A team of researchers from Western university in Canada, Institute of Chemistry, Chinese Academy of Sciences (ICCAS), and Qingdao university in China has reported an acoustic metamaterial based on air bubbles that can enhance the acoustic transmission between air and water. In their paper published in Research, they have described the structure, work principle, stability, application demonstrations of the metamaterials.

A poem said that the fish and birds have the furthest distance in the world. The ‘remote distance’ is not only from their entirely different habitats, but also from their difficulty in acoustic communications. Generally, a fish can hardly hear a bird and the reverse is also true. This is because only about 0.1% of the sound energy can transmit across the water-air interface, and it corresponds to about 30 dB loss of the sound. In this recent effort, the researchers provide an acoustic metamaterial called “Fluid-type acoustic metasurface” that can enhance the acoustic transmission over 20 dB compared to the bare water-air interface.

The structure of the metasurface is very simple. They just locked a layer of air bubbles below the water surface. The air bubble layer serves as a spring, and the above water layer works as a mass. The mass-spring system has a resonant frequency, where they demonstrated that the acoustic transmission can be largely enhanced. By adjusting the location of air bubbles, the mass of the above water can be varied, and thus the resonant frequency can be tuned. Therefore, the frequency of enhanced transmission can be flexibly tuned. “Not like the usual solid metasurface, in this metasurface, the mass (water layer) and spring (air bubble layer) are both fluids, thus called Fluid-type acoustic metasurface”, said Zhandong Huang, the first author of this paper.

In their experiments, the air layer is locked by a hydrophobic solid frame that is fabricated by 3D printing. The hydrophobicity can prevent the water from penetrating the solid frame, thus reserving the air bubbles. As stated above, the enhanced frequency can be varied by changing the position of the air bubbles, which can be achieved by adjusting the immersion depth of the solid frame.

In practical applications, the metasurface might face challenges of bubble instability from water pressure, air solubility, and temperature change. But fortunately, the bubbles are very near the water-air interface. The static pressure is very small, and the air has been almost already saturated in water. Therefore, the bubble can be stable over a long time. In the aspects of temperature change, they show that when the temperature varies from 60℃ to 5℃, the operating frequency increase by about 10%, and it almost has no influence on the enhanced transmission.

In acoustic experiments, they also made a music signal whose fundamental frequencies are near the resonant frequency of the metasurface. They show that music signal can be enhanced to cross the water-air interface. Therefore, the metasurface might be possible for acoustic communications between objects in water and those above the surface. The metasurface seems to open a window at the water-air interface for the acoustic transmission, where the sound can pass through freely.

Tag: Advanced Materials

Original source:

Zhandong Huang, Shengdong Zhao, Yiyuan Zhang, Zheren Cai, Zheng Li, Junfeng Xiao, Meng Su, Qiuquan Guo, Chuanzeng Zhang, Yaozong Pan, Xiaobing Cai, Yanlin Song, Jun Yang, "Tunable Fluid-Type Metasurface for Wide-Angle and Multifrequency Water-Air Acoustic Transmission", Research, vol. 2021, Article ID 9757943, 14 pages, 2021. https://doi.org/10.34133/2021/9757943