The ability on the part of researchers to manipulate sound waves has led to the development of critical technologies, for example, enabling ultrasonic transducers to image the interior of the human body (aka "ultrasound"). A team of researchers at the California Institute of Technology in Pasadena think they've found a way to make sound waves even more powerful with the help of a new type of acoustic lens. Caltech researchers Alessandro Spadoni and Chiara Daraio describe how they create "sound bullets" in a study published in the April 5 issue Proceedings of National Academy of Sciences.

Acoustic lenses are used to focus sound in much the same way that an optical lens focuses light. Instead of using glass and mirrors, the researchers designed an acoustic lens made of 21 rows of stainless-steel spheres, with each row having 21 spheres. "We generate acoustic solitary waves by striking the top portion of the lens (or the first sphere in each row) so that we send a compressive wave down each stack or row," Spadoni wrote in an e-mail to Scientific American. These waves are then transmitted into whatever is on the other side of the lens.

"It's the same thing as an ultrasound exam at the hospital," according to Spadoni. "The doctor puts a probe on a person's body. The waves generated by this probe are sent into the body and are modulated such that they are reflected properly." One idea for using the compact acoustic pulses, or sound bullets, that Caltech created in the lab might be as a safer alternative to radiation-based imaging, such as magnetic resonance imaging (MRI) scans.

Sound bullets might also be used as a non-intrusive scalpel for surgical operations, the researchers say. "In particular, tissue temperature at the focal point can be increased with high acoustic-energy density, which results from a compact focal volume and high pressure induced by sound bullets," Spadoni adds. "This effect is at the core of a surgical procedure known as hyperthermia [therapy]. In this sense, our nonlinear acoustic lens could aid the treatment of tumors by increasing the temperature of cancerous tissue without affecting adjacent healthy tissue."

Spadoni and Daraio created a two-dimensional acoustic lens for the purposes of their research, but Spadoni says it's possible to create a 3-D model by stacking spheres (see image). "The bottom line is the more rows of spheres, the higher the amplitude of the sound bullet (or pressure at the focal point)," he adds.

Image of a possible 3-D lens configuration and the resulting pressure field courtesy of the National Academy of Sciences PNAS