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.
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