Sound waves can also steer objects inside living things. Daniel Ahmed, an engineer at ETH Zurich in Switzerland, recently used ultrasound to move hollow plastic beads inside a living zebrafish embryo. By conducting these experiments, Ahmed aims to demonstrate the potential of using sound to direct drugs to target sites in animals such as tumors. Similar to acoustic tweezers, ultrasound creates a repeating pattern of low and high pressure regions in the embryo, allowing Ahmed to use pressure pockets to push the beads away. Other researchers are investigating the ability to steer sound to treat kidney stones. For example, in a 2020 study, ultrasound was used to move stones in the bladder of a living pig.
Other researchers are developing a technique known as acoustic holography that forms sound waves to more accurately design the location and shape of pressure zones in a medium. Scientists project sound waves through a patterned plate called an acoustic hologram. Acoustic holograms are often 3D printed and computer designed. It forms sound waves in a complex and predefined way, much like optical holograms do for light. In particular, researchers are investigating how acoustic holography can be used to study the brain to focus ultrasound and target the exact location of the head. This can be useful for imaging and therapeutic purposes.
Andrea Alù is also exploring new ways to shape sound waves, but they are not necessarily tailored to a particular application. In one recent demonstration, his team controlled the sound with Lego.
His team stacked plastic blocks on a platter in a grid pattern, protruding like a forest tree, in order to control sound propagation in new ways. By shaking the platter, sound waves are generated on the surface. However, the sound was strangely transmitted on the platter. Normally, the sound waves need to be symmetrically dispersed in concentric circles, like the ripples of pebbles falling into a pond. Alù can only convey sound in a specific pattern.
Alù’s projects are inspired by electrons, not light. Electrons are both waves and particles, according to quantum mechanics. In particular, Lego was designed to mimic the crystal pattern of a type of material known as twisted double-layer graphene that limits the movement of electrons in a unique way. Under certain conditions, electrons flow only at the edges of this material. Under others, the material becomes superconducting and the electrons form pairs and move through it without electrical resistance.
Since the electrons move so strangely in this material, Alù’s team predicted that the shape of the crystal scaled up to the LEGO size would also limit the movement of the sound. In experiments, the team discovered that the sound could be emitted in the form of an elongated egg, or in ripples that bend outwards like the tip of a slingshot.
These anomalous acoustic trajectories show amazing similarities between sound and electrons, suggesting more diverse ways to control sound propagation. This can be useful for ultrasound imaging and acoustic technology where mobile phones rely on communication with the cell tower. For example, Alù created a device with a similar principle that allows sound to propagate in only one direction. Therefore, the device can distinguish between the transmit signal and the return signal. This means that technology can send and receive signals of the same frequency at the same time. This is different from sonar, which sends a sound wave, waits for the echo to return, and then pings the environment again.
But applications aside, these experiments changed the way scientists think about sound. It’s not just about blasting from the rooftop, whispering to someone’s ears, or mapping the undersea environment. It is becoming a precision tool that scientists can shape, direct, and manipulate to suit their needs.