We live in an incredibly noisy world. The relentless hum of traffic, industrial machinery, and motorized tools has become a constant presence—and source of irritation—in our daily lives. Yet, ambient noise is more than a mere inconvenience; it impacts mental and physical health.
The World Health Organization rates environmental noise second to only air pollution on a list of health impacts. It’s a leading cause of stress, sleep disturbances, and even cardiovascular disease. Equally unsettling, a study conducted by a group of researchers in Germany found that even moderate levels of indoor noise undermine children’s ability to learn.
“We have made a lot of progress in addressing air pollution and water quality, but high levels of sound remain a problem,” said Xin Zhang, Distinguished Professor of Engineering at Boston University. “At home, at school, at work and as we sleep, we increasingly treasure silence.”
As a result, researchers are exploring new and novel ways to dial down the volume. They are turning to highly specialized materials and structures designed to buffer, baffle, and block sound waves. These include new types of fabrics that can serve as lightweight sound barriers, and acoustic metamaterials that can modify and even cloak sound waves.
Beyond the Noise
The quest for quieter environments is not new. Centuries ago, people planted trees or constructed thick walls from masonry to block sound. Today, insulation and other types of soundproofing materials are commonly used in hotels, office buildings, and along busy freeways. Planes, trains, and public places are typically filled with people wearing noise cancelling ear buds and headsets.
Yet these various forms, structures, and devices can go only so far in decreasing the cacophony of sounds that exist in the modern world. Oftentimes, there’s no practical way to block noise and, when it is possible, physical structures are expensive to construct and may provide marginal benefits. Walls and barriers can also cause other problems, such as blocking a physical path or interfering with light or ventilation.
Physics and materials science allows scientists to boldly go where sound mitigation has not gone before. “It’s possible to engineer metamaterials and use structural components to break the symmetry with which sound waves propagate,” said Andrea Alù, Distinguished Professor and founding director of the Photonics Initiative at the City University of New York. “It’s possible to create materials that have very different acoustic properties than natural materials.”
Acoustic circulators are one of Alù’s breakthroughs. Constructed from metamaterials, the devices can control the directionality of acoustic energy flow. By disrupting the inherent symmetry of sound propagation in space, it’s possible to direct sound waves, optimize sound propagation, and reduce or increase sound scattering, depending on the design of an object. These systems also can create a one-way path for sound waves, or cloak the source entirely.
Other researchers are finding other ways to reduce noise. Zhang, for example, has worked with fellow researchers at Boston University to develop metamaterial structures of various shapes that can cancel up to 94% of ambient sound. The structures can serve as walls or indoor partitions; they can take on different shapes and designs to address different frequencies.
“There is no limit on how you can design these structures,” Zhang said. “You can use a 3D printer to produce different colors and shapes to fit the practical and design requirements of a space. You can produce beautiful walls that are highly functional.” In fact, designers can fabricate these structures from different materials, which makes them ideal for airports, subways, industrial sites, and other noisy or hazardous locations.
Meanwhile, at the Massachusetts Institute of Technology (MIT), researchers have developed a noise-blocking silk sheet that is both portable and pliable. This invention, which is enhanced with a special fiber to add strength and resilience, transforms acoustic sounds into electricity. Essentially, the silk sheet serves as a microphone and produces vibrations that block sound. It reduces the amplitude of vibration waves by 95% and attenuates transmitted sound by up to 75%.
Good Vibrations
Artificial intelligence (AI) is also making waves—and helping researchers diminish sound across a broader spectrum of frequencies. At New Zealand’s University of Auckland, a group of researchers has developed two-dimensional honeycomb structures that guide sound waves along engineered paths. This makes it possible for the sound to avoid target objects. The group used machine learning along with a deep learning tool called Conditional Variational Auto-encoder (CVAE) to design these acoustic metamaterials.
The practical gains extend outside the lab and into the physical world. “These acoustic metamaterials can be used for vibration and noise control measures in various places, such as industrial environments, apartments, vehicles, airports, roadways, and even next to ventilated windows,” said Lihua Tang, an associate professor in the University of Aukland’s Department of Mechanical and Mechatronics Engineering. “AI is a game changer.”
At Pusan National University in South Korea, associate professor Sang Min Park of the School of Mechanical Engineering is using a deep learning method that taps inverse design to create sound-dampening acoustic metamaterials that can reduce noise while preserving air flow. These ventilated acoustic resonators (VARs) expand the sound-blocking bandwidth by nearly 29%.
A New Tune
In the years ahead, engineered materials and entirely different microstructures will reshape the way we think about sound and noise, Alù said. Among other things, sound abatement could extend into new areas and spaces. It could, for example, lead to specialized soundproofing partitions in hospitals, new types of surfaces and furniture in offices, and adaptive walls and windows. The technology could also transform product design in areas as varied as automotive tires and gardening equipment.
Sound science is also taking new turns. For instance, researchers are now exploring magnets and electromagnets that would dampen sound. It might be possible to tune these devices with vibration-damping properties that could reduce the noise produced by industrial machinery and bridges. The technology might also introduce noise-cancelling walls, floors, and ceilings. Meanwhile, active acoustic metamaterials might adapt their acoustic response, based on conditions.
“There are many practical applications for this area of science,” Zhang said. “Sound reduction technologies have advanced from conceptual to feasible. They will become part of our lives in the years ahead.”
Samuel Greengard is an author and journalist based in West Linn, OR, USA.
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