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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: 2) by RS3 on Friday May 11 2018, @01:25AM
Light travels in a relatively straight line through space and atmosphere. If the night sky object absorbs light (is opaque), you observe a loss of starlight- a dark shadow. If that object is clear, you might not see it, but you might notice some kind of disturbance, especially around the edges.
Sound waves are not like light- they spread out and wrap around objects. Yes, they will be disturbed, but casual listening might not detect it. But with sensor arrays and processing one can get a pretty good idea of the size and shape of the object.
The Penn State research attempts to get the sound waves to reconnect coherently- very little net disturbance or distortion- so the object is pretty much invisible even to advanced sensing systems.