(a) The acoustic Purcell factor (black solid curve) by a line source located at the center of the enclosure shown in Fig. 1(a). Two peaks are identified in the Purcell factor spectrum around 5300 Hz and 12300 Hz. As a comparison, the Purcell factor predicted by the effective medium theory is plot as red dashed curve. (b,c) Near-field intensity distribution of the pressure wave at 5300 Hz (b) and 12300 Hz (c), respectively. It can be seen that at 12300 Hz the wave is mostly localized inside the central cavity, while at 5300 Hz, the wave is mainly distributed inside the central cavity, the annular cavity, and the narrow slits between them.