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Sunday, June 2, 2013

kitchen cling wrap used in important sound transmission discovery

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.