New device can transmit underwater sound to air

Don’t expect to play a game of Marco Polo by shouting from beneath the pool’s surface. No one will hear you because, normally, only about 0.1 percent of sound is transmitted from water to the air. But a new type of device might one day help.

Researchers have designed a new metamaterial — a type of material that behaves in ways conventional materials can’t — that increases sound transmission to 30 percent. The metamaterial could have applications for more than poolside play. A future version might be used to detect noisy marine life or listen in on sonar use, say applied physicist Oliver Wright of Hokkaido University in Sapporo, Japan, and a team at Yonsei University in Seoul, South Korea, who describe the metamaterial in a paper accepted to Physical Review Letters.

Currently, detection of underwater sounds happens with hydrophones, which have to be underwater. But what if you wanted to listen in from the surface?

Enter the new device. It’s a small cylinder with a weighted rubber membrane stretched across a metal frame that floats atop the water surface. When underwater sound waves hit the device, its frame and membrane vibrate at finely tuned frequencies to help sound transmit into the air.

“A ‘hard’ surface like a table or water reflects almost 100 percent of sound,” says Wright. “We want to try to mitigate that by introducing an intermediary structure.”

Both water and air resist the flow of sound, a property known as acoustic impedance. Because of its density, water’s acoustic impedance is 3,600 times that of air. The greater the mismatch, the more sound is reflected at a boundary.

Adding a layer of material one-fourth the thickness of an incoming wave’s wavelength can reduce the amount of reflection. This is the principle at work behind anti-reflective coatings applied to lenses of cameras and glasses. While optical light has a wavelength in the hundreds of nanometers, necessitating a thin coating only a few atoms thick, audible sound waves can be meters long.

Even though it’s only one-hundredth the thickness of the sound’s wavelength, instead of the conventional one-fourth, the metamaterial still transmits sound.

“It’s a tour de force of experimental demonstration,” says Oleg Godin, a physicist at the Naval Postgraduate School in Monterey, Calif., who was not involved with the research. “But I’m less impressed by the suggestions and implications about its uses. It’s wishful thinking.”

One major problem that the researchers would have to overcome is the device’s inability to transmit sound that hits the surface an angle. In the lab, the device is tested in a tube — effectively a one-direction environment. But on the vast surface of a lake or ocean, the device would be limited to transmitting sounds from the small area directly below it. Additionally, the metamaterial is limited to transmitting a narrow band of frequencies. Noise outside that range reflects off the water’s surface as usual.

Still, the scientists are optimistic about the next steps, and even propose that a sheet of these devices could work in concert.