Sleep better thanks to noise cloak

Spanish scientists have designed a cloak of sound – the trick could, for example, solve the snoring problem in many marital beds

For a long time now, camouflage coats of arms have made the leap from the world of myths to reality. The possibility of making oneself invisible from the surrounding environment apparently appeals to both researchers and those who devise applications for it. So far, however, science has focused on electromagnetic waves, especially light (see I see that you do not see me). However, there are other wave anomalies that are accessible to our perception. We know this from the movies: The invisible hero steps on a dry branch, and the great camouflage is gone, the hiding place is blown, the hero becomes at least temporarily the pursued one.

But meta-materials, which make such a game of hide-and-seek possible in the first place, are fortunately not only available for light. They can also be constructed for sound waves – as a reminder: these non-natural "materials" are actually the smallest technical works of art, which make the effect, an overall negative refractive index, possible only in the sum. They simply refract waves in the wrong direction and are therefore suitable for exciting technical gadgets.

However, the sense of hearing differs from the sense of sight in one small detail: it is based primarily on the interception of sound waves emitted by an object. We can only see an object (which is not luminous) if it reflects light waves. And this is exactly what the concepts for optical cloaks, already presented by researchers, protect against.

In acoustics the problem is much more complicated. A cloaking device could be used to prevent an object from reflecting sound waves. The cloak is powerless against the sound waves emitted by the object. The movie hero who steps on a twig will continue to betray himself to the minions of the evil one. Nevertheless, the technology is also interesting for practical application. First of all for the military: warships hunt submarines with active sonar, torpedoes sight their targets by sonar. If the sound reflection on the submarine could be prevented, the locating screens of the search ships would remain empty.

In the civilian field, however, the acoustic cloak could be used for armor protection by causing the sound waves to be directed around the object to be protected. A house in the flight path of the airport, in which it is nevertheless heavenly quiet, an annoyed sleeper who no longer has to listen to his snoring partner, a concert-goer who is not distracted by the chattering neighbor… However, it will be a long time before such applications become reality.

That’s because the concept, now presented in the New Journal of Physics by Spanish researchers Daniel Torrent and Jose Sanchez-Dehesa of the University of Valencia, still requires about 200 layers of the meta-material to achieve optimal camouflage.

It is based on so-called sonic crystals, which have little to do with crystals in the solid-body physical sense. Instead, they consist of asymmetrical arrays of sound wave scattering cylinders made of two different materials. In this way, the mass anisotropy necessary for an acoustic meta-material is achieved. Compared to optical cloaks, the proposed (as yet untested) technique has the advantage that it is hardly wave-length dependent – above a certain entry frequency, which is determined by the properties of the cylinders, it acts on the complete spectrum. It is also relatively tolerant of manufacturing deviations in the meta-material.

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