It has been an exciting couple of years for theorists devoted to exploring what waves can be made to do. First, it was shown that light could be induced to bend around an object, effectively rendering it invisible—provided the viewer only observes one color of light. Then, it was shown that, although light could be made to do this, it was by no means certain that all wave phenomena could be induced to do similar things. In particular, it was thought that acoustic cloaking would be confined to two dimensions because acoustics in 2D is mathematically identical to light in 2D, but the two are different in 3D. Now, however, theorists have pushed the boundary back on that particular subject, showing that 3D objects can be acoustically cloaked as well.
As with all other cloaking materials, some anisotropy—direction dependence—is required. In optics, this is induced by constructing the cloak out of oriented rings or bars that have the effect of slowing the wave in one direction, causing it to bend smoothly around the interior of the cloak. In the case of acoustics, mass anisotropy is required, meaning that the acoustic wave has to see a material that is denser in one direction than in another.
This sounds impossible, but it can be constructed in the same manner as optical meta-materials. The cloak could be constructed from a structure that repeats itself on the same length scale as the sound wavelength and is acoustically active (e.g., a spring). The sound wave would then experience the properties of the springs rather than the material that the spring is manufactured from. If the wave comes into the material from a different direction, then the springs are not set into motion by the sound wave, so the wave experiences the properties of the material from which the springs are made. This provides an effective mass anisotropy.
Given this mass anisotropy, it is simply a matter of solving some equations to find out if there is a region where the sound wave doesn't go and figure out what properties the mass anisotropy would need to achieve particular cloaks. It turns out that a spherical region is cloaked when the effective density of the cloak increase in the radial direction as the wave moves into the cloak. This increasing radial density, compared to the density experienced as one tries to circulate around the sphere, causes the wave to smoothly bend around an interior spherical region.
Interestingly, because light cloaking is described by bending coordinate space, there are strict limits to the range of colors that can be bent while still making sure that cause follows effect. In other words, because we accept that cause must follow effect, a single cloaking structure works for only a limited range of colors. Acoustic cloaking does not require bending coordinate space, but rather on varying physical properties of the medium, meaning that cause and effect are preserved for all possible structures. The greater freedom of design implies that structures that cloak over a broad range of sound frequencies might be possible.
Since a cloaking structure is possible, I would expect that a containment structure is also possible, which would interest certain people in the military. However, I think applications such as improved noise shielding for apartments and sasers are more likely to find big and lucrative markets.
Physical Review Letters, 2008, DOI: 10.1103/PhysRevLett.100.024301