By Jon Cartwright
The past few years has seen a steady stream of proposals for cloaking objects, whether it’s from light, heat, water waves, magnetic fields or even time. Now, physicist Sang-Hoon Kim at the Mokpo National Maritime University in Korea is adding to this list, first off with a cloak that could protect buildings from earthquakes.
An earthquake cloak has been proposed before using – as is common in invisibility cloaks – elaborately structured “metamaterials” to guide seismic waves safely around a building. However, Kim, together with Mukunda Das at the Australian National University in Canberra, has put forward a different approach: a metamaterial barrier that dissipates seismic energy as sound and heat. The idea is that many buildings could hide in the “shadow zone” of the barrier. This could be a boon for city planners, who would not have to make cloaks for individual buildings. Kim and Das’s paper has been accepted for publication in Modern Physics Letters B and is available as a preprint entitled “Artificial seismic shadow zone by acoustic metamaterials“.
Kim has also been working with colleague Seong-Hyun Lee at the Korea Institute of Machinery and Materials in Daejeon on an entirely different metamaterial concept – silencing noisy neighbours. Their idea for a soundproof window is based on a perforated acrylic block and “is so simple that any carpenter can make it”, they claim. And it’s tuneable, too. “For example, if we are in a combined area of sounds from sea waves of low frequency and noises from [machinery] operating at a high frequency, we can hear only the sounds from sea waves with fresh air.”
That sounds lovely in the summer, but it might get a bit draughty come winter with all those holes. The window is described in a preprint entitled “Air transparent soundproof window“.
Loyal readers of physicsworld.com will know that this is not the first time someone has noticed that sound can be blocked by punching holes in a material. Back in 2008 we published the story “Holes prevent sound from passing through plate“.