Theodore F. Argo IV (left) and Matthew D. Guild (right).
Journal articles written by The University of Texas at Austin graduate students Matthew D. Guild and Theodore F. Argo IV reached the positions of first and second most downloaded papers in The Journal of the Acoustical Society of America last April. Both are Mechanical Engineering Ph.D. students in the Acoustics program conducting research at the Applied Research Laboratory, which primarily studies aquatic acoustics for military applications related to submarines and sonar. Guild's two doctoral supervisors are Assistant Professor Andrea Alù (ECE) and Lecturer Michael Haberman (Mechanical Engineering and Applied Research Laboratories), and Argo's supervisor is Mechanical Engineering's Associate Professor Preston Wilson.
Matthew Guild: Cancellation of Acoustic Scattering
These figures, supplied by Matt Guild, illustrate the total pressure field for an aluminum sphere in water when uncloaked (top) and cloaked (bottom).
The first most-downloaded paper for April 2011, "Cancellation of acoustic scattering from an elastic sphere," lead-authored by Matthew Guild, explored a way to "cloak" an underwater object from incoming sound waves, making them undetectable by sonar.
Cloaking
The idea of cloaking, making an object partially or wholly invisible, has been around for decades and depicted in various popular fictional works such as Star Trek and Harry Potter. In recent years, the topic has received significant attention from the scientific community, beginning with theoretical works describing how waves could be effectively "bent" around an object, thereby cloaking it. Due to the mathematical technique used to determine the cloaking properties, this is commonly referred to as a "coordinate transformation-based cloak," or simply "transformation-based cloak." Although cloaking has traditionally been thought of as manipulation of the electromagnetic spectrum (visible light, infrared, radio, etc.), it can also be applied to waves of sound.
Scattering Cancellation
More recently, another concept of cloaking has been developed in which the waves used for detection (either light or sound) interact with an object rather than being bent around it. By surrounding an object with layers of material(s) with certain physical properties, the different scattering modes of the object (ways that it scatters incoming waves) could systematically be cancelled, a technique known as "scattering cancellation." This essentially means that, rather than bouncing back in multiple directions as would normally happen, the light or sound waves that hit the object behave as though they had passed through it (as in the above image). This type of cloaking, unlike transformation-based cloaking, has the significant advantage of allowing the waves to interact with the interior of the object, meaning that someone or something inside the cloaking field can observe the outside without being seen or heard. The produced effect is called a "plasmonic cloak," a method originally developed by one of Guild's advisors, Andrea Alù of The University of Texas at Austin's Department of Electrical and Computer Engineering.
Guild's Research
Alù's work deals with electromagnetic waves, but the same concept of scattering cancellation can be applied to waves of sound and used to cloak an object from acoustical methods of detection like sonar. The name "plasmonic cloak" comes from the materials used, called "plasmonic metamaterials," to create the effect, called "metamaterial cloaking." Although metamaterials are necessary to achieve scattering cancellation with light, sound waves are much easier to manipulate in the same way and do not require the same complex materials. This is because, to produce a scattering cancellation effect, the properties and thickness of the applied layers of material(s) must counteract the properties and thickness of the object being cloaked so that the complete contraption "feels" the same to the light or sound waves. In the case of sound, this means that if the object is softer and lighter than the medium, then the cloaking layer must be something harder and heavier than the surrounding medium and vice versa. For example, if one wanted to cloak a metal sphere (heavier than water) from sonar, the cloaking layer would have to be something lighter than water such as air or foam.
While it is easy to apply layers that "feel" harder or softer to sound waves, it is very difficult to create a material that "feels" softer to light than the object to be cloaked; such materials must have what is called a refractive index of less than one (the materials used in plasmonic cloaking actually have a negative refractive index), whereas all known naturally occurring materials have refractive indices greater than one. However, since the movement of sound only depends on the weight and density of the materials it travels through and there are many materials more or less dense than water, an acoustical plasmonic cloak in water can be created using common, natural materials such as metals, plastics, ceramics, and foam.
One of Guild's composites made of metal and ceramic powders in epoxy that will sonically cloak enclosed air when submerged in water.
