Altering Sound Waves Could Make Objects Invisible to Sensors
Matthew Greenwood posted on July 12, 2018 |

Anyone who's had difficulty in getting smartphone signal reception inside buildings knows how difficult it is for electromagnetic waves to travel through solid obstacles without signal degradation or failure. The reality is that most naturally-occurring materials have a disordered atomic structure that interferes with sound and electromagnetic waves. When waves come into contact with these materials, they bounce and disperse, losing energy and signal clarity.

From two labs at the Ecole Polytechnique Federale de Lausanne (EPFL) and from the University of Crete, a team of researchers has developed a system that allows sound waves to travel across naturally occurring materials while retaining intensity and signal strength.

The system uses tiny speakers as acoustic relays that offset the wave scattering. The speakers can be controlled to amplify, attenuate or shift the phase of the sound waves to offset the diffusion that occurs when the waves hit obstacles. As a result, the speakers can reproduce the sound exactly on the other side of the obstacle.

“We realized that our acoustic relays had to be able to change the waves’ amplitudes and phases at strategic locations,” said Romain Fleury, head of EPFL’s Laboratory of Wave Engineering and co-author of the study recently published in Nature Physics.

The researchers tested their system by creating an air-filled tube with various kinds of obstacles such as walls, porous materials and chicanes; this created a highly disorganized medium through which no sound waves could pass cleanly. They then placed the speakers between the obstacles. The speakers feature programmable circuits that are controlled electronically to alter their acoustic properties.

“We had to develop a new control mechanism so we could also amplify [sound waves], like how we can already amplify optical waves with lasers,” said Etienne Rivet, co-author of the study.

The system allows sound waves to maintain their integrity regardless of what obstacle lies in their path—meaning that it could eventually be used to hide objects such as submarines by eliminating the sounds that bounce off of them. Conversely, it could also be used to reveal details inside opaque materials, which would be useful in fields such as medical diagnostics.

Read more about using engineering technologies to manipulate sound waves at Origami Lattice Helps Quell Traffic Noise.

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