A new metamaterial that transports sound along its edges and localizes it at its corners, could yield improved sonar, ultrasound devices and other technologies that use sound waves.
The material—which was developed by researchers from the Advanced Science Research Center (ASRC) at The Graduate Center of The City University of New York and at the City College of New York (CCNY)— features a robust acoustic structure that controls in unusual ways the propagation and localization of sound, even when there are fabrication imperfections.
The team developed the material using topology, a mathematical field that involves studying the properties of an object that are not affected by continuous deformations.
The researchers utilized these principals to predict and eventually discover topological insulators—materials that conduct electric currents only on their edges and not in the bulk. These properties are caused by the topology of their electronic band gap, making these materials unusually resistant to continuous changes like disorder, noise or imperfections.
“There has been a lot of interest in trying to extend these ideas from electric currents to other types of signal transport, in particular to the fields of topological photonics and topological acoustics,” Andrea Alù, director of the ASRC’s Photonics Initiative, said in a statement. “What we are doing is building special acoustic materials that can guide and localize sound in very unusual ways.”
The researchers 3D printed a series of small trimers, comprised of three acoustic resonators that were arranged and connected in a triangular lattice. The rotational symmetry of the trimers and the generalized chiral symmetry of the lattice gave the structure the unique acoustic properties desired.
The acoustic modes of the resonators were hybridized to give rise to an acoustic band structure for the entire object, enabling sound played at frequencies outside of the band gap to propagate through the bulk of the material.
However, when sound is played at frequencies inside of the band gap, the sound only travels along the triangle’s edges or are localized at its corner, a property that is not impacted by disorder or fabrication errors.
“You could completely remove a corner, and whatever is left will form the lattice’s new corner and it will still work in a similar way, because of the robustness of these properties,” Alù said.
After reducing the symmetry of the material by changing the coupling between resonator units, the researchers were able to break these properties and change the topology of the band structure.
“We have been the first to build a topological metamaterial for sound supporting different forms of topological localization, along its edges and at its corners,” Alexander Khanikaev, a professor in the electrical engineering and physics departments at CCNY who is also affiliated with the ASRC, said in a statement.
“We also demonstrated that advanced fabrication techniques based on 3D printed acoustic elements can realize geometries of arbitrary complexity in a simple and flexible platform, opening disruptive opportunities in the field of acoustic materials,” he added. “We have been recently working on even more complex 3D metamaterial designs based on these techniques, which will further expand the properties of acoustic materials and expand capabilities of acoustic devices.”
The study was published in Nature Materials.
