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