With acoustic lenses having many different applications, such as in biomedical imaging and surgery, defense systems and damage detection in materials, this is an exciting breakthrough.

The device, which consists of a line of identical balls suspended from a frame by wires in a similar way to the Newton's cradle toy, makes superior acoustic images, and could also operate as a non-invasive sonic scalpel that homes in on and destroys cancerous tissues located within the body, or examine the interior of objects such as bridges, ship hulls and airplane wings for damage.

The study, which builds on work into the fundamental dynamics of one-dimensional granular chains, was undertaken by two researchers from the California Institute of Technology (Caltech). Published in the Proceedings of the National Academy of Sciences (PNAS), [Spadoni, A., and Daraio, C., PNAS (2010) doi:10.1073/pnas.1001514107], they made their acoustic lens by assembling 21 parallel chains of stainless steel spheres into an array, with each chain being strung with 21 9.5-millimeter-wide spheres.

As in Newton's cradle, the balls can only move in one plane, and only just touch one another. As 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 impact of the striker excites a pulse at one end, producing nonlinear waves within each chain. The compact acoustic pulses produced are known as solitary waves.

These waves maintain the same spatial wavelength in any given system, and can have very high amplitude without undergoing any distortion within the lens, unlike signals that are produced by existing technology.

By changing the amount of precompression of the waves, the team were able to vary the speed of the solitary wave, allowing for a target material to be hit at a focal point. It is this action that forms the sound bullet. Altering the parameters of the system can even produce a rapid-fire barrage of bullets that all hit the same spot.

Although years from being commercially viable, the device has the ability to control the focal points, which means that sound bullets are easy adapted to a range of applications. The study also has the potential to open up further enquiry into which new materials can be designed by leveraging on nonlinear effects, rather than trying to remove them.