METAMATERIASL FOR ACOUSTIC CLOAKING post by Nick Meligari

Since World War II, the world’s prominent superpowers have used Sonar technology to detect and track the whereabouts and locations of their enemies at sea. This technology uses sound waves emitted at specific frequencies (an increasingly widening range of frequencies in today’s systems) to detect structures that, when impacted by the signal, cause a sort of ‘rebound’ and send their own sonic waves echoing back to the sonar receiver. These ‘pings’ allow the Sonar-wielding vessel to gain a relatively clear idea about the location of the vessel being ‘pinged’ (an ability that has proved to be tactically advantageous in times of conflict).

The ability to decrease the ‘rebound’ amplitude, or sonic signature, of sound waves that a structure emits during this ‘pinging’ process has the potential to render a maritime opponent blind. This is due to the simple fact that the rebound signal received by the opponent’s sonar will not be interpreted as that of an enemy submersible or weapon, but instead, due to its significant reduction, as something much smaller or less significant, such as a sea creature or even acoustic noise. This is where research being done by a small group of engineers at an engineering consulting firm called Weidlinger Associates Inc. comes in- they hope to create a practical method of reducing this acoustic rebound signature.

Led by Dr. Jeffrey Cipolla, the project involves developing a method of signature reduction based on the use of what he calls ‘Pentamode’ Materials. These materials are nothing new; they’ve been around in R&D environments for over 10 years. It is the analytical methods and proprietary software that he and his team are applying to the design of these materials that are leading to the recent progress in the field. These Pentamode materials are unique in the fact that the localized stiffness and density of the material at specific points can be manipulated and changed with respect to the material properties at another point in the same material. Since material stiffness and material density are the two primary factors influencing the rate at which sound waves travel through a medium, they are critical properties to be able to manipulate if you are interested in controlling the path of the sonic waves.

By changing local stiffness and local density at specific points throughout the material, the team hopes to actually propagate and accelerate the sound waves passing through it- similar to how water propagates sound waves. By accelerating the waves through the medium, it will eliminate the usual reduction in wave velocity that occurs when the sound waves transition from a liquid medium (seawater) to a solid one (the structure of a submersible/weapon, etc.). By eliminating the reduction in wave velocity that occurs at this boundary you, in practice, eliminate (or at least significantly reduce the intensity of) the rebound signature that is then perceived and interpreted by the opponent vehicle’s Sonar system. Think traffic jam- when it clears up, the constant acceleration of the pack of cars keeps everyone’s foot off the brake, so any slight changes in acceleration magnitude are harder to track as they echo down the line. Before the jam clears up however, while the roadway is still constricted, every time a brake is applied by one of the cars the resulting start and stop can be witnessed reverberating and echoing down the road, right to the last car. So that last driver, though experiencing a temporally distorted version of the events taking place, knows that someone stepped on the breaks all the way up front.

The first thing that comes to mind is to apply these methods to common acoustical engineering problems like noise reduction in cars and aircraft as well as naval vessels and military vehicles. The Pentamode materials that Dr. Cipolla talks about using in his research are made of aluminum, so the stiffness and weight makes it well-suited for these kinds of mobile applications. Concert halls and auditoriums could benefit from the ability to redirect and manipulate sound impacting the structure as well. Cool!

For more info on the development of these metamaterials, check out Dr. Cipolla’s paper on the matter: http://asadl.org/jasa/resource/1/jasman/v128/i4/p2375_s3?bypassSSO=1