Underwater acoustic metalens for enhanced sound transmission penetration through an air layer

Supercavitation technology is an underwater drag reduction technique that generates a sustained gas envelope around an underwater vehicle to overcome viscous drag, thereby achieving approximately 90% drag reduction. However, severe impedance mismatch inhibits sound waves from penetrating the underwater air layer, resulting in acoustic transmission losses of over 60 dB which critically limits the communication capabilities of supercavitation underwater vehicles. Owing to the need to overcome the constraints imposed by the two water-air interfaces, nearly no acoustic devices can enhance acoustic transmission without disrupting the cavity. To address this challenge, we propose a design method for a laminated acoustic metalens based on the effective medium theory. This method enables efficient sound transmission with a certain operating bandwidth through the underwater air layer, which is primarily attributed to the control of phase differences. In this design, the laminated acoustic metalens independently achieves two key functionalities: transmission enhancement with a certain operating bandwidth through the underwater air layer and enabling tunability of the operating frequency. Compared with the bare underwater air layer, experimental results show that the laminated acoustic metalens achieves sound transmission enhancement of more than 30 dB within a 400 Hz operating bandwidth. Additionally, the proposed laminated metalens can realize transmission enhancement within the range where the incident angle is less than 80°,and the transmission frequency can be flexibly adjusted. Its exceptional performance demonstrates that the proposed metalens design methodology holds significant potential for navigation and guidance applications in underwater supercavitation vehicles.