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Caltech researchers have built a device that produces highly focused, high-amplitude acoustic signals dubbed ‘sound bullets’.
The non-linear acoustic lens has “the potential to revolutionize applications from medical imaging and therapy to the nondestructive evaluation of materials and engineering systems,” says Chiara Daraio, assistant professor of aeronautics and applied physics at Caltech.
Her team built the acoustic lens by assembling 21 parallel chains of stainless steel spheres into an array. Each of the 21 chains was strung with 21 9.5-millimeter-wide spheres, just like a Newton’s cradle toy.
In the lens, a pulse is excited at one end by an impact with a striker, and nonlinear waves are generated within each chain. These chains, Daraio says, “are the simplest representation of highly nonlinear acoustic waveguides, which exploit the properties of particle contacts to tune the shapes of the traveling acoustic signals and their speed of propagation, creating compact acoustic pulses known as solitary waves.”
“The solitary waves always maintain the same spatial wavelength in a given system,” she adds, “and can have very high amplitude without undergoing any distortion within the lens, unlike the signals produced by currently available technology.”
The chains are precompressed using fishing line. By changing the amount of precompression, Daraio and Spadoni were able to vary the speed of the solitary wave.
When a series of those waves exit the array, they coalesce at a focal point, which can be located in a gas, a liquid or a solid.
This focus forms the sound bullet — a highly compact, large-amplitude acoustic wave. Varying the parameters of the system can produce a rapid-fire barrage of sound bullets, all trained on the same spot.
In the current design, the spheres are assembled in a two-dimensional arrangement, with each row independent of its neighbors. “Three-dimensional arrangements will be just as easy to create and will allow 3-D control of the sound bullets’ appearance and travel path,” Spadoni says.