Acoustic lenses are employed in a variety of applications, from biomedical imaging and surgery to defense systems and damage detection in materials. Focused acoustic signals, for example, enable ultrasonic transducers to image the interior of the human body. Currently however the performance of acoustic devices is limited by their linear operational envelope, which implies relatively inaccurate focusing and low focal power. Here we show a dramatic focusing effect and the generation of compact acoustic pulses (sound bullets) in solid and fluid media, with energies orders of magnitude greater than previously achievable. This focusing is made possible by a tunable, nonlinear acoustic lens, which consists of ordered arrays of granular chains. The amplitude, size, and location of the sound bullets can be controlled by varying the static precompression of the chains. Theory and numerical simulations demonstrate the focusing effect, and photoelasticity experiments corroborate it. Our nonlinear lens permits a qualitatively new way of generating high-energy acoustic pulses, which may improve imaging capabilities through increased accuracy and signal-to-noise ratios and may lead to more effective nonintrusive scalpels, for example, for cancer treatment.
The simple set up 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 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.
The beauty of this system is that it’s just a bunch of ball bearings that we control with weights,” said Chiara Daraio, a member of the research team. Caltech’s acoustic lens relies on the same principle as Newton’s cradle-that toy your high school science teacher probably kept on his or her desk with metal balls on strings that demonstrated the conservation of energy.
In this design, 21 parallel chains each contain 21 bearings. When the team strikes one end, it starts a compression wave that carries through the system. But instead of having the last ball swing out like a pendulum and bring the momentum back into the system, like the toy does, the acoustic lens focuses all the energy at the end of the system onto one spot, just a few inches away from the metamaterial.
The paper also hints at use in defense systems, though it leaves the implications of that to the imaginations of others. 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.
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