Boston University engineers have developed a material that can “mute” noise without reducing airflow. Known as an acoustic metamaterial, it combined math and 3D printing to keep sound out but not air and light. It’s, in essence, a giant mute button.
How Silencing Sound Works
When sound—which is defined as the vibration of air molecules, which travels in a wave—hits a material, three things can happen. A surface can reflect it, which amplifies the sound. It can also transmit it, although it does this with varying degrees of clarity—the thinner the material, the sharper and louder the sound. Finally, a surface can also absorb it, where the sound waves get trapped in the microscopic imperfections of the receiving material. The sound, which is a form of energy, is converted to a small amount of heat. You can see (hear?) absorption in action when you go outside when it’s snowing. The snow, with all its surface flaws, absorb the sound, making the environment sound muffled.
Sound barriers use the third principle (absorption) to reduce noise, but these are typically just thick materials that deaden sound waves before it reaches the listener’s ear. Designers usually compromise between letting air through or keeping sound out. Some loud sound sources cannot be controlled this way, for example, so those who listen to these sources should protect their ears (such as using earplugs) instead of dealing with the source of the sound itself.
What This Acoustic Metamaterial Offers
Not so this newly developed material. Using precise math, the engineers made a donut-shaped device with dimensions that can nullify the waveform of the incoming sound. The shape of the device interferes with the sound and sends it back to where it came from. They used 3D printing to arrive at a ring-shaped device for this purpose.
The engineers then inserted this device into a PVC pipe attached to a loudspeaker playing loud music. The results were astounding: the material reduced noise by as much as 94%, making the difference with it and without it like “night and day,” said the co-author of the study. They went on to say that while the computer models predicted it with high accuracy, it’s still one thing to hear the results yourself.
Future Applications
The possibilities are endless for the application of this material, but the biggest is likely in industrial settings, especially in dense urban areas. It can reduce the risk of hearing loss among workers exposed to repetitive and loud sounds, like in the construction, air travel, and military sectors. On things that are somewhat closer to home, it can also be used to moderate loud appliances like HVAC units or garden tools like chainsaws.
This can also find uses in modern interior design, especially in fields where natural lighting is an important aspect. The material’s engineers said that the donut shape is just proof of concept, so production runs of the material can make it look anywhere from a ring to a hexagonal brick that can be scaled up to build a porous wall.
As noise pollution is becoming more notorious worldwide, an oasis of quiet provided by this material may well be one of the best inventions to come from this decade.
