Acoustic focusing through two layer annuluses in air
Guan Yi-Jun1, Sun Hong-Xiang1, 2, †, 
, Liu Shu-Sen1, Yuan Shou-Qi1, ‡, , Xia Jian-Ping1, Ge Yong1
, Liu Shu-Sen1, Yuan Shou-Qi1, ‡, , Xia Jian-Ping1, Ge Yong1 Pressure–field amplitude patterns for (a) free space at 42.63 kHz and focusing lenses with two-layer annuluses at (b) 42.63 kHz, (c) 29.50 kHz, and (d) 55.32 kHz, with (e) single layer annulus at 42.51 kHz, and with (e) three-layer annuluse at 42.59 kHz, respectively.
