Managing traffic noise pollution has vexed researchers in large part because of the broad range of frequencies we encounter on the road. Currently, only heavy, wall-like barriers can effectively dampen all of these various sounds.
Researchers at the University of Michigan in Ann Arbor have brought a new method into the fold, demonstrating an origami lattice prototype that can potentially reduce acoustic noise on roadways. The technique allows researchers to selectively dampen noise at various frequencies by adjusting the distance between noise-diffusing elements. They report their work this week in the Journal of Applied Physics.
"Our main contribution we're developing is an adaptive structure that can change its periodicity between different Bravais lattices with distinct symmetry properties," said Kon-Well Wang, one of the study's authors. "It is known that if you reconfigure the lattice structure in such a manner, you will change the wave propagation characteristics significantly."
In the origami sonic barriers, noise-diffusing cylinders called inclusions are placed on an aluminum sheet bent into a Miura fold, a common origami folding method. As the resulting lattice folds, the inclusions are drawn closer together or farther apart, diffusing noise in different frequency ranges.
"The lattice contains only one degree of freedom, making it particularly easy to collapse and expand," said Manoj Thota, another author of the paper.
Manipulating a lattice of inclusions might allow traffic experts to adjust noise-dampening devices to particular frequency ranges. Heavier vehicles produce noise at lower frequencies than lighter vehicles. Cars traveling quickly during off-peak times skew toward higher frequencies than cars stuck in traffic jams.
Concrete walls that line the roadway are effective across a broad spectrum of noise frequencies, but the wind they block can add unwanted force on their foundations. Since they feature flat surfaces, Thota said, the reflected wave is not diffusive enough to reduce the sound on the road. With a straight top edge, the incidence of oblique waves onto these barriers leads to higher diffraction and increases the propagation across the barrier.
The researchers' work draws on periodic sound barriers, in which a series of inclusions dampen sound at certain frequencies while allowing wind to pass through. The drawback, Thota said, is that these systems are fixed designs. If a system is designed to alleviate noise from traffic jams, it is not as useful when cars are moving fast.
As more drivers take to the road, there has been growing concern about the impact noise has on blood pressure, hearing loss, and attentiveness at work and school. In a prototype, Thota and Wang found their barrier reduced acoustic pressure by 10 decibels, or 90 percent.
"The traffic noise that could otherwise be heard as far as a mile away would now only be perceived from a distance of 0.3 miles with these barriers," Thota said.
Benefits of origami structures might extend out of traffic noise reduction, Wang said, citing its potential for wave guiding and acoustic diodes.
"Overall, origami structure gives us an effective platform to adapt to environmental change," he said.
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"Reconfigurable origami sonic barriers with tunable bandgaps for traffic noise mitigation," Journal of Applied Physics (2017). DOI: 10.1063/1.4991026