Researchers have constructed a “metamirror” device capable of perfectly reflecting sound waves in any direction. The proof-of-principle demonstration is analogous to looking directly into a mirror and only seeing the person next to you instead of your own face. With a regular mirror, the light follows the Law of Reflection — it must bounce off of it at the same angle at which it entered. The same rules generally apply to sound. To break the law of reflection with sound waves, the researchers engineered a device that could precisely control amplitude (loudness) and speed throughout the entire wave.
To achieve this, metamaterials — artificial materials that manipulate waves like light and sound through their structure rather than their chemistry — were used. While the particular metamaterial that was designed is made out of plastic, it’s not the properties of the plastic that are important; it’s the shapes of the device’s features that allow it to steer sound waves in any direction.
The surface of the metamaterial looks much like a wave, etched with a series of channels of various depths. Those depths are engineered to precisely control the speed at which the sound wave reflects off various points of the meta-mirror. The wavelike positioning controls the sound wave’s amplitude. As a sound wave hits the meta-mirror, it reflects off of its curved surfaces and interferes with itself. Between the metamirror’s shape and the depth of its channels, this interference pattern results in the sound wave reflecting in a desired direction without any of its energy being absorbed or scattered in an unwanted direction.
In the proof-of-concept demonstration, the metamaterial device takes a sound wave traveling directly toward it at 3,000 hertz, a very high pitch not dissimilar to getting a “ringing in your ears,” and perfectly reflects it at an angle of 70 degrees.
While the prototype device is specifically tailored to one frequency and angle of reflection, the researchers plan to pursue a dynamic device that could change shape to reflect different frequencies in different directions. They also plan to work on similar devices for underwater acoustics applications. A similar device could also be created to control light waves, though its features would have to be engineered on a much smaller scale because light wavelengths are shorter. Such a device would not only be able to reflect light in different directions, it could also split a single wave into two arbitrary directions.