Last May, researchers at the U.S. Naval Research Laboratory in Washington, DC revealed that elastic response and engineered gradient materials could be combined to make extremely efficient acoustic metamaterials, according to Physics Today.
What are acoustic metamaterials? They are advanced materials that can effectively block out sound using only their geometry. The question is, can they do it without disrupting airflow?
In March of 2019, 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, submitted a paper demonstrating an acoustic metamaterial that could effectively cut out sounds while maintaining airflow.
"Today's sound barriers are literally thick heavy walls," said at the time Ghaffarivardavagh. The researcher decided that there had to be a better more material-efficient way to silence noise and proceeded to engineer it.
This new method is particularly useful in situations where thick heavy walls cannot be used like a jet engine's exhaust vent. Barricading a jet engine is not an option, so, the crew surrounding it wear earplugs to protect their hearing from the powerful roar instead.
But, what if there was a way to allow the airflow of the jet engine while blocking the sound? Zhang and Ghaffarivardavagh invented an acoustic metamaterial that could do just that.
They used 3D printing to materialize an open structure made of plastic and proceeded to test it with a loudspeaker. The trial was a hit as the loudspeaker blasted at an irritatingly high noise level but nothing at all could be heard! The noise-canceling acoustic metamaterial was working.
Zhang's team reported being ecstatic about the success of their test. "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," said Jacob Nikolajczyk, a study co-author and former undergraduate researcher in Zhang's lab.
The team's further explorations indicated that they could block out 94 percent of the noise of absolutely anything. The new development has many applications from airplanes to drones to construction. The study is published in the journal Physical Review B.
For more acoustic metamaterial examples be sure to read our article on the topic.