Researchers create metamaterial that can block sound relatively well

Scientists at Boston University have developed so-called acoustic metamaterial that, in the laboratory, appears to be very capable of attenuating sound to a large extent. These are structures made with a 3D printer and which interfere with sound waves.

Multidisciplinary professor Xin Zhang and fellow scientist Reza Ghaffarivardavagh, two mechanical engineers from Boston University, argue that it is possible to almost completely attenuate or eliminate noise or sound with an open, ring-like structure. They claim that it is possible with certain materials to block the sound so that it can hardly be heard, without affecting the air movement.

The latter is important, because, for example, with aircraft engines you should not have any damage to the air movement. Otherwise, a jet aircraft would not be able to take off. However, the movement of the air also generates sound and the scientists argue that this can also be damped without using thick sound barriers such as walls.

For this they printed a 3D structure of plastic that allows air movement, but at the same time blocks the sound waves. This is meta material. These are synthetic materials that are made with a property that does not occur naturally in materials. It usually concerns metals or plastics that are arranged in repeating patterns that are smaller than the wavelengths they are intended to affect.

To test whether they could block the small vibrations that sound can produce, the researchers made a test set-up in a laboratory with a speaker and mounted a hollow PVC tube that consists of a ‘noise-cancelling structure’ on it. The acoustic metamaterial was only placed all the way to the front of the tube on the inside, and how much could be heard.

“Standing in space, based purely on your hearing, you wouldn’t know that a loudspeaker was producing a loud, irritating, high-pitched tone,” the researchers write. They describe removing this material as using a mute button. “It was a night and day difference,” said one of the researchers. They were found to be able to muffle 94 percent of the sound from the loudspeaker, making the sound from the loudspeaker inaudible to the human ear.

The researchers mainly see possibilities for use with drones, where people often complain about the noise. Ghaffarivardavagh says it is possible to eliminate the noise heard on the ground if sound-damping open structures are placed under the drone propellers. The technology can also offer a solution for air conditioning systems and fans. “The idea is that we can now mathematically design an object that can block the sound of anything,” Zhang says.

According to the researchers, their structure is very light, open and beautiful, so that it is also easy to apply in walls, for example. The shape of the acoustic metamaterial can be completely adjusted based on the application; it doesn’t have to be a ring and can take all kinds of shapes. For a wall, they think of a hexagonal shape that can be put together like an open-air honeycomb structure. This could also be used with MRI machines in a hospital to remove the intense vibrations and noise.

This research has been published in the scientific journal Physical Review B, under the title Ultra-open acoustic metamaterial silencer based on Fano-like interference.

In the outer ring is a helical pattern that interferes with the sound. The sound waves are blocked and thus do not pass through the open hole of the ring. At the same time, the air can still move through the ring.