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Super soundproofing is all about little balls

Ever wish you could really crank your home theater system without bothering the neighbors? The answer is little balls, and a bonus is making electricity.

Ever wish you could crank your home theater system so the explosions in Michael Bay's movies rock you out of your chair but don't put you out of favor with the neighbors? How about providing your kid with a truly soundproof room while she grapples with a saxophone?

graphic of one-way sound material
Sound waves can go one way but not the other. Georgios Theocharis/Caltech

Fill your walls with ball bearings of a certain size and your wishes will come true. Well, it's not quite that easy, but that's the idea behind acoustic research out of Caltech. The researchers found that the right arrangement of ball-bearing-size particles lets sound move in one direction, but not the reverse.

In addition to keeping apartment dwellers and concertgoers happy, the one-way sound material could be used to sharpen medical ultrasound imaging. And because it can lower the frequency of a sound, it could be used to filter out high-frequency noises.

The material could also be used to harvest energy from sound waves by efficiently channeling the waves to devices that convert vibrations to electricity.

The researchers, who detail their mechanism in the July 24 issue of the journal Nature Materials, made a prototype by cramming a line of steel ball bearings into a frame.

When they replaced a ball bearing at one end of the line with a smaller, lighter one--a defect, in research parlance--sound waves traveling down the line split and spread across a broad range of frequencies, which stopped the sound waves from traveling any farther. The upshot: Send sound waves through the chain in one direction, and they pass through normally. Send them through the other way, and they're stopped in their tracks.

The spheres can be made of ceramics, hard plastics, or metals. To work with sounds in the audible range, the spheres need to range in size from a few millimeters in diameter to a few centimeters, said Chiara Daraio, a Caltech professor of aeronautics and applied physics.

Other researchers have built one-way sound materials, but those materials halt sound waves gradually. The Caltech technique halts sound waves more abruptly, which makes it more appropriate for practical uses.

Acoustic materials made with technology could be ready for simpler uses like energy harvesting within a few years, Daraio said. Sound control uses could take longer, she said.