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Metamaterials are artificially structured materials designed to control and manipulate physical phenomena such as light and other electromagnetic waves, sound waves and seismic waves in unconventional ways, resulting in exotic behavior that’s not found in nature. They are predicted to be able to protect the building from earthquakes by bending seismic waves around it, Similarly, tsunami waves could be bent around towns, and sound waves could be bent around a room to make it soundproof.
More recently, the metamaterial concept has been extended to acoustic waves in a variety of scenarios of interest such as acoustic clocking, super-lensing and sound focusing and confinement. Acoustic metamaterial is defined as an artificial composite material displaying novel acoustic properties unavailable in naturally occurring materials and radically different from those of any of the constitutive components. The novel acoustic properties derive from closely spaced constituent elements with specifically chosen geometrical and mechanical characteristics.
Acoustic metamaterials are designed to control, direct, and manipulate sound waves. These types of materials are used for noise canceling, sonic cloaking for submarines, acoustic focusing for manipulating small objects, and abnormal acoustic refraction to guide acoustic wave directions. They in particular have shown great promise in the field of sound attenuation. Acoustic metamaterials composed of thin elastic membranes decorated with or augmented with designed patterns of rigid platelets, have been utilized to absorb large percentage of acoustic waves and can be made to create acoustically “dark” materials as well as vibration dampening devices
Some of the military applications of these materials are acoustic isolator (acoustic diode), acoustic circulator, acoustic switch, acoustic cloaking, acoustic sensors and thermal management.
Acoustic Metamaterials
Acoustic waves are a type of longitudinal waves that propagate by means of adiabatic compression and decompression. Longitudinal waves are waves that have the same direction of vibration as their direction of travel. Important quantities for describing acoustic waves are sound pressure, particle velocity, particle displacement and sound intensity. Acoustic waves travel with the speed of sound which depends on the medium they’re passing through. According to the oscillation frequency, acoustic waves have been classified to different fields that cover the audio, ultrasonic and infrasonic frequency range, or seismic waves at much larger scale which are waves of energy travelling through the Earth’s layer.
The material properties of interest for acoustic metamaterials are the effective mass density ρ and the effective bulk modulus κ, which is analogous to a spring’s stiffness. Of particular interest for acoustical applications is the ability to manipulate an acoustic wave’s speed. Because acoustic metamaterials have a broad range of effective properties, they can produce propagating waves with extremely high, zero, or even negative speeds, not to mention the purely imaginary values that correspond to nonpropagating evanescent waves.
Materials with either negative mass density and positive bulk modulus or positive mass density and negative bulk modulus are called single negative. Single-negative materials cannot support propagating waves, so any acoustic wave in those materials will exponentially decay. That makes them superior sound absorbers.
Double-negative acoustic metamaterials enable significantly improved refractive properties through negative refraction. ”They permit us to construct superlenses that amplify evanescent waves and provide subwavelength resolution for acoustic imaging devices. Double-negative materials can also be fabricated into hyperlenses, which use a hyperbolic dispersion relation between frequency and wavenumber, rather than the elliptical dispersion found in traditional anisotropic materials, to produce negative group velocity and a negative index of refraction. Hyperlenses provide a different means to improve the resolution of acoustic imaging devices. They can also be constructed from single-negative anisotropic acoustic metamaterials, but those hyperlenses lack certain advantages arising from negative refraction,” write Michael R. Haberman and Matthew D. Guild in Physics today.
Recent advances in acoustic metamaterials have made it possible to design engineered metamaterials and associated structures for practical applications. Transformation acoustics provides precise control over acoustic wave propagation and this coupled with metamaterials gives unprecedented control in controlling, manipulating and directing sound waves. This coupled with advent of fabrication technology and development of simulation techniques such as finite element method (FEM) and finite difference time domain method (FDTD) have led to a revolution of metamaterials in controlling and manipulating acoustic waves in new ways not previously imagined . For instance, in acoustics, it is now possible to design acoustic lenses for sub-diffraction imaging or design acoustic cloaking which is able to make an object acoustically invisible by bending the waves. Also, an assembly of rubber-coated spheres into a bulk metamaterial can exhibit locally designed resonant structures.
Acoustic Metamaterial applications
Lightweight material in theory translates to a high sound transmission, particularly at low frequencies. Chinese researchers, including Ms. N. Sui and Y. Jing, have designed and experimentally demonstrated a lightweight and yet sound-proof honeycomb acoustic metamaterial. The use of no-mass-attached membrane-type acoustic metamaterials leads to excellent acoustical performance with minimum weight-penalty.
