As we continue to explore the mysteries contained in the world’s oceans, it is essential to have a robust communication system capable of sending information through the water’s surface. However, because the impedance of water is 3600 times that of air, about 99.9% of an underwater sound signal reflects into the water without being transmitted.

Not only is this inefficient, but constant noise reflecting in the water could be harmful to marine life. Zhou et al. designed a water-air impedance gradient that can be placed at the boundary to reduce reflected signals and enable broadband communication.

“If we analogize the water and air as two steps with a height difference of 3600m, the gradient impedance matching layer designed is equivalent to building some new steps between water and air, which allow sound waves in the air and water to be transported to each other,” said author Jun Yang. “The more steps constructed, the wider the frequency band of sound waves that can pass through between water and air.”

The authors used metafluids to build their device that adjusts the impedance gradually to reduce reflection. According to their calculations, the received signal strength could be up to 470 times that of the bare water-air interface. It is even possible to encode colored images using this technique.

“The achieved broadband water-air gradient impedance matching layer has important application prospects for numerous ocean exploration applications, such as ocean network development, ocean geological surveys, and marine life research,” Yang said. “We also want to emphasize the potential of our device in removing underwater reverberation noises and maintaining the marine acoustic ecological environment.”

Source: “Water-air acoustic communication based on broadband impedance matching,” by Ping Zhou, Han Jia, Yafeng Bi, Yunhan Yang, Yuzhen Yang, Peng Zhang, and Jun Yang, Applied Physics Letters (2023). The article can be accessed at