According to the information published in your article, similar tests have previously been run. How does your research build on existing knowledge of acoustic camouflage?
It is the first time we successfully ran this test on three-dimensional objects, particularly on a sphere covered with rings, but we are also responsible for a previous publication where we explained how we camouflaged a two-dimensional object.
What led you to seek to improve the system?
We have to take into account that we live in a three-dimensional world. If we wanted to apply the previous findings, they had to be effective on three-dimensional objects. The basis of this research lies on a study we carried out on acoustic lenses, which divert the sound path at will. Just as we get a shining spot if we place a magnifier under the sun we can make sound waves gather at a particular point in space and thus eliminate it.
What is the invisibility mechanism made up of and how does it work?
To be specific, we should point out that, instead of «invisibility» —a term related to light perception— we should talk about «undetectability». Sound waves bounce on objects, and what we have achieved is neutralising this phenomenon in such a way that sound passes through the objects as if they were not there. That is why we say they are acoustically undetectable. The experiment was run on a sphere covered with 60 plastic rings, and what we have done at the University of Valencia is calculating the sphere’s radius and position in relation to the sphere. For this purpose we have used an optimisation software based on genetic algorithms.
What is a genetic algorithm?
It is based on Darwin’s laws of evolution: only the fittest survive, the ones capable of transmitting their genetic material to future generations. In this sense, our software has evolved into finding a «specimen» that does what we want it to do: gathering sound waves.
How long can the calculations to obtain this effect take?
We have needed a «super computer» that works in parallel, which means that we have used up to 48 microprocessors working together. We need a strong computing power to find a «fit» specimen among so many generations. Even so, five days of intensive calculations have been necessary to obtain the invisibility cloak for the chosen sphere.
That would have been unconceivable twenty years ago…
Of course. It is like operating 48 computers all at once for five whole days.
What type of object does the invisibility cloak work with nowadays?
We have been working with a sphere of radius 4 cm, but actually, the size of the object is not important. The invisibility cloak can be bigger or smaller, but its structure is what matters, as well as the wave-length and the sound frequency of the object. We chose the sphere because it is the simplest three-dimensional object. In future we aim to work with more complex objects.
One possible use of the invisibility cloak could be fighting acoustic pollution?
Of course, in fact, the research carried out at the University of Valencia was focused on that direction. However, the purposes of invisibility are not yet clear, that is why we are focused on the idea of the acoustic lens, which is more important in the short-term. We divert sound waves to gather them in a specific point and then destroy them, that is, transform them into heat. It would be a way of «absorbing» pollution.
Would it be useful in airports?
The problem with acoustic pollution is that waves spread very quickly, so we have to find specific areas where this tool can be useful and in addition, the human inner ear senses many frequencies at the same time. In an airport, sound is not concentrated at one point, and you can’t set up acoustic lenses just everywhere. Maybe in the future we can think about a structure to achieve this goal, but for the moment we are focused on experimenting with specific points, like air conditioning and extraction systems.