing forword theapplication of the acoustic metamaterial.