Microphone in a soundboothOne-way mirrors, which many of us know from watching police procedurals on TV, seem a bit magical—how does the mirror know which light to let through and which to reflect? The truth is, it doesn't. The one-way mirror and its smaller cousin, the mirrored sunglass lens, rely on lighting imbalances for efficacy. If the cops behind the one-way mirror were as brightly lit as the interrogation room, the suspect would be able to see them just fine.

But materials that genuinely discriminate between the direction of light or sound might be possible, according to a new study. That could make for true one-way mirrors or for directional soundproofing—imagine, for instance, a wall through which sound can enter but not escape.

Stefano Lepri of the Italian National Research Council and Giulio Casati of the University of Insubria in Italy and the National University of Singapore have worked out the theoretical groundwork for materials that transmit waves in an asymmetric way, which they report in the April 22 issue of Physical Review Letters.

Their proposal relies on the use of nonlinear materials, in which the response of the material depends on the attributes of the wave passing through it. "When you introduce nonlinear interactions and forces, many of the intuitions we have are no longer valid," Lepri told Physical Review Focus, an American Physical Society publication that highlights studies from affiliated journals and explains them to a wider audience. "We can use this nonlinear interaction to break this fundamental result of reciprocity theory," which demands that all waves get the same transmission treatment regardless of the direction from which they arrive.

By stacking layers of nonlinear materials along with ordinary linear layers in an asymmetric fashion, the researchers have calculated, a wave would be able to pass through in one direction but would almost completely bounce off when it arrives from the other direction. The one-way bias isn't universal, however—the researchers note that each particular implementation would have a sweet spot of wave amplitudes and frequencies for which it would work best.

So far, the finding is based only on numerical simulations rather than laboratory experiments. But if those simulations prove to be a good approximation of real materials, the researchers report, the "results may open the way to novel strategies to control and optimize wave propagation and to design devices for sound or light rectification."

Soundproofing photo: © iStockphoto/Simone Becchetti