Study Shows Spiral Sound Can Shift Sideways

Likun Zhang, associate professor of physics and astronomy and senior scientist at the National Center for Physical Acoustics, published his team's study on behavior of spiral sound waves in Physical Review Letters, a premier journal for physics research.

The experiment is the first measurement of the Hall Effect as it applies to acoustics. The Hall Effect occurs when something traveling forward – traditionally an electric current – is deflected slightly to the side by an external influence such as a magnetic field.

"About five years ago, our group extended the concept of the Hall Effect to acoustics, where we predicted that this would be the case," Zhang said. "But this follow-up is the first time that we've been able to say, experimentally, 'Here is that shift, and we can prove that it's there.'"

Understanding this effect could help researchers better control structured sound waves, which could improve medical instruments, communications and other technology that rely on sound. Acoustic tweezers, for example, use sound waves to manipulate particles as small as a nanometer. For comparison, a single human hair is about 100,000 nanometers wide.

In cases where such extreme precision is necessary, understanding how the Hall Effect can influence sound could aid in designing more deft equipment.

"This is important in understanding sound wave physics, but also in practical applications," Zhang said. "For instance, we can use this beam to rotate objects like a sonic wrench.

"In biomedical engineering, this would allow us to mix fluids, move micro-particles in cells or tissue, but it could also be used in acoustic communication like in underwater environments."

Using vortex beams – sound that spirals forward – Zhang and his team were able to shift the direction of the beam slightly by passing it through a specially engineered surface, shifting the beam's angular momentum and demonstrating the acoustic orbital Hall Effect for the first time.

"The Hall Effect was discovered in the late 19th century and is considered one of the foundational experimental results in solid state physics," said Kevin Beach, Ole Miss chair and associate professor of physics and astronomy. "Dr. Zhang and his coauthors have demonstrated that an entirely analogous effect can be produced in the context of acoustic waves using metamaterials.

"This is an astonishing achievement. And it's a delight to see Hall Effect physics appear in such an unexpected place."

As the team predicted in their 2021 publication, the shift in the vortex beam is extremely small and difficult to measure. Unlike their previous research, however, this experiment was not a digital model in an idealized scenario, but a physical experiment in the real world, where any minute change might affect the outcome.

"This is a very interesting interaction between sound and materials that has never been noticed before," Nathan Murray, director of the acoustics center. "Understanding subtle effects like this one can ultimately unlock potential designs for new technologies that use sound to measure or even move tiny objects.

"Research like this strengthens the scientific foundation that future acoustic technologies can build on."

Top: Beams of sound that spiral forward can be shifted sideways by passing through some surfaces, an Ole Miss acoustics team has discovered. This shift, known as the Hall Effect, promises to allow scientists to develop new biomedical devices or even improve underwater communication. Graphic by Cole Russell/University Marketing and Communications