摘要：Sound can't normally travel from beneath water's surface to the air above due to a mismatch in densities, but a new material changes that

A material that can transport underwater sounds into the air, allowing them to be heard clearly above the water’s surface, could be used for monitoring aquatic environments or eavesdropping on underwater communications.

Sound normally can’t travel between water and air because of a mismatch in densities between the two mediums. It instead reflects at the boundary.

Now, Bin Liang at Nanjing University in China and his colleagues have developed a material that allows almost all of the sound to pass from water to air.

“The interface between the water and air is acoustically transparent, it’s just like it’s not even there,” says Liang. “Our technique lets a sound sensor in one medium easily detect the sound signal coming from the other.”

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The technique involves using a metamaterial, a class of materials with artificial structures that imbue them with unusual properties. The team’s metamaterial is a sheet made from resin with a grid of air-filled passages that have a winding, labyrinthine structure. Each passage has a smaller entrance that sits on the water and a larger entrance open to the air.

When a sound wave enters from the water and traverses the maze-like passages, its effective speed is reduced because it emerges from the other side after a delay. This essentially allows the sound wave to travel at the same frequency in both water and air, so cross the boundary.

The material could be used for listening to underwater communications or monitoring aquatic environments, says Liang.

Roy Sambles at the University of Exeter, UK, points out that the current version of the metamaterial only works at quite high frequencies, around 8000 hertz, which is at the upper end of what humans can hear. For lower frequencies, the metamaterial would need to be much thicker, which would mean it wouldn’t function at higher frequencies. It would also be unlikely to work on very choppy seas, he adds.

Journal reference: Advanced MaterialsDOI: 10.1002/adma.202301799