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 (a) free space and (b) square annuluses at 17.19 kHz, and (c) partial zoom in panel (b). (d) Schematic diagram of acoustic directional transmitter (R1 = 23.50 mm, and D = 2.00 mm). (e) Pressure–field amplitude pattern for acoustic directional transmitter at 17.19 kHz, and (f) partial zoom in panel (e). White circular lines represent contour lines of amplitude.