Sound waves are used in many imaging applications, but they're often underpowered and hard to focus. But focus them into "sound bullets" and all sorts of interesting things happen.

A paper published in PNAS this week describes how scientists might transition from creating sound-based images with linear acoustic dynamics to using nonlinear approaches. Researchers created a system with an acoustic lens that can focus highly tunable and accurate signals into "sound bullets." Once researchers have slightly better control over them, the bullets could be used for everything from detecting objects underwater to acting as nonintrusive scalpels in certain kinds of surgery.

We use sound waves a lot to get an idea of what things look like without actually being able to see them, such as an unborn baby (ultrasound) or the underwater ruins that the History Channel leads you to believe might be Atlantis before they end an hour-long program on an ambiguous note (sonar). However, the images that these hard-to-focus signals produce are notoriously murky, and often show little more than nebulous blobs pulsating against a black background.

The quality ceiling for these images is a result of the actuators used in the imaging devices, as they cannot create compact or high-amplitude signals. Their performance can be improved somewhat by taking advantage of the geometry of transducers that can focus the signal a bit, or by using time-reversal focusing or phase lags to construct a more coherent signal.

Still, all these procedures rely on linear dynamics, meaning there are strict limits on how the signals can change over time and interact with each other.

Let's get nonlinear

In nonlinear dynamics, these rules don't apply, and signals can fly around in a bunch of new ways. While this creates more possibilities for improving wave performance, it also means that scientists have to limit the signals at more steps, and create an environment where the increased activity can be harnessed rather than allowed to dissipate or destructively interfere.

To create an acoustic lens to control a nonlinear signal, researchers needed materials that can support power law behavior, as opposed to constant linear behavior. They found that using chains of spherical particles in the lens supported even highly nonlinear signals as they moved through, and that these could scale down to linear motion.

Scientists assembled chains of spheres into an array and precompressed them so that an acoustic signal traveling through them with a fixed phase incident would experience specific amounts of phase delay at certain points. This way, the waves traveling through each chain could be directed to convene at a particular point and form a powerful signal.

Each chain of particles in the lens was able to feed a single-crested wave, with all the waves overlapping each other. At the point where all the waves intersect, they formed a "sound bullet," a small traveling region that retains a high energy density as it moves through its medium.

Once the sound bullet is formed, it can only be maintained in a linear nondispersive medium. This includes both solid and fluid substances, including human bodies, gases, and oceans. The lens' creators also found that it was possible to vary the focal length and phase velocity of the signals by varying the precompression states of the particles.

Of course, while a handful of acoustic signals arcing together to produce a sound bullet is theoretically great, that's not always the reality— sometimes the arcs are misshapen and don't all intersect where researchers want. This causes the "bullet" to list to one side or another, curving off-axis; however, scientists found that the bullet was just as powerful even when its path was crooked.

With this approach, scientists should be able to use the new acoustic lens with existing transducers, cutting down somewhat on adoption costs. Once the system provides a bit more control, possible uses include detection in defense systems, noninvasive scalpels in surgery, and many other applications in biomedical devices.

PNAS, 2010. DOI: 10.1073/pnas.1001514107 (About DOIs).

Listing image by NASA