Development of a new type of acoustic invisibility cloak

Sound waves are a group of waves that result from a specific source, and have the necessary kinetic energy that works to move the surrounding air molecules and thus travel from one place to another.

Scientists have developed a technique to mask the effect of objects on sound fields, so that sound waves do not appear to be hitting or being reflected from them. It works using an outer ring of microphones (used as acoustic sensors) and an inner ring of loudspeakers (used as audio sources). By analyzing the sound waves picked up by the microphones, the computer instructs the speakers to instantly adjust the sound field so that it behaves as if the object that was hidden wasn't there.

This opens up previously inaccessible research directions and facilitates practical applications including architectural acoustics, education, and stealth, the researchers explain in their paper.

The idea of ​​masking things acoustically is not new in itself, it has also been experimented with what are known as metamaterials, which are designed to absorb all sound waves when they reach the surface. However, this is a rather passive and inflexible approach that only works across a limited range of frequencies.

And with this new real-time approach, there is a lot of variety in making things disappear - and they can even work the other way around too, to make it look as if a non-existent object is taking up space in the room (3D imaging).

There are so-called Field Programmable Gate Arrays (FPGAs) - integrated circuits that can be encoded on-demand to ensure that audio source outputs are able to respond to speaker outputs with virtually no delay at all.

So far, the researchers have been able to operate their system for 2D objects up to 12 centimeters (4.7 inches) in size. With further study, the team expects to be able to extend the techniques to work with 3D objects that can be much larger in size. Moreover, it actually works over a wide frequency range.

Geophysicist Johann Robertson of ETH Zurich in Switzerland said, our facility allows us to process the sound field over a frequency range of more than three and a half octaves.

This technique can be used well in any field where sound waves are recorded and analyzed – covering a whole range of scientific applications, such as studying underground structures.

Moreover, the researchers hope to get a system like this that works underwater as well, where acoustics vary widely. Again, any type of sound wave scanning can be utilized where existing objects need to be hidden or virtual objects placed. This new research is further evidence of the amazing patience of many scientists as well, as the initial basis for the acoustic mantle was developed many years ago, explains mathematical geologist Andrew Curtis, of the University of Edinburgh in the UK.

The research was published in the journal Science Advances.

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