Rectangular and cylindrical PGUOM structures operate on the same underlying mechanism, offering comparable broadband silencing performance. Both geometries maintain high airflow, underscoring the metamaterial's versatility across diverse application scenarios. The samples shown were 3D-printed using a commercial printer. Credit: Zhiwei Yang and Xin Zhang
A new breakthrough from the Zhang Lab at Boston University is making waves in the world of sound control.
Led by Professor Xin Zhang (ME, ECE, BME, MSE), the team has published a new paper in Scientific Reports titled "Phase gradient ultra open metamaterials for broadband acoustic silencing."
The article marks a major advance in their long-running Acoustic Metamaterial Silencer project.
The Zhang lab is renowned in the fields of metamaterials and microsystems for its continual advancement of real-world applications. Back in 2019, their research on an Acoustic Metamaterial Silencer—or "sound shield"—aimed to "significantly block sound while maintaining airflow, based on Fano resonance effects," in the lab's words. At the time, applications focused on fans, propellers, and HVAC systems, targeting the reduction of narrowband noise while preserving air passage.
The Zhang lab has since extended its work to explore a broader range of acoustic silencing strategies—including multi-band, broadband, and tunable approaches—making the technology viable in new environments such as factories, offices, and public spaces, where diverse and unpredictable sound frequencies are common and airflow remains essential.
Their latest advance centers on broadband silencing. While this broader control came with a modest trade-off in peak silencing performance—a common challenge when shifting from narrowband to broadband suppression—it unlocked powerful new possibilities. The breakthrough was made possible through the use of phase-gradient metamaterials, giving rise to the Phase Gradient Ultra-Open Metamaterial (PGUOM).
Boston University’s customized ventilated PGUOM achieves broadband acoustic silencing while preserving airflow—reaching up to 70% openness. Credit: Zhiwei Yang and Xin Zhang
"PGUOM takes a smarter approach—more like noise-canceling headphones—effectively silencing a broadband of unwanted sounds," says Zhang.
"It remains highly effective even as the noise shifts in pitch or volume, making it far more practical in dynamic settings like open offices, ventilation systems, or transportation hubs, where sound sources are unpredictable and span a wide range of frequencies."
"Earlier designs based on Fano resonance—developed by our team—were like tuning a radio to block a single station," says Zhang. "PGUOM takes a smarter approach—more like noise-canceling headphones—effectively silencing the broadband of unwanted sounds. It remains highly effective even as the noise shifts in pitch or volume, making it far more practical in dynamic settings like open offices, ventilation systems, or transportation hubs, where sound sources are unpredictable and span a wide range of frequencies."
Further advances in the project have provided the team with greater design flexibility, enabling them to preserve airflow while adapting the structure to real-world systems.
Zhang explains that the metamaterial is composed of single or repeating supercells, each consisting of three subwavelength unit cells. Solid barriers are incorporated into the first and third unit cells to induce controlled phase shifts in the incoming sound waves, while the central unit cell remains open to allow unobstructed airflow. These engineered phase shifts generate a full 2π phase gradient across each supercell, converting incoming sound waves into spoof surface waves—acoustic counterparts to electromagnetic surface plasmons—which are trapped and dissipated along the surface.
The result: Broadband noise is suppressed efficiently, while airflow and geometric adaptability are maintained.
"Our design isn't one-size-fits-all—and that's a strength," Zhang says. "It's customizable in both frequency range and airflow level, depending on the application."
Unlike traditional phase-gradient structures with uniform unit cells, their design enlarges the central cell to accommodate varying airflow needs without compromising silencing performance.
The motivation behind the work is clear: "Chronic exposure to excessive noise—often overlooked compared to air and water pollution—can seriously impact human health, contributing to hearing loss, sleep disruption, heightened stress levels, and even cardiovascular disease," Zhang notes.
But the impact doesn't stop with humans—noise pollution also disrupts wildlife, altering mating and hunting patterns and destabilizing ecosystems. With recent design advances focused on lighter, more open, and broadband-capable materials, the team is now tackling these challenges on a broader scale—unlocking greater real-world impact.
These breakthroughs aren't just theoretical. The team has successfully transitioned from simulation to physical prototypes, and is now eyeing future deployment.
"We're focusing on integrating our designs into specific products and applications, while optimizing the metamaterials for scalable manufacturing processes," says Zhang. "We're also working to further enhance noise-blocking performance—aiming for high attenuation across even broader frequency bands, while preserving low airflow resistance and minimizing overall thickness."
Ultimately, the Zhang Lab is developing versatile, scalable solutions that can be applied across industries to make the world a quieter, healthier place.
Provided by Boston University
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