April 2019 Newsletter: Acoustic Metamaterials

Why have acoustic metamaterials appeared on the scene in the last few years, and what are they? An acoustic metamaterial is a material designed to control, direct, and manipulate sound waves in gases, liquids and solids; a common theme of acoustic metamaterial devices is an apparent defiance of the intuitive laws of physics. Now researchers from Boston University have developed a synthetic, sound-silencing material that can block 94% of sounds without blocking air! Read on to learn more about this exciting new development in the world of acoustics.

The most common method of attenuating unwanted sound today is through the use of physical barriers. Using a barrier, we can reduce any type of sound for the most part –  from traffic noise to mechanical equipment on a roof. However, these physical barriers impede visual aspects of the landscape in the case of a highway barrier, as well as airflow, which can impede the operation of mechanical equipment.

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 (March 2019) demonstrating it’s possible to silence sound using an open, ring-like structure created to mathematically-perfect specifications for cutting out sounds while maintaining airflow. The mathematically-designed acoustic metamaterial is shaped in such a way that it sends incoming sounds back to where they came from, Ghaffarivardavagh and Zhang say. They calculated the dimensions and specifications that the metamaterial would need to have in order to interfere with the transmitted sound waves thus preventing sound, but not air, from being radiated through the open structure.

As a test case, the researchers decided to create a structure that could silence sound from a loudspeaker. To manufacture the metamaterial, they used a 3-D printer. The researchers sealed the loudspeaker into one end of a PVC pipe. On the other end of the pipe, the tailor-made acoustic metamaterial was fastened into the opening. Standing in the room, based on your sense of hearing alone, you’d never know that the loudspeaker was blasting an irritatingly high-pitched note. The metamaterial, ringing around the internal perimeter of the pipe’s mouth, worked like a mute button until the moment when Ghaffarivardavagh reached down and pulled it free. The lab suddenly echoed with the screeching of the loudspeaker. 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. Check out this video of Professor Zhang testing her acoustical metamaterial here.

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. These include drones, MRIs, jet engines, HVAC systems and countless other applications.  Additionally, the outer shape does not have to be round; it can be a cube or hexagon, which will be instrumental in building a wall.  The hexagonal shapes can fit together like an open-air honeycomb structure.

So next time you are eating a donut, hold it up and see if you can block any sound!

May 2019 Newsletter: Low End Theory
March 2019 Newsletter: Recovering Sound from Silence