Sound waves can also guide objects inside living organisms. Daniel Ahmed, an engineer at ETH Zurich in Switzerland, recently used ultrasound to move hollow plastic beads in live zebrafish embryos. Through 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, the ultrasound creates a repeating pattern of areas of low and high pressure within the embryo, allowing Ahmed to use a pressure bag to push the beads around. Other researchers are studying the control power of sound for kidney stones. For example, a 2020 study used ultrasound to move stones in the bladders of live pigs.
Other researchers are developing a technique called acoustic holography to shape sound waves in order to more precisely design the location and shape of pressure zones in a medium. Scientists project sound waves through a patterned plate called an acoustic hologram, which is usually 3D printed and computer designed. It shapes sound waves in a complex, predefined way, much like an optical hologram does to light. In particular, researchers are investigating how acoustic holograms can be used for brain studies to focus ultrasound waves on precise locations on the head, which could be useful for imaging and therapeutic purposes.
Andrea Alù also explores new ways to shape sound waves, but not necessarily tailored to specific applications. In a recent demo, his team used Lego bricks to control the sound.
To control sound propagation in new ways, his team stacked plastic blocks in a grid on a plate, making them stand up like trees in a forest. By shaking the plate, they created sound waves on the surface of the plate. But the sound spreads over the plate strangely. Generally, sound waves should spread out symmetrically in concentric circles, like the ripples of a pebble falling into a pond. Alù allows sound to travel only in specific patterns.
Alù’s project draws inspiration not from light, but from electrons – which, according to quantum mechanics, are both waves and particles. In particular, the LEGO bricks are designed to mimic the crystalline pattern of a material called twisted bilayer graphene, which confines the movement of its electrons in a unique way. Under certain conditions, electrons flow only at the edges of the material. In other cases, the material becomes superconducting, with electrons forming pairs and passing through it without resistance.
Because electrons move so strangely in the material, Alù’s team predicts that the geometry of the crystals, amplified to the size of Lego, will also limit the movement of sound. In one experiment, the team found that they could make the sound come out in an elongated egg shape or in ripples that curve outward like the tip of a slingshot.
These unusual acoustic traces illustrate surprising similarities between sound and electronics, and hint at more general ways to control sound propagation, which could have implications for ultrasound imaging or the acoustic technology that phones rely on to communicate with cell towers Useful, Alù said. For example, Alù created a device with a similar principle that allows sound to travel in only one direction. As such, the device can differentiate between transmitted and returned signals, which means it could enable technology to transmit and receive signals on the same frequency at the same time. This is different from sonar, which emits sound waves and must wait for the echo to return before pinging the environment again.
But applications aside, these experiments have changed the way scientists think about sound. It’s not just a map you can blast from a roof, whisper in someone’s ear, or even map the environment under the sea. It is becoming a precision tool that scientists can shape, direct and manipulate according to their needs.