/span/span div style="float:right;width:305px;display:inline;font-size:13px;font-weight:400;font-style:normal;line-height:16px;" img src="http://news.bbcimg.co.uk/media/images/53671000/jpg/_53671958_acoustic_cloak.jpg" width="304" height="398" alt="Graphs showing acoustic 'cloaking'"span style="width:304px;"Reflections of sound off a surface (top), off an object on it (middle) and off a cloaked object (bottom)/span /div div class="story-feature related narrow"Related Stories/div p class="introduction" id="story_continues_1"Scientists have shown off a "cloaking device" that makes objects invisible - to sound waves./p pSuch acoustic cloaking was proposed theoretically in 2008 but has only this year been put into practice./p pa href="http://prl.aps.org/abstract/PRL/v106/i25/e253901"Described in Physical Review Letters/a, the approach borrows many ideas from attempts to "cloak" objects from light./p pIt uses simple plastic sheets with arrays of holes, and could be put to use in making ships invisible to sonar or in acoustic design of concert halls./p pMuch research has been undertaken toward creating Harry Potter-style "invisibility cloaks" since the feasibility of the idea was first put forward in 2006./p pThose approaches are mostly based on so-called metamaterials, man-made materials with properties that do not occur in nature. The metamaterials are designed such that they force light waves to travel around an object; to an observer, it is as if the object were not there./p pBut researchers quickly found out that the mathematics behind bending these light waves, called transformation optics, could also be applied to sound waves./p p"Fundamentally, in terms of hiding objects, it's the same - how anything is sensed is with some kind of wave and you either hear or see the effect of it," said Steven Cummer of Duke University. "But when it comes to building the materials, things are very different between acoustics and electromagnetics./p p"The thing you need to engineer into the materials is very different behaviour in different directions that the wave travels through it," he told BBC News. /p pIn 2008, Dr Cummer first described the theory of acoustic cloaking in a href="http://prl.aps.org/abstract/PRL/v100/i2/e024301"an article in Physical Review Letters/a, and earlier this year a group from the University of Illinois Urbana-Champaign demonstrated the first practical use of the theory in a href="http://prl.aps.org/abstract/PRL/v106/i2/e024301"an article in the same journal/a./p pThat work showed acoustic invisibility in a shallow layer of water, at ultrasound frequencies above those we can hear./p pNow, Dr Cummer and his colleagues have shown off an acoustic cloaking technique that works in air, for audible frequencies between one and four kilohertz - corresponding to two octaves on the higher half of a piano. /p div style="float:right;width:305px;display:inline;font-size:13px;font-weight:400;font-style:normal;line-height:16px;" img src="http://news.bbcimg.co.uk/media/images/53678000/jpg/_53678822_cloak_plastic_pic.jpg" width="304" height="171" alt="Acoustic cloaking device"span style="width:304px;"The cloaking shell is made of easily-manufactured sheets of plastic with holes through them/span /div pIt works by using stacked sheets of plastic with regular arrays of holes through them. The exact size and placement of the holes on each sheet, and the spacing between the sheets, has a predictable effect on incoming sound waves./p pWhen placed on a flat surface, the stack redirects the waves such that reflected waves are exactly as they would be if the stack were not there at all. /p pThat means that an object under the stack - in the team's experiments, a block of wood about 10cm long - would not "hear" the sound, and any attempts to locate the object using sound waves would not find it./p p"How the sound reflects off this reflecting surface with this composite object on it - which is pretty big and has a cloaking shell on it - really reflects... just like a flat surface does," Dr Cummer said./ppOrtwin Hess, a director of Imperial College London's Centre for Plasmonics and Metamaterials, called the work "a really remarkable experimental demonstration"./p p"It shows very nicely that although acoustic and electromagnetic waves are very different in nature, the powers of transformation optics and transformation acoustics are [similar] - I'm quite pleased that there's activity on both ends."/p pProfessor Hess pointed out that the demonstration was for very directed sound waves, and only in two dimensions, but the most notable aspect of the approach was its simplicity./p p"It's almost like someone could take a pencil and poke holes in a particular way in the plastic," he told BBC News./p p"It's a bit more challenging for three dimensions. I don't see any reason why it shouldn't be possible but it won't be just an afternoon's work."/p pThe work shows that an object can be hidden from sonar, and protected from incoming sound, but the same principles could be applied in the other direction - that is, containing or directing the sound within a space, for instance in soundproofing a studio or fine-tuning the acoustics of a concert hall./ppThis article is from the a href="http://news.bbc.co.uk" title="Link to BBC News"BBC News website/a. © British Broadcasting Corporation, The BBC is not responsible for the content of external internet sites./p pa href="http://feedads.g.doubleclick.net/~a/H7vbPdXNp70Oksj4NUIc-t37mBY/0/da"img src="http://feedads.g.doubleclick.net/~a/H7vbPdXNp70Oksj4NUIc-t37mBY/0/di" border="0" ismap="true"/img/abr/ a href="http://feedads.g.doubleclick.net/~a/H7vbPdXNp70Oksj4NUIc-t37mBY/1/da"img src="http://feedads.g.doubleclick.net/~a/H7vbPdXNp70Oksj4NUIc-t37mBY/1/di" border="0" ismap="true"/img/a/pimg src="http://feeds.feedburner.com/~r/BbcNewsTechnologyFullFeed/~4/L7GRIyQQUU8" height="1" width="1"/