“Circling“ around the silent center: Design (top) and intermediate step of production (bottom) of the elastic invisibility cloak. Image: AP, KIT |
Progress
of metamaterials in nanotechnologies has made the invisibility cloak, a
subject of mythology and science fiction, become reality: Light waves
can be guided around an object to be hidden, in such a way that this
object appears to be non-existent. This concept applied to
electromagnetic light waves may also be transferred to other types of
waves, such as sound waves. Researchers from Karlsruhe Institute of
Technology (KIT) have now succeeded in demonstrating for the first time
an invisibility cloak for elastic waves. Such waves also occur in
strings of a guitar or drum membranes.
It
is as if Harry Potter had a cloak that also makes him unhearable.
“Maybe a place of peace and quiet in the Christmas season,” say the KIT
researchers, who succeeded in transferring the concepts underlying the
optical invisibility cloak to acoustic waves in a plate.
“The
key to controlling waves is to specifically influence their local speed
as a function of the ‘running direction’ of the wave,” says Dr. Nicolas
Stenger from the Institute of Applied Physics (AP). In his experiment,
he used a smartly microstructured material composed of two polymers: A
soft and a hard plastic in a thin plate. The vibrations of this plate
are in the range of acoustic frequencies, that is some 100 Hz, and can
be observed directly from above. The scientists found that the sound
waves are guided around a circular area in the millimeter-thin plate in
such a way that vibrations can neither enter nor leave this area.
“Contrary
to other known noise protection measures, the sound waves are neither
absorbed nor reflected,” says Professor Martin Wegener from the
Institute of Applied Physics and coordinator of the DFG Center for
Functional Nanostructures (CFN) at KIT. “It is as if nothing was there.”
Both
physicists and Professor Martin Wilhelm from the KIT Institute for
Chemical Technology and Polymer Chemistry have now published their
results in the journal Physical Review Letters.
The
scientists explain their idea by the following story: A city, in the
shape of a circle, suffers from noisy car traffic through its center.
Finally, the mayor has the idea to introduce a speed limit for cars that
drive directly towards the city: The closer the cars come to the city
area, the slower they have to drive. At the same time, the mayor orders
to build circular roads around the city, on which the cars are allowed
to drive at higher speeds. The cars can approach the city, drive around
it, and leave it in the same direction in the end. The time required
corresponds to the time needed without the city. From outside, it
appears as if the city was not there.
