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‘Sound bullet’ could be weapon or cure

An old toy has inspired a powerful new weapon that could destroy submarines or annihilate tumors.
Image: Newton's cradle
Researchers at Cal Tech adapted the Newton's cradle toy, shown here in an illustration, for a new field of science called metamaterials. Andrea Danti /
/ Source: Discovery Channel

An old toy has inspired a powerful new weapon that could destroy submarines or annihilate tumors.

Using a modified version of Newton's cradle, a series of stainless steel balls suspended by fishing wire, scientists from the California Institute of Technology have created a powerful weapon for soldiers and doctors known as a "sound bullet."

"The beauty of this system is that it's just a bunch of ball bearings that we control with weights," said Chiara Daraio, a professor at Cal Tech and co-author of a new Proceedings of the National Academy of Sciences article. Yet by varying the pressure on the ball bearings, the scientists can dramatically amplify and focus sound waves to make them "extremely destructive," according to Daraio.

Newton's cradle is an old toy, but the Cal Tech scientists, including Alessandro Spadonia, have adapted it for a hot new field of science known as metamaterials, which includes the Harry Potter-style invisibility cloak. Metamaterials are usually difficult and time-consuming to create.

For their study, the researchers lined up 21 rows of stainless steel ball bearings in an area the size and shape of a laptop computer, with weights attached at the ends to vary the pressure on each row. Then the scientists dropped a small ball about eight inches onto the ball bearings. Using a high-speed camera and a pressure sensitive material, the scientists watched the pressure waves focus on a single spot several inches away from the metamaterial.

The simple setup belies the power of the new metamaterial. Not only did the scientists focus all of the sound waves onto one specific area; they also amplified those waves more than 100 times than what any other metamaterial had previously produced. Those numbers could easily go higher, said Daraio.

The sound waves Daraio and Spadonia manipulated were too high-pitched for human ears to detect. Properly adapted to audible sound, the new metamaterial could turn a normal sentence into a split second ear drum rupturing explosion.

If these sound bullets were actual bullets, the metamaterial would be like transforming hot lead projectiles into rocket propelled grenades, all converging on one place at one time. The damage from such concentrated waves of pressure could create would be devastating.

Like normal bullets, sound bullets can travel through air. Unlike normal bullets, sound bullets can also easily travel through liquids and solids. Sound bullets could be used by the military to create submarine-melting waves of pressure or shock waves powerful enough to destroy caves otherwise untouchable by conventional weapons.

Sound bullets won't just destroy large objects. Daraio says that powerful, focused sound waves could obliterate hard kidney stones or destroy cancerous tumors without any damage to surrounding tissue.

Other, less destructive applications are also possible, said Daraio. Focused sound waves could let engineers see weak bridge supports or future potholes in roads.

Military or medical use of the new metamaterial is still years away, said Daraio. Yet the simplicity and effectiveness of the sound bullet makes it a "significant development," according to L.B. Freund, a professor at Brown University who was not involved in the research.

"People have focused sound waves for a long time," said Freund. "But if you want to do anything with a sound wave, what you can do is determined by the intensity of the sound wave."

Now that Daraio and Spadonia have found a new way to create these powerful sound waves, the potential range of applications for sound bullets is huge.

"The research has opened a new way to developed applied devices, and was done by young researchers in a very creative way," said Freund.