The proposed metamaterials can be used to build the core material of the sandwich structures which are experimentally proven to be significantly more sound-proof particularly at low frequencies with an extremely low weight penalty. The proposed metamaterial is promising for constructing structures that are simultaneously strong, lightweight, and sound-proof, which can be extremely useful for aerospace and other transportation industries.
Recently Researchers from the University of Maryland A. James Clark School of Engineering directed by Miao Yu, have developed a new acoustic metamaterial that dramatically amplifies acoustic signals, more than 10 times past the detection limit of conventional sensors.
They developed a novel metamaterial having “graded refractive index” or GRIN for short, that would compress and amplify a sound wave before detection by a sensor. Unlike other cutting-edge technologies in acoustic amplification, the GRIN material is relatively broadband and highly compact, with a theoretical length of only 2 to 4 wavelengths of the incident wave. Applications of the research could include improving the capabilities of sonar devices and medical imaging that detects cancer.
Prof Katia Bertoldi of Harvard University also studies strange, elastic materials like this, which have a negative “Poisson ratio”. This means that when you compress them, instead of squashing out to the sides and getting both flatter and wider, they actually shrink in all directions.
Then when stretched, they expand in all directions. Prof Bertoldi’s team has engineered various useful properties into such materials, including making them absorb sound at different frequencies when squeezed. The Poisson ratio can also affect fatigue in a metal – so she has worked with Rolls Royce to design engine components with complex slits wound into them, which withstand many more cycles of compression before breaking.
Mechanical engineers have developed an “acoustic metamaterial” that can cancel 94 percent of sound
Boston University researchers, Xin Zhang, a professor at the College of Engineering, and Reza Ghaffarivardavagh, a Ph.D. student in the Department of Mechanical Engineering, released a paper in Physical Review B demonstrating it’s possible to silence noise using an open, ringlike structure, created to mathematically perfect specifications, for cutting out sounds while maintaining airflow.
As a test case, they decided to create a structure that could silence sound from a loudspeaker. Based on their calculations, they modeled the physical dimensions that would most effectively silence noises. Bringing those models to life, they used 3-D printing to materialize an open, noise-canceling structure made of plastic.
“Sound is made by very tiny disturbances in the air. So, our goal is to silence those tiny vibrations,” Ghaffarivardavagh and Zhang say. “If we want the inside of a structure to be open air, then we have to keep in mind that this will be the pathway through which sound travels.” They calculated the dimensions and specifications that the metamaterial would need to have in order to interfere with the transmitted sound waves, preventing sound—but not air—from being radiated through the open structure. The basic premise is that the metamaterial needs to be shaped in such a way that it sends incoming sounds back to where they came from, they say.
By comparing sound levels with and without the metamaterial fastened in place, the team found that they could silence nearly all—94 percent to be exact—of the noise, making the sounds emanating from the loudspeaker imperceptible to the human ear.
Now that their prototype has proved so effective, the researchers have some big ideas about how their acoustic-silencing metamaterial could go to work making the real world quieter. “Drones are a very hot topic,” Zhang says. Companies like Amazon are interested in using drones to deliver goods, she says, and “people are complaining about the potential noise.” “The culprit is the upward-moving fan motion,” Ghaffarivardavagh says. “If we can put sound-silencing open structures beneath the drone fans, we can cancel out the sound radiating toward the ground.”
Ghaffarivardavagh and Zhang also point to the unsightliness of the sound barriers used today to reduce noise pollution from traffic and see room for an aesthetic upgrade. “Our structure is super lightweight, open, and beautiful. Each piece could be used as a tile or brick to scale up and build a sound-canceling, permeable wall,” they say.
The shape of acoustic-silencing metamaterials, based on their method, is also completely customizable, Ghaffarivardavagh says. The outer part doesn’t need to be a round ring shape in order to function. “We can design the outer shape as a cube or hexagon, anything really,” he says. “When we want to create a wall, we will go to a hexagonal shape” that can fit together like an open-air honeycomb structure.
Such walls could help contain many types of noises. Even those from the intense vibrations of an MRI machine, Zhang says. According to Stephan Anderson, a professor of radiology at BU School of Medicine and a coauthor of the study, the acoustic metamaterial could potentially be scaled “to fit inside the central bore of an MRI machine,” shielding patients from the sound during the imaging process.
