The acoustic cloaking device works in all three dimensions, no matter which direction the sound is coming from or where the observer is located. By placing this cloak around an object, the sound waves behave like there is nothing more than a flat surface in their path.
Turning to Nature: METAMATERIALS
To achieve this new trick, Cummer and his colleagues turned to the developing field of metamaterials — the combination of natural materials in repeating patterns to achieve unnatural properties. In the case of the new acoustic cloak, the materials manipulating the behavior of sound waves are simply plastic and air. Once constructed, the device looks like several plastic plates with a repeating pattern of holes poked through them stacked on top of one another to form a sort of pyramid.
To give the illusion that it isn’t there, the cloak must alter the waves’ trajectory to match what they would look like had they had reflected off a flat surface. Because the sound is not reaching the surface beneath, it is traveling a shorter distance and its speed must be slowed to compensate.