Scientists in Germany have demonstrated an acoustic invisibility cloak made from material that can guide sound waves around an object.
Researchers from the Karlsruhe Institute of Technology (KIT) created a micro-structured material composed of two polymers, one hard and one soft, in a millimetre-thin plate that vibrates at frequencies around 100Hz within the audible acoustic range.
In a similar way to substances that can bend light waves around a tiny object, making it effectively invisible, the material guides sound waves around a circular area in the plate, meaning anything within that area doesn’t affect the waves.
‘Contrary to other known noise protection measures, the sound waves are neither absorbed nor reflected,’ said Prof Martin Wegener from the Institute of Applied Physics (AP) at KIT. ‘It is as if nothing was there.’
The material works by speeding up the sound waves as they travel around the outside of the circle, so, although they travel further than if they had passed straight through it, they arrive at the other side at the same time.
‘The key to controlling waves is to specifically influence their local speed as a function of the “running direction” of the wave,’ said KIT’s Dr Nicolas Stenger, also from AP.
The two physicists and Prof Martin Wilhelm, from the KIT Institute for Chemical Technology and Polymer Chemistry, have published their results in the journal Physical Review Letters.