Sep 23, 2015 | By Benedict

If you’re a naval captain using sonar, a gynaecologist conducting ultrasounds, or just a dolphin, you’ll know that reflected sound waves have long been used to construct images of objects. Sound waves are reflected off solid objects in particular ways, and those reflected waves can be measured to identify the shape and location of those objects. Now, scientists from the Swiss Federal Institute of Technology (ETH), Zurich, have used 3D printing to develop a new method to differentiate very weak and short sound waves from longer ones. 

The effect of this development is significant: whilst sonar has been used as a technological device for over a hundred years, with its use increasingly rapidly following the Titanic disaster of 1912, typical acoustic imaging techniques often fail to reconstruct a precise picture of an object’s edge. Sound waves nearest to the edges of an object are predominantly ‘evanescent waves’, which have a much shorter wavelength than the incident sound waves that produce them, and since the evanescent waves decay very fast as they propagate, they can only be measured in close proximity to the edge of the object. When used in acoustic imaging, the new 3D-printed technology makes it possible to detect only the outline of objects, by amplifying the evanescent waves and filtering out the others.

“This type of measuring method delivers similar results to the edge detection filter in an image-processing software, which allows the outline of prominent photo objects to be identified with the click of the mouse,” explains Chiara Daraio, professor of mechanics and materials at ETH Zurich and one half of the research team behind the project. The secret to this handy selective hearing can be found in the structure of the wave-measuring device.

The 3D printed instrument, which enables amplification of evanescent waves, is pipe-shaped polymer structure with a square cross-section, the inside of which is divided into five adjoining resonance chambers connected via small windows. “The resonance achieved by this structure intensifies the evanescent waves, and the successive chambers filter out the longer waves,” explains Miguel Molerón, the other half of the ETH research duo. At the head of the 3D-printed structure, four microphones measure the transmitted sound.

To create an outline image of an object, the scientists bounced sound with a specific frequency off the object through a loudspeaker. They attached the 3D printed polymer structure, along with microphones, to a robot close to the object’s surface, which enabled them to systematically scan the entire surface and generate the outline image from the measured sound data. Without the 3D printed device to amplify evanescent sound waves and minimise other noise, a less precise image would have been obtained.

Images from ETH Zurich

Molerón and Daraio believe that their tool’s greatest asset is its speed, rather than its accuracy. “We have created an acoustic imaging method with which any unnecessary information isn’t recorded,” says Daraio.

“Outlines and edges are sufficient to classify objects based on their shape and size, for example, or to identify fissures or defects on the surface of materials,” adds Molerón. Although the two researchers are happy with their progress, the work is currently just a proof of concept, and the method needs to be refined before it can be applied in practice. “Because the size of the polymer structure has to be adjusted to the operational wavelength, we need to miniaturize the structure. We now want to find out how far we can go with it,” says Molerón. We wish them the best of luck. Their article, ‘Acoustic metamaterial for subwavelength edge detection’, can be found here.

 

 

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