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"Cone of Silence" Possible Say Scientists
Ponca City, We Love You writes "The 'Cone of Silence,' once a staple of 1960's television shows, is now possible say scientists at Duke University who first demonstrated a working 'cloak of invisibility' that works at microwave frequencies in 2006. Such a cloak designed for audio frequencies might hide submarines in the ocean from detection by sonar or improve the acoustics of a concert hall by effectively flattening a structural beam. Although the theory used to design such acoustic devices so far isn't as general as the one used to devise the microwave cloak, the finding nonetheless paves the way for other acoustic devices. 'We've now shown that both 2-D and 3-D acoustic cloaks theoretically do exist,' says Researcher Steven Cummer. 'It opens up the door to make the physical shape of an object different from its acoustic shape.'"
"It opens up the door to make the physical shape of an object different from its acoustic shape."
Every time my wife asks me to do something, I don't hear it.
Every time I ask her to do something, she doesn't hear me.
Don't be apathetic. Procrastinate!
It seems they're importing ideas from photonics and metamaterials - light and sound, they're all waves. What's been done with light (in Britain and Germany IIRC) is some object has been surrounded by a 'cloaking device', and for one specific wavelength of light, the cloaking device and object become completely transparent and invisible. Light flows through the cloaking device and around the object that's being hidden (well that's the hand-wavy explanation). To do funky stuff like cloaking, you need (in optics) a material with negative refractive index (so light seems to travel backwards). People get this by arranging tiny (smaller than the wavelength of light involved) resonators in a regular pattern. The light wave "doesn't see" the individual resonators, but instead "sees" an overall medium. However, this medium can have quite abnormal properties (such as negative refractive index). Another way of looking at the device is that you surround the object with resonators that specifically cancel out any effect on the sound/light wave that the object makes. So you get no net effect on the wave, so it's as if the object wasn't there. Presumably the people at Duke have transplanted some designs for light and have worked through the corresponding acoustic wave equations to find "negative refractive index sound" (though i'm not sure what their resonators would look like, because light is more complicated than sound and most light metamaterial designs use properties of both E and H components of light. Maybe they can cheat because sound travels faster in, say, wood than air)
The devices the article talks about are not what you want from a cone of silence. What the researchers are proposing is something that will hide an object from external noises - as in the object will not affect any sound waves heading towards it, they will just pass straight through as if the object and cloaking device were not there. The proposed device WILL NOT contain noise created by whatever you're trying to hide, so the bad guys can still listen for a submarine's engines, they just won't be able to use active sonar to find the submarine.
If you want a cone of silence, then you put yourself in a noise isolation chamber. Or if you want something cooler, then you put yourself in the acoustic equivalent of a gap-defect photonic crystal, which is a series of cylindrical rods arranged in a hexagonal lattice with one removed. A particular wavelength of sound will be reflected by this lattice, so if you're in the middle of that gap and you sing at that frequency, no-one outside the lattice will be able to hear you. Of course, you will very quickly become deaf because the sound is all reflected within the defect rather than absorbed, so the noise from your singing builds up.