Sound waves can also control objects inside organisms. Daniel Ahmed, engineer at ETH Zurich in Switzerland, recently used ultrasound to move hollow plastic beads inside a living zebrafish embryo. In performing these experiments, Ahmed aims to demonstrate the potential of using sound to direct drugs to a target site in an animal, such as a tumor. Similar to the acoustic tweezers, the ultrasound creates a repetitive pattern of low and high pressure areas inside the embryo, allowing Ahmed to use the pressure pockets to push the beads around. Other researchers are examining the control of sound for the treatment of kidney stones. A study from 2020used, for example, ultrasound to move the stones around the blisters of live pigs.
Other researchers are developing a technology known as acoustic holography to shape sound waves, to more precisely design the location and shape of the pressure zones in a medium. Scientists project sound waves through a patterned plate known as an acoustic hologram, which is often 3D-printed and computer-designed. It shapes the sound waves in an intricate, predefined way, just as an optical hologram does for light. In particular, researchers are investigating how they can use acoustic holograms for brain research, focusing ultrasound waves to target a precise location in the head, which could be useful for imaging and therapeutic purposes.
Andrea Alù also explores new ways of shaping sound waves, but not necessarily tailored to specific applications. In a recent demonstration, his team controlled sound with legos.
To control the sound propagation in new ways, his team stacked the plastic blocks on a platter in a lattice pattern so that they protrude like trees in a forest. By shaking the dish, they produced sound waves on the surface. But the sound traveled bizarrely across the dish. Normally, a sound wave should be scattered symmetrically in concentric circles, like the ripple of a pebble falling into a pond. Alu could make the sound travel only in certain patterns.
Alu’s project draws inspiration not from light, but from the electron – which according to quantum mechanics is both a wave and a particle. In particular, the Legos were designed to mimic the crystal pattern of a type of material known as twisted double-layer graphene, which restricts the motion of its electrons in a characteristic way. Under certain conditions, electrons flow only on the edges of this material. Under others, the material becomes superconducting, and the electrons form pairs and move through it without electrical resistance.
Because electrons move so strangely in this material, Alù’s team predicted that the crystal geometry, scaled up to Lego size, would also limit the motion of sound. In one experiment, the team found that they could make the sound emanate in an elongated egg shape or in ripples that curve outward like the tips of a slingshot.
These unusual acoustic paths illustrated surprising parallels between sound and electrons and suggest more versatile ways of controlling sound propagation, which may prove useful for ultrasound imaging or the acoustic technology that cell phones rely on to communicate with cell towers, Alù says. For example, Alù created a device with similar principles that allow sound to propagate in only one direction. The unit can thus distinguish a transmission signal from a return signal, which means that the technology can send and receive signals of the same frequency simultaneously. This is in contrast to sonar, which emits an acoustic wave and has to wait for the echo to return before it pings the surroundings again.
But aside from applications, these experiments have changed the way scientists think about sound. It’s not just something you can blow up from rooftops, whisper in someone’s ear or even use to map an underwater environment. It is becoming a precision tool that scientists can shape, direct and manipulate according to their needs.