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Abstract Wave reflection and refraction at a time interface follow different conservation laws compared to conventional scattering at a spatial interface. This study presents the experimental demonstration of refraction and reflection of flexural waves across a temporal boundary in a continuum-based mechanical metabeam, and unveils opportunities that emerge by tailoring temporal scattering phenomena for phononic applications. We observe these phenomena in an elastic beam attached to an array of piezoelectric patches that can vary in time the effective elastic properties of the beam. Frequency conversion and phase conjugation are observed upon a single temporal interface. These results are consistent with the temporal Snell’s law and Fresnel equations for temporal interfaces. Further, we illustrate the manipulation of amplitude and frequency spectra of flexural wave temporal refraction and reflection through multi-stepped temporal interfaces. Finally, by implementing a smooth time variation of wave impedance, we numerically and experimentally demonstrate the capabilities of the temporal metabeam to realize waveform morphing and information coding. Our findings enable precise control over wave amplitude and frequency through temporally modulated mechanical systems, providing a concrete framework for designing time-mechanical metamaterials and time-phononic crystals. 

Achieving sound attenuation across a broad frequency range while maintaining adequate ventilation remains a significant challenge in acoustic engineering, as there exists a rigid trade-off between attenuation ability and ventilation. In this Letter, we propose a double-layered microperforated meta-shells to serve as broadband acoustic ventilation barrier. Multiple scattering theory is first employed to characterize sound attenuation performance of the proposed design in terms of both sound absorption and transmission loss, which is not reported before. Experimental result demonstrates a good enhancement of absorption due to the insertion of inner shell with a specific perforation rate of micro cores. The mechanism can be attributed to the inter-cell coupling, which is further utilized to devise a different configuration by wrapping the meta-shell with porous sponge. It is found that adding an extra layer of sponge can further improve the low-frequency attenuation performance. The proposed broadband sound barrier with effective ventilation can find potential applications in architectural acoustics and office noise insulation.