A new spin on guiding sound waves along a one-way route

An array of miniature turntables could offer a powerful new way to control the flow of sound.

The proposed device, reported in the March 20 Physical Review Letters, would channel sound waves in a protected one-way thoroughfare along its edge. The structure is an acoustic version of a hotly researched class of materials called topological insulators. As the name suggests, these materials are primarily insulators, yet they efficiently transport electrons along their periphery.

The new study “shows that this rather exotic physical property can emerge in something as familiar as sound propagation,” says Steven Cummer, an electrical engineer at Duke University who was not involved in the work.

For now, the design requires many moving parts and manipulates only a narrow swath of sound frequencies. But a similar structure could eventually allow engineers to redirect loud noises out of a room or perhaps to cloak submarines from sonar.

Topological insulators have exotic split personalities ( SN: 5/22/10, p. 22 ). They both insulate, blocking the movement of electrons, and conduct, moving electrons but only in special thoroughfares along the materials’ edge. All electrons with a particular spin move in the same direction, and nothing can take them off course. As they cruise along this one-way road, electrons scale bumps and leap potholes because the material’s properties don’t allow the particles to bounce or scatter backward — they can only move forward.

Recently, scientists have expanded the scope of topological insulators by creating materials that steer light instead of electrons (SN: 5/18/13, p. 8). Physicist Baile Zhang and his colleagues at Nanyang Technological University in Singapore took things a step further and designed an analogous device for manipulating sound. Their proposed structure is a lattice of metal rods, each encased in a cylindrical shell. The rods are sized and spaced to interfere with sound waves and confine the waves to the material’s edge, setting up the dual insulator-conductor behavior.

The trickiest part was thinking of a way to channel waves in only one direction. Finding inspiration in an experiment from last year, Zhang and colleagues propose spinning the metal rods like turntables to create swirling pockets of air within each shell. The counterclockwise motion of the air should force sound waves into a counterclockwise path along the edge of the device, the researchers hypothesized. Zhang suggests using small motors like those used to turn the rotors of toy helicopters to spin the metal rods.

Computer simulations confirmed that a network of hundreds of centimeter-wide spinning rods would confine audible sound waves to a narrow one-way path along the boundary of the material. And the simulated sound waves remained on course without scattering even after encountering imperfections and sharp turns. “Sound is forced to propagate along the boundary, even if you do rather tortuous things to the boundary” like making it jagged or bumpy, Cummer says.

The design has drawbacks. The device manipulates only a narrow range of frequencies, Cummer notes. And Eugene Mele, a University of Pennsylvania theoretical physicist who coauthored a pivotal paper on topological insulators in 2005, is skeptical about the practicality of so many spinning cylinders. “It’s a bit of a Rube Goldberg contraption,” he says.

An eventual prototype may not resemble Zhang’s design exactly, but it could find plenty of uses. Cummer envisions topological acoustic paneling on a wall to capture sound from a noisy generator and prevent it from reaching the rest of the room. Zhang says that a similar structure could cloak submarines by preventing sonar pulses from bouncing back, though University of Texas at Austin engineer Andrea Alù says it’s difficult to steer sound around a curved, free-floating object.

Zhang hopes to experimentally demonstrate his team’s design within a few months.