Realization of triple-negative complementary metamaterials for ultrasonic transmission through elastic barriers

We propose a triple-negative complementary metamaterial designed to achieve ultrasonic transmission through an elastic barrier. The proposed metamaterial simultaneously exhibits negative effective properties of mass density and elastic moduli, serving a pivotal role in virtually removing the barrier. Its unit cell features a monolithic aluminum structure incorporating a symmetric and crosswise arrangement of local resonators, each comprising an inertial mass connected to the host matrix by thin flexural beams that serve as elastic springs. This configuration is tailored to induce dipolar, monopolar, and quadrupolar resonances, which are the physical origins of negative mass density, bulk modulus, and shear modulus, respectively. Based on boundary effective medium theory, a metamaterial unit cell exhibiting negative effective density and elastic modulus near 50 kHz is designed, and its capability to enhance ultrasonic transmission, when attached to a single side of the barrier, is predicted via transient simulations for a 10-cycle Gaussian pulse. The metamaterial sample is fabricated by using wire electrical discharge machining, and water-tank experiments show a nearly fourfold increase in acoustic intensity transmission compared to the bare barrier, with a 14.2% relative bandwidth sufficient to encompass most of the spectral energy of a Gaussian pulse. Furthermore, the reciprocity of this enhancement is experimentally verified, implying an over 16-fold enhancement in the returning echo intensity. This research work on barrier-penetrating technologies offers broad potential applications such as transcranial therapy, ultrasonic neuroimaging, nondestructive evaluation, and underwater communication.