1. Current subs use 1-inch-thick rubber foam to reduce sonar detection. Materials scientists at Université Paris-Diderot, the University of Manitoba, and PSL Research University are working on a technique to create a much thinner sheet of rubber populated by thousands of bubbles that work to deflect sonar while saving on weight over the bulky foam. Lab tests have shown that the material cuts down on radar wave detection by 10,000 times.
In practice, a 4-millimeter film of bubble material could dampen a sonar signal by as much as 99 percent. A 0.16-inch-thick (4 millimeters) film with 0.08-inch (2 millimeters) bubbles could absorb more than 99 percent of the energy from sonar, cutting down reflected sound waves by more than 10,000-fold, or about 100 times better than was previously assumed possible.
In underwater experiments, the scientists bombarded a meta-screen placed on a slab of steel with ultrasonic frequencies of sound. They found that the meta-screen dissipated more than 91 percent of the incoming sound energy and reflected less than 3 percent of the sound energy. For comparison, the bare steel block reflected 88 percent of the sound energy.
There is the challenge of being able to economically produce durable bubble material to cover an entire submarine.
A bubble metascreen, i.e., a single layer of gas inclusions in a soft solid, can be modeled as an acoustic open resonator, whose behavior is well captured by a simple analytical expression. We show that by tuning the parameters of the metascreen, acoustic superabsorption can be achieved over a broad frequency range, which is confirmed by finite element simulations and experiments. Bubble metascreens can thus be used as ultrathin coatings for turning acoustic reflectors into perfect absorbers.
2. Imagine a material that wicks sound across its surface like water droplets sliding over a windowpane. For submarines, such a coating would mean an entirely new way to slip past sonar without detection as sound waves pass harmlessly around the vessel.
Physicist Baile Zhang and his colleagues at Nanyang Technological University in Singapore think they may have found a way to design such a coating, which could work for any 3D shape—sharp corners included.
Sound waves like sonar hit his proposed coating, they strike an acoustically tuned material called a phononic crystal. That crystal bends the waves so that when they bounce off the hull, they loops around—smacking right back onto the surface to bounce over and over again. Zhang likens the process to a professional soccer player curving the ball.
The manipulation of acoustic wave propagation in fluids has numerous applications, including some in everyday life. Acoustic technologies frequently develop in tandem with optics, using shared concepts such as waveguiding and metamedia. It is thus noteworthy that an entirely novel class of electromagnetic waves, known as “topological edge states,” has recently been demonstrated. These are inspired by the electronic edge states occurring in topological insulators, and possess a striking and technologically promising property: the ability to travel in a single direction along a surface without backscattering, regardless of the existence of defects or disorder. Here, we develop an analogous theory of topological fluid acoustics, and propose a scheme for realizing topological edge states in an acoustic structure containing circulating fluids. The phenomenon of disorder-free one-way sound propagation, which does not occur in ordinary acoustic devices, may have novel applications for acoustic isolators, modulators, and transducers.
A two-dimensional acoustic topological insulator and its band structure. (a) Triangular acoustic lattice with lattice constant a. a=0.2 m in the following calculation. Inset: unit cell containing a central metal rod of radius r1=0.2a, surrounded by an anticlockwise circulating fluid flow (flow direction indicated by red arrows) in a cylinder region of radius r2=0.4a. (b) Band structures of the acoustic lattice without the circulating fluid flow