An acoustic metamaterial with space-time modulated densitya).

Space-time modulation opens the door for unprecedented wave behavior control, such as nonreciprocal wave manipulation. Here is proposed a one-dimensional space-time modulated membrane system aiming to realize a kind of acoustic metamaterial with space-time modulated effective density. Three different approaches, namely, the effective medium method, transfer matrix method, and time-domain simulation, are applied to analyze the acoustic response of the system under a monochromatic incidence. Results show that the proposed metamaterial can support two different nonreciprocal acoustic functionalities, namely, unidirectional parametric amplification and parametric frequency conversion, when different modulation profiles are enforced.