Researchers Use Acoustic Metamaterials to Block Out Sound

Most of us are immersed in urban sound, which ranges in volume from annoying to damaging.

A simple solution is building tall, heavy walls to repel sound. Although these structures can help reduce noise levels, they are thick and solid and not well suited for areas where airflow is also critical.

This made Xin Zhang, a professor of mechanical engineering at Boston University, wonder,  “How can we design a structure that can block noise but preserve air flow?”

Zhang teamed up with Ph.D. student Reza Ghaffarivardavagh to explore this idea further. Using metamaterials, mathematical calculations, and an open, ring-like structural design, they invented an open “silencer” that can maintain efficient ventilation while also silencing unwanted acoustic waves by up to 94 percent.

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Zhang believes a possible way for a device to block sound and still have an open structure was by using acoustic metamaterials. These artificial materials consist of “meta-atoms” that have dimensions much smaller than the acoustic wavelength of interest. The unique mechanical properties of these materials enable dynamic tuning and modulation at the material level.

Zhang and Ghaffarivardavagh calculated the specification the metamaterial would need to block transmitted sound waves and modeled the physical structure tht would be most effective for silencing noise. They then fabricated it using 3D printing.
 

The “ultra-open acoustic metamaterial silencer” features a large central opening that allows 60% airflow. Six helical channels wrapped around the central port generate an out-of-phase wave on the transmitted side. When the acoustic waves hit the central and helical channels, they cancel each other out on the transmitted side, resulting in silence.

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To test their design, Zhang and Ghaffarivardavagh sealed a loudspeaker into one end of a PVC pipe. At the other end, the acoustic metamaterial silencer was fastened across the opening at the other end of the pipe. When sound was played through the loudspeaker at high volume at the closed end of the pipe, no sound could be heard at the open end. However, when the silencer was removed, the sound coming from the pipe could be heard at full volume.

By comparing sound levels with and without the metamaterial fastened in place, the two determined they could silence 94 percent of the noise, making the sounds emanating from the loudspeaker almost imperceptible to the human ear.

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Zhang has received interest from large companies across many industries, including aerospace, automotive, manufacturing, energy and healthcare. She is now working to address the viability of each potential application. Their silencer has the potential for industrial applications, in which highly efficient, air-permeable sound silencers are required, such as for fan, propeller, or engine noise reduction, as well as smart sound barriers.
 

 “The ultra-open metamaterial silencer offers the potential to mitigate environmental noise from diverse sources,” said Zhang. “We are excited to explore many new opportunities where our design could benefit people, our environment, and working conditions by reducing noise.”

Mark Crawford is a science and technology writer in Corrales, N.M.