Imagine a room where a band is playing. Neighbors can't hear the music, but if someone outside the room is talking, the musicians can hear it.
The concept — a kind of one-way mirror for sound — seems imaginary, but two Italian scientists recently pushed this kind of sound manipulating technology closer to reality.
"Nonlinearity makes our world much richer than a linear world," said Giulio Casati, a physics professor and director of the Center for Nonlinear and Complex Systems at the University of Insubria. "It allows us much more possibility."
Casati and Stefano Lepri, a researcher for the Institute for Complex Systems at the National Research Council in Florence, came up with an unexpected acoustic mechanism. Their idea was inspired by earlier work on thermal diodes capable of transmitting heat asymmetrically. The technology they envision now is a material or device that has nonlinear properties, meaning it can bend sound waves in an unexpected way.
Theoretically, the nonlinear system would function better than a one-way mirror. Mirrors like the ones used by police aren't truly one-way because they actually rely on differing levels of light and reflectivity — one side has to be dark and the other light. Nonlinear acoustic material, on the other hand, would allow sound to travel in only one direction, blocking incoming sound.
To direct sound waves, the researchers propose alternating layers of linear and strongly nonlinear materials asymmetrically. With the right formation, when a sound wave enters from one side, it will essentially get caught in the material and then be redirected. This is because the frequency is shifting in two different directions, Casati explained.
The physicists demonstrate mathematically that such an acoustic metamaterial has the potential to be built one day. They recently published their findings in Physical Review Letters.
"Physics allows these wave diodes," Casati said. "It's a crucial point because before, this was not known."
Although the acoustic technology remains theoretical, the researchers think that it can be built. Lepri cited optics, saying that nonlinear photonics is a well-developed field now.
"The question is if that same nonlinearity can be created in acoustics," he said. "It's perhaps less easy, but there are some attempts in the literature and I think it's something that's already being realized."
Steve Cummer is an associate professor of electrical and computer engineering at Duke University, and one of his areas of expertise is engineered electromagnetic materials. "It's an interesting theoretical solution, and the kinds of materials they're talking about are not completely crazy," he said of the Italians' published paper.
Cummer added that the device concept is easy to understand, and he thinks it would have practical uses if it could be built.
"I suspect there are lots of applications where one might want to keep sound in a room from escaping out, but you'd like to be able to hear what's going on outside."