Experimental demonstration of the acoustic frequency conversions by temporal phononic crystals

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We report on the experimental investigation of the transmission spectra for acoustic waves through temporal phononic crystals (TPCs), which are designed structures with periodically time-varying density and bulk modulus. In our experiment, the TPCs are instanced as the media that swap between air and carbon dioxide repetitively, in between the sound source and the detector. The transmission spectra for monochromatic incident acoustic waves exhibit a series of equidistant peaks, manifesting novel up- and down-conversion effects while not employing material nonlinearity. These experimental results agree with the theoretical calculations based on the plane wave expansion in time domain.