How to deflect a sound wave – a new discovery

Thanks to this invention, le sound emitted by a loudspeaker could move around the pillars in a cathedral and be received intact by a person situated in their “shadow”. In the same way, the noise of jet engines could be deflected towards the sky rather than the ground, and certain types of vehicle could be rendered undetectable by sonar.

Scientists at EPFL’s Laboratory for Electromagnetism and Acoustics have succeeded in creating a device that implements the principle of negative refraction in the domain of acoustics. This was achieved by applying recent findings from the domain of electromagnetism, and the results could mean that one day objects are no longer an obstacle to sound.

“Even though it’s only at the experimental stage, the study of this phenomenon has become a burning topic in acoustics over the last three or four years”, describes Hervé Lissek, scientist and co-author of an article recently published in the journal Physical Review B*.

Already well understood in the area of optics, the phenomenon of refraction is illustrated by the deflection of a light beam when it encounters an alteration in the propagation medium. This explains why a straw, when dipped into a glass of water, appears to be broken at the point where it enters the liquid – the trajectory of the light is deflected when it passes from one medium (air) to another (water). Refraction is also responsible for mirages and rainbows.

To “flow” around an object

Negative refraction is not found in nature. It can be produced by using artificial structures called “metamaterials”, which have the property of further bending – or inverting the path of – light rays, resulting in their potential to “flow” around an object, at least theoretically. Some scientists believe that this phenomenon could one day be used to make objects invisible or transparent, a bit like Harry Potter’s cape!

For the first time, EPFL scientists have succeeded in demonstrating the possibility of obtaining negative refraction in the domain of acoustics using a discrete device; this work was based on a recent PhD thesis in the domain of electromagnetism**. Sound is defined by two essential properties of a fluid (a material medium) in which it travels: its mass, put in movement by an acoustic disturbance, the acceleration of which is normally directed according to the forces of acoustic pressure (inertial force); and its compressibility, which allows it naturally to oppose such forces (restoring force).

Using the device they conceived, the EPFL scientists obtained, simultaneously, negative values for these two parameters; that is to say, to further compress the particle of fluid subjected to excessive acoustic pressure, and to determine that its acceleration oppose the forces of pressure. This combination is a prerequisite in obtaining negative refraction, a property that has been initially observed in a digital model of acoustic metamaterial. Similar results have also been observed in a second phase, using an experimental prototype which enabled the validation of the theoretical concept.

Even though we are still a long way from a genuine acoustic “invisibility cape”, these results demonstrate the performance of a tangible physical system representing negative refraction, while constituting an important achievement for research in this area.

*“Acoustic transmission line metamaterial with negative/zero/positive refractive index”, published in Physical Review B – Frédéric Bongard, Hervé Lissek and Juan R. Mosig.
**“Contribution to characterization techniques for practical metamaterials and microwave applications”, F. Bongard,PhD thesis, EPFL, Lausanne, 2009