A New Acoustic Insulator Hides Sound Better than Ever Before
Good news for nuclear submarines and those that live by airports.
Image: UK Ministry of Defence
There are few things as deeply creepy as a nuclear submarine. This is a tool that exists almost entirely as a mobile array of missile silos, engineered so that it's as silent as a heartbeat and capable of staying underwater for decades at a time. The millisecond demise of the United States eastern metropolis in a "hurricane of fire" might exist underwater, just miles away undetectable in Earth's best hiding place: the ocean.
The quest for a quieter submarine is a sort of shadow arms race. The current state of the art might be the UK's Astute class of nuclear submarine, the result of an intensive computer modeling of every possible sound a sub might make, no matter how diminutive it might seem to us surface dwellers. Engineers are just getting started with submarine stealth, however, and this week saw the unveiling of a new "acoustic topological insulator" that promises to make subs still even more silent and ghostly.
The technology, which is described in the current Physical Review Letters, comes courtesy of a research group at Nanyang Technological University led by electrical engineering professor Baile Zhang. It turns out to be not so exotic, the result of a lattice of spinning metal cylinders. The idea is an extension of the topological insulators more commonly explored in electronics, where electricity is conducted only along the surface of a material, offering a strange and very useful resilience to material defects.
"One example of a topological insulator is a two-dimensional material with a strong magnetic field applied along the 3rd dimension," Philip Ball explains in an American Physical Society summary. "Under the right conditions, in response to the field the electrons move in circles arranged in a periodic pattern. But at the edges of the material, electrons can only execute half-circles. These semicircular routes can link up to form a circuit around the edge, a path that electrons can travel in only one direction (because of the direction of their orbits). In addition, current through this channel is resistant to the scattering that would usually occur if there were any flaws present in the crystal lattice, because the one-way motion prevents all backward scattering."
Zhang and his team wanted to see if this idea could extend to sound waves. They found that the same insulation/conductivity effects, "can be achieved in a 'meta-atom' containing a ring of circulating fluid," Zhang and co. write. So, imagine the nucleus of an atom within the lattice of a topological insulator is a macroscale spinning metal cylinder. Surrounding it, taking the place of electron orbits, is a ring of circulating air or water or really any fluid. This ring is then encased by an outer shell, which is itself surrounded by a stationary, non-circulating fluid medium.
Acoustic waves are prevented from traveling through such a lattice for the same reason that electrical waves are: instead of scooting signals around the material, they offer instead closed rings. The acoustic wave can never get a foothold.
At the surface of the acoustic insulator, the rings are allowed to link up and acoustic waves are allowed to propagate, but only along this surface layer. They're shuttled away from the stealth object (the sub), without ever penetrating deeper.
"Our proposed system should be quite practical to realize," Zhang and his group write. "Acoustic devices based on these topological properties can be useful for invisibility from sonar detection, one-way signal processing regardless of disorders, and environmental noise control, which will greatly broaden our interest in military, medical, and industrial applications."