In our increasingly noisy world, it can be hard to find some quiet time. Now, a team of mechanical engineers at Boston University has developed a new device that is specially designed to block up to 94 percent of incoming sound waves, while still letting air pass through.
Current technology can be pretty effective at blocking sound. The walls of concert halls or recording studios are stuffed with thick cladding and large cavities to muffle outside noise, and newer metasurfaces could work the same way in a fraction of the space. But either way, that’s not going to allow much airflow. So the Boston team set out to design an acoustic metamaterial that could effectively block sound without blocking the passage of air.
“Sound is made by very tiny disturbances in the air,” say the researchers. “So, our goal is to silence those tiny vibrations. If we want the inside of a structure to be open air, then we have to keep in mind that this will be the pathway through which sound travels.”
Their design is a 3D-printed, ring-shaped device that’s made to some very precise mathematical standards. The shape is specifically designed to interfere with incoming sound waves and send them bouncing back the way they came. Material in the outer ring coils around like a helix, reducing the sound that can pass through the open center.
To test the device, the researchers placed a prototype in the end of a PVC pipe, and hooked a speaker up to the other end. The speaker blasted a tone through the pipe, but from the outside it was inaudible to the human ear. When the metamaterial was removed, the tone was suddenly reverberating through the room. According to the researchers, the device was able to block 94 percent of the sound.
“The moment we first placed and removed the silencer … was literally night and day,” says Jacob Nikolajczyk, co-author of the study. “We had been seeing these sorts of results in our computer modeling for months – but it is one thing to see modeled sound pressure levels on a computer, and another to hear its impact yourself.”
To test the device, the researchers placed a prototype in the end of a PVC pipe, and hooked a speaker up to the other end. The speaker blasted a tone through the pipe, but from the outside it was inaudible to the human ear. When the metamaterial was removed, the tone was suddenly reverberating through the room. According to the researchers, the device was able to block 94 percent of the sound.
“The moment we first placed and removed the silencer … was literally night and day,” says Jacob Nikolajczyk, co-author of the study. “We had been seeing these sorts of results in our computer modeling for months – but it is one thing to see modeled sound pressure levels on a computer, and another to hear its impact yourself.”
Acoustic metamaterial silences sound without blocking airflow [New Atlas]