This Sound Breakthrough Could Lead to New Class of Acoustic Devices
By LIDA TUNESI
By using ideas from the studies of symmetry and topology, researchers at the Advanced Science Research Center at The Graduate Center, CUNY, have shown how they can get sound waves to travel in one direction along the edges of an object, even if the object has defects.
This physics breakthrough will help create cheaper, lighter acoustic devices that use less battery power and can function in harsh environments.
Andrea Alù, founding director of the Photonics Initiative, and postdoctoral research associate Xiang Ni were authors on the study, published in Science Advances.
The field of topology looks at how certain properties of an object don’t change when the object is continuously deformed. In their new study, the researchers created a topological insulator—an object that allows electrical current or sound waves to flow along its boundaries even if the object has imperfections.
Alù and Professor Alexander Khanikaev created another topological object about a year and a half ago. In their 3D-printed, triangular metamaterial, sound waves could travel along edges and around corners, but a problem remained: The waves could travel either forward or backward. This is a problem because reflections from defects can interfere with the signal itself and affect the sound purity.
Their new design uses time-reversal symmetry breaking, rather than the geometrical asymmetries used before, to create topological order. This solves the problem, restricting the waves to travel in one direction only and avoiding reflections. The design also leaves room for programmability: They can guide the waves along different paths, with minimal loss.
The device consists of circular piezoelectric resonators arranged in hexagons and bonded to a thin disk of polylactic acid. This is connected to external circuits which send out a time-modulated signal to break time-reversal symmetry.
Ultrasound imaging, sonar, and electronic systems that use surface acoustic wave technology could all benefit from this work, Alù said.