Tunable acoustic metasurface for broadband asymmetric focusing based on Helmholtz resonator

This work presents an innovative type of acoustic metasurface structure based on the Helmholtz resonator in order to generate an acoustic metasurface with multifunctional effects, such as configurable asymmetrical focusing. With the aim to accomplish a flexible regulation of the complete phase of the sound waves, a new cell structure is constructed by incorporating the Helmholtz resonator height gradient change, resulting in the multifunctional design of an acoustic metasurface via simulation. The metasurface structure parameters, which include the aperture width and the Helmholtz cavity width, are adjusted to attain improved sound focusing and a higher transmission rate. According to the results, the metasurface has an asymmetric focusing influence on a greater frequency range of 2845 similar to 3620 Hz, which is raised by 10.71% compared to the highly uniform symmetric cell structure. The transmission of the cell structure is also higher than 0.96. In the meantime, the sound waves can be actively controlled by adjusting the interlayer spacing and incident angle. This results in a specific adjustable range along the x-axis within 0.22 similar to 0.87 m and the y-axis within -0.278 similar to 0.278 m of the focusing location. The outcomes of this investigation may find use in ultrasonic treatment and ultrasonography.