Lattice-resonant phononic crystals with subwavelength gaps

The local resonance mechanism is a renowned methodology for obtaining subwavelength gaps with respect to acoustic waves. In locally resonant phononic crystals, low-frequency acoustic waves are localized around scatters, and the hosts only provide weak interaction between scatters. However, it is difficult to obtain phononic crystals with subwavelength gaps using liquid hosts via the local resonance mechanism. In this paper, a new mechanism, i.e., the lattice resonance mechanism, is proposed for the formation of subwavelength gaps with respect to acoustic waves. The lattice resonance mechanism is a nonlocal resonance mechanism. The type of phononic crystal obtained based on this novel mechanism is called a lattice-resonant phononic crystal. Research results indicate the presence of omnidirectional and directional subwavelength gaps in lattice-resonant phononic crystals. However, the low-frequency waves in these phononic crystals are extended to whole lattices and not localized around scatters. The lattice resonance mechanism enables the formation of subwavelength gaps in phononic crystals with liquid hosts. By introducing the concept of degree of localization, more general conditions are obtained for the formation of subwavelength gaps in the case of acoustic waves. Further, the abnormal properties of the lattice-resonant phononic crystals are discovered and explained systematically. In addition, the acoustic impedances of the liquid hosts and lattice-resonant phononic crystals are considerably similar. This new type of phononic crystal can be used to control the low-frequency acoustic waves in hydroacoustic fields, where the locally resonant phononic crystals do not fit.