In this work, we theoretically design and numerically realize topological pumpings in 2D and 3D spatially modulated acoustic metamaterials. By adiabatically changing the pumping parameters along the synthetic dimension, we observe topologically protected states, which are shown to be pumped from one edge to the other in 2D materials, and from edge to edge and corner to corner in 3D materials. This spatially modulated metamaterials could pave the way for acoustic topological wave trapping and manipulation and serve as a platform for the study of higher-dimensional topological effects among different physical systems.
Inspired by the optical and acoustic non-Hermitian 𝒫𝒯 symmetric system, a 𝒫𝒯 symmetric metamaterial beam for flexural waves is proposed here, based on shunted piezoelectric patches. Positive and negative shunting resistances are the key to constructing the balanced loss and gain components. The associated asymmetric flexural wave scattering, 𝒫𝒯 phase transition and exceptional points are then investigated in both analytical and numerical ways. In addition, properties of exceptional points are tunable in our proposed system, simply owing to the variable shunting parameters. Our design may contribute to asymmetric wave control, enhanced sensing, amplification, and localization of flexural waves.
The way in which objects interact with various fields often allows us to detect their presence. Thus, imaging techniques based on reflections, echoes and shadows, as well as any form of scattering and localization more generally, inform us on the location, shape and composition of an object even if buried deep under thick layers of materials. Conversely, to hide an object, we say to “cloak” it, we must suppress any interaction it might have with probing fields. Care must of course be taken so that the cloaking device itself is cloaked as well. In this talk, we show how to cloak solids against stress waves.
Real-time manipulating elastic waves in solid materials is crucial for several applications ranging from active noise and vibration cancellation to inverse methods aiming to either reveal or dissimulate the presence of foreign bodies. Here, we introduce a programmable elastic metasurface with sensing-and-actuating units following feedback control loops. The active units governed by local transfer functions encoded into a digital controller and offering highly flexible and independent phase and amplitude engineering of both transmitted and reflected waves. Through numerical and experimental demonstrations, the programmable metasurface can be a perfect absorber with flexural waves incident from left to right and a perfect transparent mirror with flexural waves incident from right to left. Various other significant demonstrations include steering transmitted (reflected) waves without reflection (transmission) and simultaneously control in both transmitted and reflected domains. Finally, we unveil the relations of the programmable elastic metasurface with nonreciprocity to an effective Willis medium. The design will pave a new efficient way in the field of manipulating of elastic waves.
Reciprocity is a fundamental principle in acoustics, posing constraints on the way we process acoustic signals. Breaking reciprocity with spatiotemporal modulations provides an opportunity to design compact, low-energy, integrated non-reciprocal acoustic devices. Here, we design and experimentally demonstrate a space-time modulated programmable metamaterial beam with electromagnet resonators controlled by currents. A numerical approach based on the finite element method is developed for wave dispersion calculations of space-time modulated programmable metamaterials with complex geometries. Unidirectional band gaps are demonstrated experimentally and numerically in a good agreement. We quantify effects of the modulation amplitude and material damping in terms of band gap width and attenuation factor of the unidirectional band gaps in the space-time modulated metamaterial beam. Lastly, the unidirectional band gaps due to the second-order mode coupling caused by strong modulations are identified and examined numerically. Our design as well as the numerical approach provide a practical solution for the applications of non-reciprocal acoustic devices with spatiotemporal modulations.
We design and experimentally demonstrate a linear active elastic metasurface for real-time and simultaneously multifunctional wave control on a steel plate. The metasurface consists of an array of circuit-controlled piezoelectric patches bonded on the plate separated by thin slots for active wave phase modulations. Our experiments illustrate that by properly programming digital circuits of metasurface unit cells, wave steering directions and paths can be arbitrarily tuned in real-time, which also has an excellent agreement with numerical simulations. We further explore that multiple wave control functions can be integrated into one within the circuits to achieve a simultaneously multifunctional wave control device by using only one metasurface layer. Our numerical results prove the feasibility of the design for broadband and oblique incident applications. The active metasurface breaks the time-revisal symmetry and behaves nonreciprocal propagations of elastic waves. Our design can be simply extended for other elastic wave mode control and wave mode conversion. We believe that the proposed active elastic metasurface could open new avenues for novel and unconventional real-time elastic wave control applications.
A great deal of research has been devoted to controlling the dynamic behaviors of phononic crystals and metamaterials by directly tuning the frequency regions and/or widths of their inherent band gaps. Here, we present a novel approach to achieve extremely broadband flexural wave/vibration attenuation based on tunable local resonators made of piezoelectric stacks shunted by hybrid negative capacitance and negative inductance circuits with proof masses attached on a host beam. First, wave dispersion relations of the adaptive metamaterial beam are calculated analytically by using the transfer matrix method. The unique modulus tuning properties induced by the hybrid shunting circuits are then characterized conceptually, from which the frequency dependent modulus tuning curves of the piezoelectric stack located within wave attenuation frequency regions are quantitatively identified. As an example, a flexural wave high-pass band filter with a wave attenuation region from 0 to 23.0 kHz is demonstrated analytically and numerically by using the hybrid shunting circuit, in which the two electric components are connected in series. By changing the connection pattern to be parallel, another super wide wave attenuation region from 13.5 to 73.0 kHz is demonstrated to function as a low-pass filter at a subwavelength scale. The proposed adaptive metamaterial possesses a super wide band gap created both naturally and artificially. Therefore, it can be used for the transient wave mitigation at extremely broadband frequencies such as blast or impact loadings. We envision that the proposed design and approach can open many possibilities in broadband vibration and wave control.
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