Acoustic Cloaking
Acentech has completed a three year, $750,000 U.S. Navy research project, which sought to evaluate the possible application of metamaterials, especially those forming an acoustic cloak for underwater naval vehicles. A cloak is a layer surrounding an object that bends incident acoustic wavers around it, and reconstructs the incident waves on the opposite side. A cloaked object could thus be invisible to active sonar at any angle.
Scientists on the project from Pennsylvania State University are designing a metamaterial that allows sound waves to bend around an object as if it’s not there. The unit cell, which is described as the smallest component of the metamaterial, must be smaller than the acoustic wavelength. Lead researcher Amanda D. Hanford says that the math shows the team that the properties are possible. Prior to the new research, most acoustic metamaterials were designed to deflect sound waves in air.
This team took that a step further by trying to do the same thing underwater. Acoustic cloaking underwater is more complex because water is denser and less compressible than air. After multiple attempts to create an underwater acoustic cloak, the team was able to develop a 3-foot tall pyramid that uses perforated steel plates.
When placed on the bottom of an underwater research tank with a hydrophone that makes acoustic waves between 7,000 and 12,000 Hz along with several receivers to monitor the acoustic waves. The wave reflected by the pyramid device matched the wave reflected from the surface. The result was that the material could make an object appear invisible to underwater sonar. The team says that the results show potential for real-world applications. Those applications include acoustic materials to dampen sound and make objects appear invisible underwater.
Duke engineers also built 3-D acoustic cloaking device, from metamaterials under research supported by Multidisciplinary University Research Initiative grants from the Office of Naval Research and Army Research Office. Cummer and his team built a pyramid structure from perforated sheets of plastic, that could alter the trajectory of sound waves striking the structure, so as to appear to have bounced off from a flat surface beneath it.
The acoustic cloaking device works in all three dimensions, no matter which direction the sound is coming from or where the observer is located, and holds potential for future applications such as sonar avoidance and architectural acoustics. The technology could allow any object, for example a submarine, to become inaudible by bending sound waves, such as sonar signals, around the object.
In 2014 researchers created a 3D acoustic cloak from stacked plastic sheets dotted with repeating patterns of holes. The pyramidal geometry of the stack and the hole placement provide the effect.
Prof Wegener works on cloaking, but his aim is not to make things invisible. He wants to hide them from physical forces, and last year his lab produced a honeycomb-like material that made an object beneath it unfeelable. This particular metamaterial was a solid lattice that acts like a fluid in certain ways, deflecting pressure around its hidden cargo.
Now the tiny, hidden cylinder was very small in that case (less than 1mm) but related work by Prof Wegener’s team was picked up by French physicists and engineers, who showed that a careful pattern of drilled holes could divert damaging earthquake vibrations.
But an invisibility cloak needn’t be a sinister tool of war. Metamaterials could also absorb and emit light with extremely high efficiency — for example in a high-resolution ultrasound — or redirect light over a very small distance. This, says Anthony Vicari of Lux Research, “could be used to improve fibre optical communications networks, or even for optical communications within microchips for faster computing.”
Active Metamaterials
A major limitation of current acoustic metamaterials is that their acoustic properties are either locked into place once fabricated or only modestly tunable, tying them to the particular application for which they are designed. Researchers from Department of Electrical and Computer Engineering, Duke University presented a design approach that yields active metamaterials whose physical structure is fixed, yet their local acoustic response can be changed almost arbitrarily and in real-time by configuring the digital electronics that control the metamaterial acoustic properties.
They demonstrated experimentally by designing a metamaterial slab configured to act as a very thin acoustic lens that manipulates differently three identical, consecutive pulses incident on the lens. The slab can be configured to implement simultaneously various roles, such as that of a lens and beam steering device. The slab is suitable for efficient second harmonic acoustic imaging devices capable to overcome the diffraction limit of linear lenses. These advantages demonstrate the versatility of this active metamaterial and highlight its broad applicability, in particular to acoustic imaging.
