New research proves that if you’re bothered by low-frequency city noises (think traffic, airplanes, and other urban noise), the humble ping-pong ball offers a cheap way to block them out.

Low-frequency noise is known to be harmful to our health, but it is difficult to prevent. It usually comes from different directions and is not blocked by walls or structures like high-frequency noise. Researchers from the University of Lille in France and the National Technical University of Athens in Greece used ping-pong balls as Helmholtz resonators to combat low-frequency noise; containers specially designed to absorb sounds of certain frequencies.

Named after German physicist Hermann von Helmholtz, who created the original Helmholtz resonator, the devices consist of an empty chamber resembling a ping-pong ball with a small hole.

“Ping-pong balls are well-known everyday objects found in large quantities around the world,” says physicist Robin Sabat of the University of Lille.

“Our motivation was to use these ready-made objects to create a low-frequency isolation panel. Ping-pong balls therefore represent an economical alternative to acoustic isolators, both in terms of low cost and possible recyclability.”

As the name suggests, Helmholtz resonators work through resonance by aligning the oscillations of sound waves in a way that absorbs them. The volume of the container and the size of the opening determine the sound frequency the resonator can absorb.

While Helmholtz resonators have been studied extensively before, the researchers wanted to see how they might work and interact when brought together to form an acoustic metasurface; Materials specifically designed to manipulate sound waves in a variety of ways.

Such coupling or coupling has been shown to increase the number of resonant frequencies absorbed by the acoustic metasurface of the ping-pong ball. In other words, more sound can be blocked by using multiple Helmholtz resonators together.

“The Helmholtz resonator has the unique ability to accurately capture sound waves of the medium at its natural frequency and can be represented as cavities connected to the environment via a narrow throat,” says Sabat.

“The originality of the work was to consider the effect of the coupling between two resonators, which led to the emergence of two resonant frequencies.”

Using a combination of mathematical modeling and real-world experiments, the researchers demonstrated how the interaction of multiple HRs can be manipulated to control locked sound frequencies.

While there are no large-scale field trials of ping-pong balls blocking certain sounds, there is potential for these ubiquitous and affordable objects to be used to protect against noise pollution and more.

“The potential of this metasurface goes beyond sound insulation,” says Sabat. “It can be extended to achieve a variety of functions similar to other metasurfaces.” “These features include audio focusing, non-traditional audio mapping, audio manipulation, and more.”