14 DAY TRIAL // Researchers from the University of Illinois at Urbana Champaign (UI) have managed to create a new type of metamaterial that is able to guide sound waves through very tiny structures. The innovation could have significant applications in the field of medicine, as well as for new military devices. Doctors could use it to create clearer ultrasound images of babies in the womb, and also to detect cancer tumors, while Navy engineers could employ it to conceal large, nuclear-powered submarines from enemy sonars.
“Our focus is not about dampening noise, but to guide sound waves around structures. If we have a coating on a submarine that bends acoustics waves before they hit the surface, guiding them around the submarine smoothly, then you won't be able to detect a submarine using sonar,” UI Professor Nicholas Fang said, quoted by ABC News. Details of the innovative new material appear in the latest issue of the journal Physical Review Letters.
The main operating principle of metamaterials, be they for sound or light, is to manipulate pressure waves and light waves, respectively, around a given object, without breaking it. If this is done correctly, then the targeted object becomes invisible, in all senses of the word. That is to say, if an invisibility cloak is placed on top of an object, it guides light waves around it, in such a manner that an onlooker sees what is behind the concealed object without realizing that another thing is in front of what they perceive. Such cloaks that bend light already exist, but, at this point, they only work in a 2D environment.
One of the good things about the new material is that it's a lot easier to manufacture than optical cloaking devices, for the simple reason that sound waves are larger than electromagnetic waves. “If you need to build an ultrasonic metamaterial, the dimension of the physical structure is tens or hundreds of microns. Compare that with optical metamaterials, and you are talking about hundreds of nanometers. That makes it a lot more amenable for research,” Fang explained. This is made possible by the fact that, in any type of metamaterials, it is not the chemical structure that matters, but the physical one.
In addition to being able to bend sounds around an object, using octagon- and cube-shaped structures, the new metamaterial can also focus sound waves on a sub-millimeter-sized area, which gives a better “resolution” than any imaging technique in existence today. High-performance ultrasound scanners could thus become possible, and they could picture babies inside the wombs with unprecedented clarity. “We have seen some very exciting demonstrations. But to make this as a practical structure we need another three to five years,” Fang added.
“This is one of the very first metamaterials that has been designed to work and actually do something useful for sound waves instead of electromagnetic waves. I would say that the Illinois design for two-dimensional acoustic metamaterials is pretty straightforward to manufacture. I certainly hope that it can be transitioned to an actual application,” Duke University Professor Steven Cummer, who was involved in creating the first invisibility cloak, back in 2005, shared.