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Cloaking devices -- it's not just 'Star Trek' anymore
During the 175th Meeting of the Acoustical Society of America, being held May 7-11, 2018, in Minneapolis, Minnesota, [Amanda D.] Hanford will describe the physics behind an underwater acoustic shield designed in her lab.
Hanford and her team set out to engineer a metamaterial that can allow the sound waves to bend around the object as if it were not there. Metamaterials commonly exhibit extraordinary properties not found in nature, like negative density. To work, the unit cell -- the smallest component of the metamaterial -- must be smaller than the acoustic wavelength in the study.
[...] To date, most acoustic metamaterials have been designed to deflect sound waves in air. Hanford decided to take this work one step further and accept the scientific challenge of trying the same feat underwater. Acoustic cloaking underwater is more complicated because water is denser and less compressible than air. These factors limit engineering options.
(Score: 5, Informative) by requerdanos on Friday May 11 2018, @01:13AM
That kind of goes beyond oversimplification into legendary nonsense.
First, you can't just cook up a batch of random metamaterials and have it exhibit magical properties because it "commonly" does so. There is quite a bit of science and also some engineering involved.
Second, the "Negative Density [issp.ac.ru] [PDF Warning]" is not a property of any metamaterial (or indeed, any material), such that the more you add, the less you have.
Rather, a single (uncommon) metamaterial, used as a shield in a very particular way, causes what it's covering to appear, to measurements taken under certain very specific circumstances, to have less mass than it does. (The "dual-resonator metamaterial exhibits its negative effective mass density over a" certain frequency spectrum, according to the source linked above, which is admittedly still pretty cool.)