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
       

Pressure–field amplitude patterns for focusing lens by magnifying the parameters ((a)–(d)) R2 and r2 and ((e)–(h)) R1 and r1 n times.