The Study Of Sound Fields Manipulation Via Acoustic Metamaterials

Acoustic metamaterials(AMMs)are artificial structures and composed of sub-wavelength unit cells(such as Coiling-up structures,Membrances structures and Helmholtz resonators),which are periodically or aperiodically distributed in three dimensional(3D)space.Two dimensional(2D)ultrathin AMMs,called acoustic metasurfaces(AMSs),can achieve various unusual functionalities such as anomalous reflection and refraction,by adjusting equivalent acoustic parameters including effective density or bulk modulus.In this paper,spoof surface acoustic waves(SSAWs)and AMSs are investigated through theoretical analysis,numerical simulations and experimental verifications.A series of novel acoustic devices are proposed.The main contents and innovations are summarized as follows:1)We introduce the concept of spoof surface plasmon polariton for acoustic waves,and propose an acoustic grating with a sub-wavelength thickness.By optimizing the geometrical parameters of the element,high conversion efficiency from propagating waves to SSAWs can be achieved.The operating bandwidth of the acoustic grating ranges from 5500 Hz to 6100 Hz.On this basis,a dual passband acoustic filter is designed and measured.The acoustic filter can support the transmission of SSAWs between 5000 Hz-6296 Hz and 7650 Hz-8775 Hz,while suppressing it out of the two bands.2)The asymmetric transmission of acoustic waves in a 2D waveguide is realized based on the gradient index acoustic metamaterials(GIMs).By using a snowflake-like unit cell and adjusting its geometric parameters to meet the requirements of the index distribution,while ensuring impedance matching between the unit cell and air,high conversion efficiency of propagating waves to surface waves can be achieved.Thanks to the unidirectionality of the mode conversion,two sets of identical GIMs are placed inside the waveguide to realize the asymmetric sound transmissioin.Meanwhile,based on the transmission characteristics of surface waves,an acoustic cloak and a sound mode converter can be achieved by adjusting the index of the GIM,respectively.3)Based on the theory of coding metasurfaces,an acoustic diffuser is designed and processed.The acoustic diffuser has a sub-wavelength thickness and is composed of only two type of unit cells,which can disperse the reflected energy into various directions from 5800 Hz to 6200 Hz.At the same time,the acoustic diffuser is insensitive to the incident angle when it is less than 60°,the specular reflections can be effectively suppressed due to the induced diffusion.4)A programmable multifunctional AMS is designed and processed.The reflected phase distributions can be controlled in real time through a motor array.Based on the convolution and addition operations of the coding digits,the AMS can realize continuous beams scanning in upper space,lossless superposition of multiple beams,and independent,continuous adjustment of the multi-beams.Finally,a 3D programmable acoustic focusing meta-lens is designed.The position of the focal point can be arbitrarily moved in front of the meta-lens.