Physicists design acoustically invisible walls
May 31, 2013 1 comment
A rigid wall can be transformed from a total reflector of sound to an almost perfect transmitter by perforating it with tiny, regularly spaced holes covered by a thin elastic membrane, say researchers in Japan and South Korea. The discovery, an acoustic analogue to extraordinary optical transmission (EOT), could potentially be used in microscopes, noise filters, new types of windows, acoustic concentrators and many other applications.
EOT was discovered by Thomas Ebbesen of the University of Strasbourg and colleagues in 1998. It allows electromagnetic waves to pass almost unhindered through a lattice of sub-wavelength holes in a barrier that would otherwise be opaque in some metamaterials. In Ebbesen's discovery, this was down to coupling between photons and electrons on the surface of the barrier.
Following Ebbesen's discovery, in 2006 a different team of researchers led by Nader Engheta at the University of Pennsylvania in Philadelphia discovered another mechanism that can lead to EOT. If the holes contain a material with a refractive index close to zero, the wavelength in the holes becomes extremely long, and thus the velocity becomes extremely large. The faster a wave travels, the more energy it carries, allowing the energy of the entire wavefront to squeeze through the tiny holes. Such materials are called epsilon-near-zero (ENZ) materials because the refractive index of a material depends on its permittivity – written as epsilon (ε) – a kind of electrical inertia that is representative of the resistance that is encountered when forming an electric field in a medium. An ENZ material offers almost no resistance to such displacement.
Same for sound
Now, Sam Lee of Yonsei University in Seoul, South Korea, Oliver Wright of Hokkaido University, Japan, and colleagues have produced an analogue of Engheta's metamaterial for sound waves. Just as electromagnetic waves propagate as vibrations in a material's electromagnetic field, sound waves travel as physical oscillations of the atoms. Sound waves cannot pass through a rigid barrier because the atoms cannot oscillate. Making tiny holes in the barrier will barely increase transmission. Lee explains that if, for example, the holes make up 3% of the volume of the barrier, "to ensure continuous volume flow across the barrier, the air in the holes has to move 30 times faster than the air outside the wall. The inertia of the air does not allow for the huge accelerations needed for motion of such amplitude." To solve this problem, the researchers needed the air in the holes to have almost zero inertia – the acoustic equivalent of an ENZ material.
They achieved this, paradoxically, by covering the holes with a thin membrane of shop-bought kitchen cling film. With the tension tuned so that the membrane's resonant frequency is the same as the frequency of the incident waves, the membrane's resonance amplifies its oscillations. The resonance moves the air through the holes as though the air has no inertia, allowing it to move in response to even a small displacement and sucking almost all the energy of the incident waves through the barrier.
On the other side of the barrier, Huygens' principle dictates that each hole produces spherical wavefronts. The separation between the holes is much less than the wavelength of the sound, which means the interference pattern of the waves reconstructs the plane wave in much less than one wavelength and the barrier is effectively invisible to the propagating waves.
The researchers tested their acoustic-metamaterial design by placing an acrylic barrier perforated with four small holes in a tube. Loudspeakers producing waves of a single frequency were placed at one end and the researchers measured the intensity of the waves on either side of the barrier. The results were remarkable – they found, for example, that with the barrier perforated by bare holes, only 9% of the waves' energy was transmitted. With a membrane placed over the holes, this proportion jumped to 81%. The metamaterial worked just as well when placed at an oblique angle to the incident wavefronts.
Ebbese, who was not involved in this work, views the result as a significant contribution to acoustics with numerous potential uses. "They're talking about transmission, but in the same way you can also block waves at other frequencies by adding this membrane," he says, listing noise filters as one application where this could be useful.
According to the team, its method can be used over a range of frequencies such that it would work equally well for ultrasound. This, the researchers say, could be exploited to concentrate acoustic energy through tiny holes, forming novel lenses. They are currently working on the potential application of the idea to near-field scanning acoustic microscopy, where the properties of an object are studied by the sound waves it reflects. The researchers believe the concentration of the radiation's energy into tiny holes during the process of transmission could allow them to achieve both very high signal intensity and spatial resolution.
The research is to be published in Physical Review Letters.
About the author
Tim Wogan is a science writer based in the UK