Asymmetric acoustic metamaterials open up a new field for multi-directional sound wave manipulation. The controllability of most asymmetric metamaterials on sound waves is restricted by their fixed structure and material parameters. Here, we propose a double-layer piezoelectric metamaterial that comprises two identical membrane-type piezoelectric acoustic metamaterials and a tunable shunt circuit. For incident waves in a narrow band, one side of the metamaterial matches the air impedance to achieve perfect absorption while the other side mismatches the air impedance to completely reflect the sound waves. The proposed metamaterials can separately manipulate the absorption frequencies and coefficients on both sides of the metamaterial by tuning resistances in the shunt circuit. Both the theory and experiment show that the maximum absorption coefficient can reach over 0.98, and the tunable frequency range has a 60% bandwidth over the center frequency of 1059 Hz.
Effective direct control of the sound source is the fundamental solution to the problem of noise. Herein, we propose a passive, non-closed and remote scheme for omnidirectional reduction of the sound power radiated from vibrating sources. The physical mechanism of this scheme is to design an acoustic superscatterer based upon the idea of transformation media so that the virtual boundary of the acoustic superscatterer can overlap with the radiation boundary of the sound source to construct drastic multiple scattering effects. Through theoretical analyses and numerical simulations, we confirm the effectiveness of adopting an acoustic superscatterer to significantly suppress the sound radiation power generated by some typical dipolar sources in air. Our study shows that by arranging no more than two acoustic superscatterers at designated positions away from a dipolar thin rod, about 90% of the sound radiation power, i.e. 10 dB, can be suppressed in all directions of the dipole axis. This preliminary work could aid research into the use of passive methods to achieve non-contact omnidirectional noise control of vibrating sources.
This paper proposes an approach to identify the equivalent rotational stiffness of rail cracks based on the reflection of guided waves. The identified rotational stiffness can be adopted to detect the crack and evaluate the safety of the rail. The quasi-bending guided waves propagating in the rail head and web are found and chosen as the detecting guided waves. Considering these guided waves, the relationship between the dynamic parameters of cracks and the power reflection coefficients are deduced theoretically. Cracks are modelled and their rotational stiffness concerning geometric parameters is evaluated. Simulation results indicate that the depth and width of cracks result in the decrease of the rotational stiffness significantly. Field experiments showed that discontinuities in a long distance can be detected by the selected guided waves in the rail head and web with relative errors less than 1% in 100 meters. And artificial cracks were made to validate the proposed method for the rail crack evaluation.
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