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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: 3, Informative) by bob_super on Friday May 11 2018, @01:01AM (5 children)
For a directional sonar, it would show as an area or infinite depth (equivalent to seeing a black object). The problem is that in most places underwater, the surrounding area is also beyond detection range, and therefore infinite depth (unlike stars).
(Score: 2) by Immerman on Friday May 11 2018, @01:27AM (2 children)
That's complete absorption though - not invisibility. I believe most "invisible" meta-materials cause the wavelengths affected to bend around the target, so that you can still "see" things behind it - i.e. a meta-material invisible to visual light would NOT blot out the stars, though there might be some distortion. Nobody refers to vanta-black and other ultra-black coatings as "invisibility paint" - because it's not invisible, just near-fully absorbent.
(Score: 2) by bob_super on Friday May 11 2018, @01:39AM (1 child)
Bending around so it's not reflected, I can fathom.
Bending around with such precision and so little interference that it is emitted from the back exactly as if the object wasn't there, and back for the return path if reflected (or emitted in the case of stars), without noticeable distortion ? Sci-Fi.
Even with perfect material, somehow interference-free bending (esp hard in the sonar range), the mechanical constraints on the object would be amazing and applicability unrealistic.
(Score: 3, Insightful) by mhajicek on Friday May 11 2018, @04:20AM
There would be distortion, like the sonar equivalent of predator camo. It may be possible for advanced processing to pick up the outline.
The spacelike surfaces of time foliations can have a cusp at the surface of discontinuity. - P. Hajicek
(Score: 2) by RS3 on Friday May 11 2018, @01:33AM (1 child)
Reflecting sonar is already defeated by anti-reflective coatings they use on mil subs. However, with passive sonar, sensor arrays and processing detect the sonar shadow / disturbance in the background acoustic noise caused by an object in the water. The idea here is to minimize that disturbance.
(Score: 2, Interesting) by Ethanol-fueled on Friday May 11 2018, @05:31AM
I dunno, an effective spectrum of 7-12 KHz might bamboozle Zimbabwean subs, but won't fool the Chink subs or anything else constructed in the past 40 years.