Boundary-induced medium mapping enables air-equivalent acoustic propagation and perfect absorption in water
The extreme water-air impedance contrast (~3600) has long acted as a fundamental barrier separating airborne and underwater acoustics. Here, we overcome this barrier with flexible boundaries, which establish a direct physical mapping between disparate acoustic media, enabling a water-filled channel to emulate air-equivalent wave propagation. Through vibroacoustic coupling, the effective wave velocity is rescaled in an approximately nondispersive manner, producing slow waves with tunable attenuation. Leveraging this concept, we demonstrate broadband underwater sound absorption at a deep-subwavelength thickness approaching the causal limit. Our findings reveal that acoustic media can be reshaped via boundary dynamics rather than bulk composition, paving the way for transplanting acoustic functionalities and metamaterial design across distinct media.