Ultrasonic metamaterial with triple negativity for wave transmission through elastic barrier

An ultrasonic metamaterial characterized by simultaneously negative effective properties of mass density, bulk modulus, and shear modulus is proposed for wave transmission through elastic barriers. The necessity of triple negativity in cancelling out the physical space of the blocking medium is theoretically demonstrated via the transformation acoustics. Those negativities are achieved by embedding systematically arranged inclusions within a host matrix, whose collective motions of multiple resonances cause the desired effective properties. Numerical simulations show that attaching the metamaterial on the incident side of an elastic barrier markedly increases the energy transmission of Gaussian pulses from 2 % to 72 %. The proposed metamaterial offers potential applications such as antireflection coatings, non-destructive testing, and transcranial ultrasonic neuroimaging.