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 (a) Schematic diagram of acoustic focusing lens and (b) cross section in the
