A lightweight structure endowed with broadband wave absorption capabilities is highly sought in the development of vibration and noise reduction applications. In this work, we present a type of absorbing meta-barrier via topological optimization which totally absorbs the waves near the boundaries at an extremely broadband frequency range. In order to elucidate the perfect absorption mechanism, we developed a theoretical model that incorporates a lumped mass-spring-damper system coupled to a semi-infinite thin beam. The sample is fabricated through 3D printing. The absorption performance is experimentally characterized, and a maximum absorption of more 95% is achieved over a broad frequency range from 700 Hz to over 3000 Hz in the experiment. Our approach provides an effective solution to the design of lightweight broadband noise and vibration isolation/suppression devices for practical engineering applications.
Non-Hermitian systems have brought great attention to researchers on wave manipulation by introducing losses. Here, we introduce an LR-shunted resonator for the observation of an exceptional point (EP) via tuning the shunting resistance. Through theoretical, numerical, and experimental studies, we demonstrate that by changing the circuit parameters, such as inductance and resistance, such a simple design supports the non-Hermitian degeneracy, namely the exceptional point. Moreover, near the EP, the mechanical resonance splitting has a square-root dependence on the resistance variation. We further numerically and experimentally demonstrate programmable perfect flexural wave absorption at the low-frequency range using this LR-shunted resonator. The absorption spectrum could be further enhanced with an additional shunted negative capacitor. Our approach provides alternative solutions for nondestructive structural health monitoring with enhanced sensitivity and perfect wave absorption.
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.
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