French scientists are using metamaterial technology to create earthquake “shields” that can deflect acoustic waves like those generated in an earthquake
Designing silent metamaterials
Duke University, alongside MIT, University of California, Berkeley, Rutgers University, and the University of Texas at Austin, forms part of a five-year research program sponsored by the US Office of Naval Research to develop new concepts for acoustic metamaterials with effective material parameters that can be fabricated in the real world. Steve Cummer, professor of electrical and computer engineering at Duke University, said: “Mathematical models are the starting point. The acoustic metamaterial designs are optimized through numerical simulations, which we then translate into modern fabrication techniques and experimentally test.”
One focus of the group’s current research efforts is on developing acoustic metamaterial structures that can be used in water-based environments, including the human body, to arbitrarily transform and control incoming sound waves. Acoustic cloaking structures have proven a useful testbed for demonstrating the arbitrary control enabled by transformation acoustics. Designing for aqueous environments represents a shift in metamaterial research, which has evolved from electromagnetic cloaking and transformation optics, to acoustic cloaking and transformations in 2D and then 3D structures in air.
Cummer explained: “To arbitrarily control sound using transformation acoustics, we first apply a coordinate transformation to describe how you would like to bend or twist or deform the sound field in a particular device. Once you’ve defined that coordinate transformation, then you can derive the effective material parameters you need to create that particular deformation of the sound field.”
Attention has now shifted to getting acoustic metamaterials to work in an aqueous environment, such as underwater or inside the human body. Multiphysics modeling is used as the primary design tool to first map the previously designed structures and run simulations in order to test how they will perform in water.
The problem is that the mechanical properties of air are dramatically different from those of water. Cummer explained: “That’s why in air we can get away with building acoustic metamaterials in plastic, or whatever solid is convenient, as the solid can act essentially as a perfectly rigid structure to control the sound field flow. It doesn’t really matter what it is made of.”
But the mass density and compressional stiffness of water are not so different from solid materials. “When sound waves hit a solid structure in water, the mechanical properties of that solid start to matter a lot. We need to come up with new techniques in the design phase to be able to control how that sound wave energy interacts with the solid so that we can maintain the properties we want,” he added.
The ability to easily merge acoustics and structural mechanics is essential, especially when we’re dealing with structures in water where we can’t ignore the mechanical responses of the solid material that we’re using to build the metamaterial. In airborne acoustics, we can get away with treating the solid as a material that is infinitely rigid, which is easy and computationally efficient, but for the water-based material it is essential to be able to consider fluid-structure interaction.
Researchers use 3D printing to Tune Acoustic Metamaterials on Demand
Researchers have used 3D printing to develop acoustic metamaterials that can be tuned to different frequencies—something that is a rare quality in these types of structures and opens the door to new applications of them. The metamaterials, which are capable of blocking sound waves and mechanical vibrations, were developed by a team at USC Viterbi led by Assistant Professor Qiming Wang. They also can be remotely switched between active control and passive states using a magnetic field.
Typically, they have limitations in the flexibility of their applications, however, which Wang said he and his team have solved with their new materials. “Most of the acoustic metamaterials have fixed geometries; therefore, their performance can only be turned on forever and for a certain frequency region,” he explained. “The key innovation of our materials is that we can use remote magnetic fields to deform the structures to alter the geometries on demand.” This allows the acoustic-manipulating performance of the materials to be turned on and off reversibly and rapidly, Wang said. “The function frequency can also be tuned just by modulating the external magnetic field,” he said.
3D printing plays a key role in enabling this new material function, Wang explained. By 3D printing a deformable material that contains iron particles in a lattice structure, researchers can compress the metamaterials using a magnetic field. “The 3D-printing technology can enable freeform-design of the structures and can rapidly fabricate the structures,” Wang explained.
There are two methods for 3D printing the new materials: directly using magnetic-particle-filled photoelastomers through a micro-projection stereolithography system, or inversely printing by using a water-dissolvable 3D-printed scaffold, he said. In the latter process, the elastomers—filled with magnetic particles—are cured within. “Both methods can enable the design of 3D architectures of these magnetically active elastomer lattices with high freedom,” Wang said.
They are eyeing two types of noise-cancellation and targeted-sound applications for the new metamaterials—switchable acoustic module and switchable acoustic filter, Wang said. The former can be used to block out noise or target music directly for sound optimization. “When the environment is noisy, you can turn on the magnetic control and the noise can be shielded; when the environment is music, you can turn off the magnetic control and the music can pass to your ears,” Wang said. The latter is for when the environment has both noise and music at the same time, he stated. “You can filter out the noise to leave the music just by tuning the magnetic control.”
