Environmental noise, particularly from traffic, is recognized as a significant factor contributing to health problems, underscoring the need for effective sound insulation. Sonic crystals (SCs) have attracted attention as promising materials capable of providing sound insulation while allowing airflow. Nevertheless, conventional SCs face limitations, including narrow sonic bandgap (SBG) widths, anisotropic acoustic performance, and the need for large structures to achieve low-frequency attenuation, all of which hinder practical applications. In this study, the acoustic characteristics of two-dimensional triangular-lattice hierarchical sonic crystals (HSCs)—formed by applying a hierarchical configuration to SCs—were investigated through both theoretical and numerical analyses. The results indicate that incorporating multiple lattice constants facilitates the formation of SBGs associated with each lattice scale. Furthermore, second-order SBGs generated by HSCs exhibit isotropic properties, offering a potential approach to overcoming the incident-angle dependence observed in conventional SCs. In addition, partial modification of the unit cell geometry can further broaden the SBG width. These isotropic and broad bandgap characteristics enhance the suitability of SCs for sound insulation in architectural and urban environments. Overall, the findings confirm the effectiveness of hierarchical designs in SCs and provide useful insights into their potential for advanced acoustic applications.
Izumi et al. (Wed,) studied this question.