Raw material for acoustic cloaking

Using a technique they developed themselves, the researchers have succeeded in generating a so-called metafluid with unusual acoustic properties. These properties are determined by the mechanical behaviour of a material, which is expressed in terms of compression and shear parameters. For example, water in a cylinder can hardly be compressed, but it can be stirred with a spoon in all directions (shearing). If all five shear parameters of a material equal zero, as is the case with water, it is characterised as a pentamode material. A solid material that exhibits the same parameters as water is described as a pentamode metafluid. “The Karlsruhe prototype was manufactured from a polymer,” explains Dr. Muamer Kadic. To produce the prototype, the team used dip-in laser writing, a method derived from the direct laser writing technique developed by the firm Nanoscribe in the KIT incubator. With this method, the researchers were able to manufacture elements shaped like sugar loaves from the polymer and to arrange them in a threedimensional crystalline structure. Within this structure the “sugar loaves” touch one another only at their tips and thus form an extremely light and open matrix. The mechanical behaviour of the material is determined by the acuteness and length of the individual sugar loaves. Producing such a material is a challenge. As Dr. Kadic explains, “On the one hand, we have to be able to construct tiny sugar loaves in the nanometre range and connect them to one another at the correct angle. On the other hand, the entire structure must ultimately be as large as possible. The material itself makes up only 1 percent of the entire volume, and the resulting composite is therefore extremely light.” The new pentamode metamaterial has made it possible for the first time to influence acoustic waves selectively in three dimensions. This has already been achieved with optical waves with the help of natural materials. This development could well open the way for realising numerous ideas in the fields of transformation acoustics, such as inaudibility cloaks, acoustic prisms and new loudspeaker concepts.

Uta Deffke