Ciencia ficción?…

invisibilidad, «inaudibilidad»…

New scientist Technology nos informa de esto

«Metamaterials are all the rage in engineering and it’s not hard to see why. An entirely new class of substance with all kinds of exotic properties, metamaterials are being groomed as the the building blocks of a new age of super lenses, agile antennae and invisibility cloaks.

Until recently, though, all the focus had been on the way metamaterials can modify light. But what about other kinds of waves, such as sound? An acoustic version of this stuff would do the same for sound as it does for light: lenses, invisibility, and all.

At first glance, this sounds like a tricky ask. Metamaterials were first dreamed up by the Russian physicist Victor Veselago, who imagined how a material might behave if its permittivity and permeability – factors that determine how a substance interacts with electric and magnetic fields – were both negative. Such materials never occur in nature, but have recently been constructed by engineers.

Making a metamaterial for sound means identifying the acoustic analogues to permittivity and permeability than working how to build a material in which they are both negative.

It turns out that the acoustic analogues in question are a material’s mass density and its elastic constant. And this week, a group of physicists at Wuhan University in China describe how it could be done.

Their proposed metamaterial is truly weird. It consists of a periodic array of rubber-coated gold spheres along with spheres of water containing air bubbles, all embedded within an epoxy resin.

Yigun Ding and colleages’ big discovery is how to make the two key numbers both negative at the same time, at the same frequency.

Previously, only one aspect at a time has been made negative. Physicists have long known that a periodic array of water spheres containing bubbles can have a negative elastic constant at certain frequencies. And recently, they have discovered that a periodic array of hard spheres coated in soft rubber and embedded in a hard plastic can have a negative effective mass density at particular frequencies.

Now all somebody has to do is go ahead and build it. That might happen sooner rather than later. If acoustic metamaterials really can do the same for sound as they do for light, that could mean better lenses for ultrasound machines, and perhaps even cloaks that can hide submarines from sonar. It’s not hard to think of organisations that might be interested in doing that.»