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Making things invisible is the stuff of science fiction. Our fascination is with the idea of not being able to see things that are before our eyes, but invisibility in a different sense may be useful too. Harry Potter's invisibility cloak allowed him to disappear at will, but the sounds he made could easily have given him away when he was wandering those quiet nighttime corridors. He needed to be 'acoustically invisible' too: he needed the sounds he made to disappear. In fact researchers have been working on the technology to do that for almost a century and it hasn't been totally science fiction for a long time. Since World War II technology has existed to make submarines all but disappear from sonar. A new material may even make it possible to make an invisibility cloak that makes things truly acoustically invisible.
A Deathly Silence
Back in World War II U-boats were central to the German superiority in the Atlantic, but sonar was giving them away. Remember all those naval war films where everyone in the submarine fall silent as we listen to the pings of the sonar. The German Kriegsmarine badly wanted technology to make their U-Boats invisible to those deadly pings.
There are two kinds of sonar: active and passive. In passive sonar, the submarine killer just uses underwater microphones to listen for any noise coming from the submarine to work out where to drop the depth charges - that's why they shut off the engines and everyone falls silent in the movies. Any sound could give them away. Active sonar removes even this possibility of escape by sending out sound waves - the sonar 'pings' - and listening for their echoes. That way, even a totally silent sub can be found. When the pings hit it they bounce back to the listeners, revealing the sub's location in the same way that bats find their prey.
The technology the Kriegsmarine developed involved covering the submarines with special tiles. These so-called 'anechoic' tiles are covered with lots and lots of small bubble-like holes. They absorb the sonar pings, turning the sound energy into heat rather than reflecting it. That means the ping doesn't return to the submarine killer, giving the game away. Different sound frequencies at different depths are best distorted by different sized holes. That means the tiles have to have a specially engineered mixture of holes if the submarine is to stay hidden. They can also be designed to absorb specific sounds of the submarine like the engine noise, or maybe even that spanner dropped by the clumsy sailor at the critical moment in the films! World War II ended before the technology was perfected, but the Soviets went on to make the idea work during the cold war. Now everyone's military submarines are covered with anechoic tiles.
This kind of technology works well for submarines because there is only deep ocean and so silence around them. Up on the surface, silence itself could give the game away. Imagine a person walking through a park covered in an acoustic cloak. The park is full of birdsong and the whispering of the leaves. As the person passes by the cloak absorbs the sounds hitting it and so a person they pass would hear the birds and trees beyond momentarily fall silent. It is the sonic equivalent of a person being invisible to the eye but still leaving a shadow - just a shadow of no sound rather than no light. A ghostly silence would pass by.
True acoustic invisibility involves the sounds around a cloaked object not just being absorbed but apparently passing through the object as though it wasn't there. How could that be done? Well essentially the sound waves would need to be bent around the object. The cloak would need to take the waves in and spit them out the other side in exactly the direction, not to mention frequency and volume, they had been originally traveling.
José Sánchez-Dehesa and Daniel Torrent at the Polytechnic University of Valencia have invented a material that may one day be the basis of cloaks able to do just that, and it isn't actually all that hi-tech. It involves sheets of perspex covered with a hexagonal pattern of holes. Whilst the material itself isn't that hi-tech it was actually inspired by some rather hi-tech material physics: that of graphene. Graphene is a form of carbon where the carbon atoms are laid out in sheets of hexagons. Electrons move through graphene in an unusual way, being drawn into the hexagons as they flow over the surface. That inspired the Spanish researchers to see if sound would behave in the same way as electrons...and it turns out it does. Rather than just distorting sound, turning it into heat, as the rows of holes in the early anechoic tiles do, the special hexagonal pattern means the sound is drawn in to them unchanged. That's what a sonic invisibility cloak would need to do, pull the sound into holes around it then transport it down specially positioned tubes that release it unchanged at the other end to continue on its way. Whilst a cloaking device hasn't yet been engineered out of a material like this, in principle at least it could, so acoustic cloaking may one day be a reality.
In future, miscreants wandering school corridors after curfew may not have to worry about leaving a ghostly silence as they invisibly pass by...if of course we can make them invisible to the eye too...but even that isn't as science fiction as you probably thought.