Triple-negative complementary metamaterial for ultrasound transmission through elastic barrier.

We propose an ultrasonic metamaterial characterized by triple-negative effective material properties of mass density, bulk modulus, and shear modulus for wave penetration through elastic barriers. A theoretical framework based on a transformation method with folding and compression mapping reveals that triple negativity is pivotal for metamaterials to serve as elastic complementary media. Our metamaterial features a rotationally symmetric and crosswise arrangement of coated mass inclusions embedded within a base matrix. This design is tailored to attain triple negativity originating from the dipolar, monopolar, and quadrupolar resonances induced by collective motion between the inclusions and the base matrix. Due to its triple negativity, ultrasonic waves undergo near-zero reflections and near-zero phase changes as they traverse the metamaterial and barrier, acoustically canceling out the physical space of the blocking medium. Numerical simulations under an ultrasonic beam demonstrate that the metamaterial, when partially attached to the incident side of an elastic barrier, remarkably enhances the acoustic intensity from 2% to 72% in the transmitted region. The proposed metamaterial holds broad potential for diverse applications, including antireflection coating, non-destructive testing, underwater communication, and ultrasonic neuroimaging.