Sound scatters from most surfaces and creates echoes that can be distracting to listeners and dangerous if you happen to be in a submarine trying to evade detection. Now, researchers have proposed a new “acoustic topological insulator” that could help alleviate such problems by transmitting sound in certain directions without any backscattering. If the material can be built in the lab, it could herald the development of new acoustic devices that have a range of medical and military uses including improving hearing aids and making objects invisible to sonar.

Topological insulators are materials that do not conduct electricity through their bulk volume but are very good conductors on their surfaces. This is possible because of the existence of special “edge states” in which electrons cannot backscatter for topological reasons. Recently, several research groups have been looking at how an acoustic version of a topological insulator – whereby sound waves will travel on the surface of a material but not through its bulk – could be made from a periodic acoustic medium called a “phononic crystal”.

Spinning air

Now, a team led by Baile Zhang at Nanyang Technological University in Singapore has unveiled a new design for an acoustic topological insulator made from a regular array of spinning cylinders.

The design is based on a triangular lattice with unit cells 20 cm in size. Each unit cell contains a rigid solid cylinder at its centre that is spinning at 400 revolutions per second. Each cylinder is surrounded by a concentric shell that is transparent to sound. The rotation of the cylinders causes the air in each shell to rotate, while the remainder of the lattice is filled with stationary air.

Calculations done by the team suggest that sound waves at frequencies between 914–1029 Hz will be guided around the edges of the lattice. Furthermore, the waves move with ease across any defects, disorders, sharp corners or protrusions on the edges of the lattice. This is the same behaviour seen in the electric conductivity on the surface of a topological insulator.

“This structure can guide acoustic waves around its surfaces smoothly without reflection, even in the presence of defects or disorders,” explains Zhang.

One-way travel

Another important feature of these “acoustic edge states” is that sound will only propagate in one direction. This direction depends on whether the cylinders are rotating clockwise or anticlockwise, and therefore can be reversed by altering the rotation. The calculations also suggest that such lattices could be tuned to offer this unidirectional, reflection-free propagation across a range of audible and ultrasonic frequencies.

Zhang told physicsworld.com that the phononic crystals could be used to improve hearing aids by creating systems that are very efficient at channelling sound through the ear canal. He also believes that the technology could be used to create acoustic “invisibility cloaks” that would guide sonar sound waves around the surface of objects such as submarines, thereby hiding them from detection.

“This work constitutes credible proof of the principle of acoustic insulators,” says Thomas Brunet of the University of Bordeaux in France who was not involved in the study. José Sánchez-Dehesa of the Polytechnic University of Valencia in Spain adds that “The challenge now is making this theoretical proposal feasible in a simple and cheap manner.”

This research is described in Physical Review Letters.