Novel Physical Effects Of Sonic Crystals And Acoustic Metamaterials

Recently, the artificially structured acoustic material (ASAM), with unconvential acoustic properties beyond natural materials, is a very hot and important research field, including the study of sonic crystals and acoustic metamaterials, which are consisted of two or more component materials with different acoustic parameters according to the design of frequency bands and effective-medium theory. The period of sonic crystals is comparable to the sound wavelength with the characteristics of Bragg bandgaps, however the scale size of acoustic metamaterials is much smaller than the sound wavelength with exotic effective acoustic parameters retrieved by the effective-medium theory. This thesis is mainly focused on studying, both in theory and experiment, the propagation of acoustic waves in ASAM showing many promising exotic properties or phenomena, such as acoustic surface evanescent waves (ASEWs) and the associated extraordinary acoustic transmission (EAT), novel physical effects in ASAM with acoustic cavities (AC), and acoustic Chern insulators (CIs):sonic crystals consisted of circulating air flow.ASEWs and the associated EAT:We demonstrate both theoretically and experimentally the physical mechanism that underlies EAT and collimation of sound through a one-dimensional decorated plate. A microscopic theory considers the total field as sum of the scattered waves by every periodically aligned groove on the plate, which divides the total field into far-field radiative cylindrical waves and ASEWs. Different from the well-known acoustic surface waves like Rayleigh waves and Lamb waves, ASEW is closely analogous to surface plasmon polariton in the optical case. By mapping the total field, the experiments well confirm the theoretical calculations as well as the fact that EAT and sound collimation take place at the proper frequency when ASEWs satisfy the phase matching condition. The establishment of the concept of ASEW provides a new route for the integration of subwavelength acoustic devices with structured solid surface.Novel physical effects in AS AM with AC:AC are important acoustic devices, by adjusting whose structural parameters the frequency and quality factor of the supported local resonance mode can be effectively modulated leading to feasible manipulation of sound propagation.1) We design a slit structure with AC to realize Rabi splitting in acoustics. We develop rigorous analytical models discovering that the splitting effect is caused by the coupling of the slit’s Fabry-Perot (FP) fundamental mode with the resonant mode of AC.2) We study the phase-modulation propeties of an acoustic metasurface based on AC. By adjusting the structural parameters of AC, the phases of transmitted sound can be effectively modulated leading to the observation of acoustic focusing.3) We have investigated the ultrasonic properties of a novel acoustic grating made of AC. Both full and zero acoustic transmission is observed due to the bonding and anti-bonding state formed by the coupling of AC or not.4) We realize acoustic tunnelling through a thin plate with arrays of split-ring AC. The observed tunnelling peaks are due to the resonance modes formed by the combination of the AC array and the plate.5) We study the tunable imaging effect of a sonic crystal consisted of AC. By setting the resonant frequency of AC to be in the Bragg bandgap, we observe an additional passband with associated negative refraction leading to flat-plate imaging, which can be switched on and off by rotating the AC rod.6) We have realized acoustic rainbow trapping effect by tapping an air waveguide with high refractive-index space-coiling AC, which reduces the physical dimension of the device with compact size. Such device with the capability of dropping different frequency components of a broadband incident temporal acoustic signal into different channels can function as an acoustic wavelength division de-multiplexer. Above all, these novel physical effects in ASAM with AC improve the understanding of the connection between quantum and classical problems, and might inspire unique acoustic device designs, such as acoustic microscope, acoustic hyperlens, acoustic filters, acoustic collimators, acoustic switch and artificial cochlea.Acoustic CIs (sonic crystals consisted of circulating air flow):Recent explorations of topology in physical systems have led to a new paradigm of condensed matters characterized by topologically protected states and phase transition, for example, topologically protected photonic CIs enabled by magneto-optical effects. However, in other wave systems such as acoustics, topological states cannot be simply reproduced due to the absence of similar magnetics-related sound-matter interactions in naturally available materials. Here, we propose an acoustic topological structure by creating an effective gauge magnetic field for sound using circularly flowing air in the designed acoustic ring resonators. The created gauge magnetic field breaks the time-reversal symmetry, and therefore topological properties can be designed to be nontrivial with non-zero Chern numbers and thus to enable an acoustic CI, in which the topologically protected acoustic edge-state transport is observed, featuring robust one-way propagation characteristics against a variety of topological defects and impurities. Our results open a new venue to non-magnetic topological structures and promise a unique approach to effective manipulation of acoustic interfacial transport at will.Above all, we systematically study the novel physical effects of acoustic propagating waves in ASAM via the manipulation of symmetries, phase control and modes coupling, which improves the understanding of sound propagation through artificial structures and may underpin the future growth of novel acoustic devices.