- Researchers build a lightweight sound-canceling device using sub-wavelength metamaterial structures.
- It blocks 94% of noise but preserves air passage.
- It can block a wide range of noises, including intense vibrations of an MRI machine.
The types of sound barriers we use today are nothing but thick heavy walls. Most sound baffles — noise reducing barricades — can contain the movie sounds within cinema hall walls, but this clunky approach cannot be applied to scenarios where airflow is necessary.
The need of sound attenuation while preserving ventilation — which is often required in mitigating fan noise — has inspired numerous efforts, especially within the context of duct acoustics. However, with recent advances in metamaterial science, new possibilities for handling acoustic energy have emerged.
Now, researchers at Boston University have developed a ringlike structure that can cancel noise without blocking air passage. They have done this by utilizing sub-wavelength metamaterial structures, wave-front modulation, acoustic cloaking, and sub-diffraction imaging.
Sound is a result of tiny disturbances in the air. The objective was to silence those tiny vibrations without blocking the path through which air travels.
The researchers measured the dimensions and characteristics that the metamaterial would require to have in order to obstruct sound waves from radiating through the structure. They wanted to design the metamaterial in such a way that it could send incoming sound back to where it came from.
In this study, researchers created a structure to block the sound coming from a loudspeaker. They proposed a transverse bilayer metamaterial concept — inspired by the Fano-like interference phenomena — to apply destructive interference to achieve acoustic silencing.
3D-printed acoustic metamaterial | Credit: Cydney Scott/Boston University
The calculated the dimensions that would most effectively silence noises and 3D printed the plastic structure. To test this acoustic metamaterial in the lab, the team used PVC pipe and sealed its one end into the loudspeaker and fastened its other end into the tailor-made structure.
Reference: Physical Review B | doi:10.1103/PhysRevB.99.024302 | Boston University
When they played a high-pitched note on the loudspeaker, it came out as a faint, soft noise in the lab. The open-acoustic silencing metamaterial successfully redirected the sound to the loudspeaker through the pipe. As soon as they pulled this metamaterial out, the lab echoed with the screeching noise.
The team compared the sound levels in both cases and found that the metamaterial was able to reduce 94% of the noise, making the loudspeaker sound ‘almost unnoticeable’ to the human ear.
How Is This Useful?
Since the metamaterial is lightweight and customizable, it can be used to build any type of structure. For example, each unit could be used as a brick (of any shape like hexagon or cube) to construct a noise-canceling, permeable wall. These walls would have the potential to contain a wide range of noises, including the intense vibrations of an MRI machine.
The team has some great ideas about how their device can make the real world quieter. It could be used in drones (beneath the drone fans) to cancel out the noise radiating toward the ground. Heating, Ventilation, Air Conditioning systems installed closer to office and homes could also benefit from this sound-reducing structure.