The Future Isn't Invisible, But It's Real Quiet

The idea of being invisible has captivated scientists and comic-book writers alike for many years. As Einstein told us, it's going to be pretty hard to do. But it's somewhat easier to make something invisible to sound waves. Researchers in Spain have developed  a material that directs sound waves around an object so that they re-form on the opposite side with no degradation. Of course the military would  be interested in the material; your submarine would be invisible to Sonar if it used such a material, for instance. But think of the civilian uses of the stuff. You could live next door to an airport or an Wu-Tang Clan aficionado and never know it.

In order for a material to work as an acoustic cloak, the speed of sound passing through it must be direction dependent. That is, sound waves traveling through the shielding material from one direction must move at a different speed than waves traveling in a perpendicular direction. These differences create scattering effects that should direct sound waves to flow over a shielded object like water flowing around a rock. Because the waves return to their original conformation after passing such a shielded object, the object effectively becomes invisible to sonar. And a listener inside such a shield wouldn't hear the sounds flowing around.

The scientists have demonstrated the idea, but there are technical difficulties with the engineering of the substance itself that must be overcome before you'll be getting any off the shelf at Wal-Mart.
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