Broad forbidden bands in parallel-coupled locally resonant ultrasonic metamaterials

The authors demonstrate that a class of ultrasonic metamaterial, which is composed of subwavelength resonant units built up by parallel-coupled Helmholtz resonators with identical resonant frequency, possesses broad locally resonant forbidden bands. The bandwidths are strongly dependent on the number of resonators in each unit. The broadening of bands is ascribed to the change of effective acoustic impendence. The coupling effects on the wave vector and negative dynamic modulus are discussed. Numerical simulations by finite element method further confirm the theoretical results.