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 amplitude distributions along a line passing through focal spot (a) in the propagation direction and (b) in a direction transverse to the propagation direction (shown as lines “1” and “2” in Fig.