In their paper "Cancellation of acoustic scattering from an elastic sphere," Guild et al. described the theoretical formulation of how the concept of metamaterial cloaking could be applied to waves of sound, as well as their study of what sort of material properties would be needed to achieve it. Through numerous studies they were able to show that the scattered field of sound waves can be significantly reduced, even when using relatively simple configurations of cloaking material layers. In their paper, Guild et al. focused on a single layer of cloaking materials for simplicity, but this limited the frequency range that could be cloaked—the higher the frequency of the incoming sound waves, the more scattering modes contribute to the scattered field of sound waves. This creates a wider range of frequencies to be cancelled, and each cloaking material can only cancel a finite range of frequencies. However, the concept of acoustic scattering cancellation has since been expanded to include multiple layers which allows for the reduction of a scattered field over a wider range of frequencies. Guild is currently conducting experiments based on this new work in the hopes of demonstrating that this method is much more capable and advantageous than the alternative method, transformation-based cloaking.
Guild also hopes to eventually look into applying the same technique to objects surrounded by air rather than water.
Theodore Argo: Sound Speed in Water-Saturated Glass Beads
The second most-downloaded paper for April 2011, lead-authored by Theodore Argo and coauthored by Matthew Guild, was titled "Sound speed in water-saturated glass beads as a function of frequency and porosity." The study sought to further understanding of the ways in which sound waves travel through a sediment in water depending on the frequency of the sound waves and porosity of the sediment.
Acoustical Oceanography
Aerial view of a large amount of sea floor sediment in the Gulf of Mexico churned up by Tropical Storm Ida. In view is the entire coast of Texas (Houston circled in red).
The way sound waves interact with sediment is important in today's world. In industry, for example, oil and gas exploration companies work to map mineral deposits under the ocean floor, and fisheries use sound waves pointed toward the ocean floor to determine the presence of fish. In addition, objects such as shipwrecks or explosive mines can be buried deep under the ocean floor. Since it is impractical to physically explore every point and depth of the ocean, engineers use sonar to locate objects and materials of interest. Knowledge of the sound speed and attenuation (reduction of volume) of sound as it passes through the ocean bottom is necessary to understand what you are seeing far below the surface using sonar.
To accurately locate objects buried underneath water and sediment, predictions of the sound speed and attenuation need to be made using knowledge of the type of sediment in that area. There are multiple models describing how sound travels through sand at the bottom of the ocean, but none of them can accurately describe both the speed of sound and the attenuation of sound as it travels through this medium. All of these models have a number of different physical properties that they have to take into account (e.g. the density of sand grains and the viscosity of water) to make their predictions, but there is not a good understanding of what effect each of these individual properties has on the sound propagation.
Argo's Research
The fluidized bed used to measure the speed of sound through a water-saturated sediment.
In order to better understand the specific effects of individual factors in sound travel through water and a sediment, Argo et al. set out to test the effect of the porosity (the amount of space for water in between grains) of the sand upon the speed of sound through the medium. In the past, researchers have not able to test this property directly since they have no way to control how tightly packed the grains are and, instead, they have had to use the single porosity naturally occurring in their sand sample. To overcome this, Argo et al. used a device called a fluidized bed, developed for unrelated experiments at The University of Texas at Austin's Physics Department by two of the coauthors, to perform tests on tiny glass beads which simulate a sand sediment. The glass beads used, roughly the size of sand grains, were put into the fluidized bed and adjusted for porosity by pushing water through the system. Once the beads were settled, the authors were able to perform acoustic measurements using ultrasonic transducers stuck to the outside of the fluidized bed.
After comparing their data to a model for the speed of sound through ocean sediments, Argo et al. found that the speed of sound measured in the fluidized bed matched the speed of sound predicted by the model, verifying that it accurately accounted for the effects of porosity independently of other factors. However, they also found that the speed of sound decreases as frequency increases, an occurrence that is not accounted for by the model they used. In their next steps forward, the laboratory is looking into measuring the speed of sound through different sized beads and through natural sand using the fluidized bed.
