Caltech researchers have created a new device that shoots out highly focused single waves of sound. The system utilizes a radically new way of creating these “sound bullets” over traditional sound focusing like in current high intensity focused ultrasound (HIFU) machines. Additionally, the ability to generate individual waves that are not part of a larger vibration cycle, should allow for highly precise medical applications of this technology.
Caltech press ofice explains:
Daraio [Chiara Daraio, assistant professor of aeronautics and applied physics] and postdoctoral scholar Alessandro Spadoni, first author of the paper, crafted their 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.
The device is akin to the Newton’s cradle toy, which consists of a line of identical balls suspended from a frame by wires in such a way that they only move in one plane, and just barely touch one another. When one of the end balls is pulled back and released, it strikes the next ball in line and the ball at the opposite end of the cradle flies out; the balls in the middle appear to remain stationary.
The chains of particles in Daraio’s and Spadoni’s acoustic lens are like a longer version of a Newton’s cradle. 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 chains are squeezed closer together—or "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 particular location—a focal point—in a target material (which can be a gas, like air; a liquid; or a solid). This superposition of solitary waves at the focal point forms the sound bullet—a highly compact, large-amplitude acoustic wave. Varying the parameters of the system can also produce a rapid-fire barrage of sound bullets, all trained on the same spot.
Press release: Caltech Researchers Create “Sound Bullets”…
Abstract in Proceedings of the National Academy of Sciences: Generation and control of sound bullets with a nonlinear acoustic lens