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
       

(a) Schematic diagram of the simplified model, and (b) the real part of effective density ρeff and the absolute value of effective acoustic velocity |veff| each as a function of frequency. (c) Pressure field amplitude pattern for the simplified model at 42.63 kHz.