Deep learning and metamaterials helped scientists make sound visible

Visualization allows you to depict an object by analyzing the far field of light and sound waves,

which it transmits or emits.The shorter the wavelength, the higher the image resolution. However, so far the level of detail is limited by the size of the wavelength in question. Researchers from EPFL's Wave Engineering Laboratory have successfully proven that a long and therefore imprecise wave (in this case a sound wave) can reveal features that are 30 times smaller than its length. To do this, the research team used a combination of metamaterials—specially designed elements—and artificial intelligence. Their research opens up exciting new possibilities, particularly in the fields of medical imaging and bioengineering.

The team's pioneering idea was tocombine two separate technologies that previously separated the boundaries of rendering. One of these is the technology of metamaterials: specially created elements that can, for example, focus wavelengths precisely. However, they lose their effectiveness due to accidental absorption of signals, which makes them difficult to decipher. Another technology is artificial intelligence or, more specifically, neural networks that can process even the most complex information quickly and efficiently, although this requires training.

To exceed the diffraction limit (minimumthe value of the spot size, which can be obtained by focusing electromagnetic radiation) the research team conducted the following experiment. First, they created an array of 64 miniature speakers, each of which can be activated according to the pixels in the image. They then used a lattice to reproduce sound images of numbers from zero to nine with precise spatial detail. The pictures of the numbers entered in the lattice were taken from a database of about 70,000 handwritten examples. Opposite the lattice, the researchers placed a bag with 39 Helmholtz resonators (spheres 10 cm in diameter with a hole at one end), which formed the metamaterial. The sound produced by the grating was transmitted by the metamaterial and picked up by four microphones located several meters away. Algorithms then decoded the sound recorded by the microphones to learn how to recognize and redraw the original digital images.

The team achieved almost 90% success in their experiment.

In medical imaging, the use of long wavelengths to observe very small objects could be a big breakthrough.

Long waves mean doctors canuse much lower frequencies, with the result that acoustic imaging techniques are effective even through dense bone tissue. When it comes to imaging using electromagnetic waves, long waves are less hazardous to the patient's health. In applications, we will not train neural networks to recognize or reproduce numbers, but rather organic structures.

Romain Fleury, Head of Research Group at EPFL

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